core/macros/

mod.rs

#[doc = include_str!("panic.md")]
#[macro_export]
#[rustc_builtin_macro(core_panic)]
#[allow_internal_unstable(edition_panic)]
#[stable(feature = "core", since = "1.6.0")]
#[rustc_diagnostic_item = "core_panic_macro"]
macro_rules! panic {
    // Expands to either `$crate::panic::panic_2015` or `$crate::panic::panic_2021`
    // depending on the edition of the caller.
    ($($arg:tt)*) => {
        /* compiler built-in */
    };
}

/// Asserts that two expressions are equal to each other (using [`PartialEq`]).
///
/// Assertions are always checked in both debug and release builds, and cannot
/// be disabled. See [`debug_assert_eq!`] for assertions that are disabled in
/// release builds by default.
///
/// [`debug_assert_eq!`]: crate::debug_assert_eq
///
/// On panic, this macro will print the values of the expressions with their
/// debug representations.
///
/// Like [`assert!`], this macro has a second form, where a custom
/// panic message can be provided.
///
/// # Examples
///
/// ```
/// let a = 3;
/// let b = 1 + 2;
/// assert_eq!(a, b);
///
/// assert_eq!(a, b, "we are testing addition with {} and {}", a, b);
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "assert_eq_macro"]
#[allow_internal_unstable(panic_internals)]
#[cfg(not(feature = "ferrocene_certified_runtime"))]
macro_rules! assert_eq {
    ($left:expr, $right:expr $(,)?) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    let kind = $crate::panicking::AssertKind::Eq;
                    // The reborrows below are intentional. Without them, the stack slot for the
                    // borrow is initialized even before the values are compared, leading to a
                    // noticeable slow down.
                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
                }
            }
        }
    };
    ($left:expr, $right:expr, $($arg:tt)+) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    let kind = $crate::panicking::AssertKind::Eq;
                    // The reborrows below are intentional. Without them, the stack slot for the
                    // borrow is initialized even before the values are compared, leading to a
                    // noticeable slow down.
                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
                }
            }
        }
    };
}

/// Asserts that two expressions are equal to each other (using [`PartialEq`]).
///
/// Assertions are always checked in both debug and release builds, and cannot
/// be disabled. See [`debug_assert_eq!`] for assertions that are disabled in
/// release builds by default.
///
/// [`debug_assert_eq!`]: crate::debug_assert_eq
///
/// On panic, this macro will print the values of the expressions with their
/// debug representations.
///
/// Like [`assert!`], this macro has a second form, where a custom
/// panic message can be provided.
///
/// # Examples
///
/// ```
/// let a = 3;
/// let b = 1 + 2;
/// assert_eq!(a, b);
///
/// assert_eq!(a, b, "we are testing addition with {} and {}", a, b);
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "assert_eq_macro"]
#[allow_internal_unstable(panic_internals)]
#[cfg(feature = "ferrocene_certified_runtime")]
macro_rules! assert_eq {
    ($left:expr, $right:expr $(,)?) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    panic!("assertion `left == right` failed")
                }
            }
        }
    };
    ($left:expr, $right:expr, $msg:literal) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    panic!(concat!("assertion `left == right` failed: ", $msg))
                }
            }
        }
    };
    ($left:expr, $right:expr, $($arg:tt)+) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    panic!("assertion `left == right` failed")
                }
            }
        }
    };
}

/// Asserts that two expressions are not equal to each other (using [`PartialEq`]).
///
/// Assertions are always checked in both debug and release builds, and cannot
/// be disabled. See [`debug_assert_ne!`] for assertions that are disabled in
/// release builds by default.
///
/// [`debug_assert_ne!`]: crate::debug_assert_ne
///
/// On panic, this macro will print the values of the expressions with their
/// debug representations.
///
/// Like [`assert!`], this macro has a second form, where a custom
/// panic message can be provided.
///
/// # Examples
///
/// ```
/// let a = 3;
/// let b = 2;
/// assert_ne!(a, b);
///
/// assert_ne!(a, b, "we are testing that the values are not equal");
/// ```
#[macro_export]
#[stable(feature = "assert_ne", since = "1.13.0")]
#[rustc_diagnostic_item = "assert_ne_macro"]
#[allow_internal_unstable(panic_internals)]
#[cfg(not(feature = "ferrocene_certified_runtime"))]
macro_rules! assert_ne {
    ($left:expr, $right:expr $(,)?) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if *left_val == *right_val {
                    let kind = $crate::panicking::AssertKind::Ne;
                    // The reborrows below are intentional. Without them, the stack slot for the
                    // borrow is initialized even before the values are compared, leading to a
                    // noticeable slow down.
                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
                }
            }
        }
    };
    ($left:expr, $right:expr, $($arg:tt)+) => {
        match (&($left), &($right)) {
            (left_val, right_val) => {
                if *left_val == *right_val {
                    let kind = $crate::panicking::AssertKind::Ne;
                    // The reborrows below are intentional. Without them, the stack slot for the
                    // borrow is initialized even before the values are compared, leading to a
                    // noticeable slow down.
                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
                }
            }
        }
    };
}

/// Asserts that two expressions are not equal to each other (using [`PartialEq`]).
///
/// Assertions are always checked in both debug and release builds, and cannot
/// be disabled. See [`debug_assert_ne!`] for assertions that are disabled in
/// release builds by default.
///
/// [`debug_assert_ne!`]: crate::debug_assert_ne
///
/// On panic, this macro will print the values of the expressions with their
/// debug representations.
///
/// Like [`assert!`], this macro has a second form, where a custom
/// panic message can be provided.
///
/// # Examples
///
/// ```
/// let a = 3;
/// let b = 2;
/// assert_ne!(a, b);
///
/// assert_ne!(a, b, "we are testing that the values are not equal");
/// ```
#[macro_export]
#[stable(feature = "assert_ne", since = "1.13.0")]
#[rustc_diagnostic_item = "assert_ne_macro"]
#[allow_internal_unstable(panic_internals)]
#[cfg(feature = "ferrocene_certified_runtime")]
macro_rules! assert_ne {
    ($left:expr, $right:expr $(,)?) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if *left_val == *right_val {
                    panic!("assertion `left != right` failed")
                }
            }
        }
    };
    ($left:expr, $right:expr, $msg:literal) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if *left_val == *right_val {
                    panic!(concat!("assertion `left != right` failed: ", $msg))
                }
            }
        }
    };
    ($left:expr, $right:expr, $($arg:tt)+) => {
        match (&$left, &$right) {
            (left_val, right_val) => {
                if *left_val == *right_val {
                    panic!("assertion `left != right` failed")
                }
            }
        }
    };
}

/// Asserts that an expression matches the provided pattern.
///
/// This macro is generally preferable to `assert!(matches!(value, pattern))`, because it can print
/// the debug representation of the actual value shape that did not meet expectations. In contrast,
/// using [`assert!`] will only print that expectations were not met, but not why.
///
/// The pattern syntax is exactly the same as found in a match arm and the `matches!` macro. The
/// optional if guard can be used to add additional checks that must be true for the matched value,
/// otherwise this macro will panic.
///
/// Assertions are always checked in both debug and release builds, and cannot
/// be disabled. See [`debug_assert_matches!`] for assertions that are disabled in
/// release builds by default.
///
/// [`debug_assert_matches!`]: crate::assert_matches::debug_assert_matches
///
/// On panic, this macro will print the value of the expression with its debug representation.
///
/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
///
/// # Examples
///
/// ```
/// #![feature(assert_matches)]
///
/// use std::assert_matches::assert_matches;
///
/// let a = Some(345);
/// let b = Some(56);
/// assert_matches!(a, Some(_));
/// assert_matches!(b, Some(_));
///
/// assert_matches!(a, Some(345));
/// assert_matches!(a, Some(345) | None);
///
/// // assert_matches!(a, None); // panics
/// // assert_matches!(b, Some(345)); // panics
/// // assert_matches!(b, Some(345) | None); // panics
///
/// assert_matches!(a, Some(x) if x > 100);
/// // assert_matches!(a, Some(x) if x < 100); // panics
/// ```
#[unstable(feature = "assert_matches", issue = "82775")]
#[allow_internal_unstable(panic_internals)]
#[rustc_macro_transparency = "semitransparent"]
#[cfg(not(feature = "ferrocene_certified_runtime"))]
pub macro assert_matches {
    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
        match $left {
            $( $pattern )|+ $( if $guard )? => {}
            ref left_val => {
                $crate::panicking::assert_matches_failed(
                    left_val,
                    $crate::stringify!($($pattern)|+ $(if $guard)?),
                    $crate::option::Option::None
                );
            }
        }
    },
    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => {
        match $left {
            $( $pattern )|+ $( if $guard )? => {}
            ref left_val => {
                $crate::panicking::assert_matches_failed(
                    left_val,
                    $crate::stringify!($($pattern)|+ $(if $guard)?),
                    $crate::option::Option::Some($crate::format_args!($($arg)+))
                );
            }
        }
    },
}

/// Asserts that an expression matches the provided pattern.
///
/// This macro is generally preferable to `assert!(matches!(value, pattern))`, because it can print
/// the debug representation of the actual value shape that did not meet expectations. In contrast,
/// using [`assert!`] will only print that expectations were not met, but not why.
///
/// The pattern syntax is exactly the same as found in a match arm and the `matches!` macro. The
/// optional if guard can be used to add additional checks that must be true for the matched value,
/// otherwise this macro will panic.
///
/// Assertions are always checked in both debug and release builds, and cannot
/// be disabled. See [`debug_assert_matches!`] for assertions that are disabled in
/// release builds by default.
///
/// [`debug_assert_matches!`]: crate::assert_matches::debug_assert_matches
///
/// On panic, this macro will print the value of the expression with its debug representation.
///
/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
///
/// # Examples
///
/// ```
/// #![feature(assert_matches)]
///
/// use std::assert_matches::assert_matches;
///
/// let a = Some(345);
/// let b = Some(56);
/// assert_matches!(a, Some(_));
/// assert_matches!(b, Some(_));
///
/// assert_matches!(a, Some(345));
/// assert_matches!(a, Some(345) | None);
///
/// // assert_matches!(a, None); // panics
/// // assert_matches!(b, Some(345)); // panics
/// // assert_matches!(b, Some(345) | None); // panics
///
/// assert_matches!(a, Some(x) if x > 100);
/// // assert_matches!(a, Some(x) if x < 100); // panics
/// ```
#[unstable(feature = "assert_matches", issue = "82775")]
#[allow_internal_unstable(panic_internals)]
#[rustc_macro_transparency = "semitransparent"]
#[cfg(feature = "ferrocene_certified_runtime")]
pub macro assert_matches {
    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) =>{
        match $left {
            $( $pattern )|+ $( if $guard )? => {}
            ref left_val => {
                panic!("assertion `left matches right` failed");
            }
        }
    },
    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $msg:literal) => {
        match $left {
            $( $pattern )|+ $( if $guard )? => {}
            ref left_val => {
                panic!(concat!("assertion `left matches right` failed: ", $msg));
            }
        }
    },
    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => {
        match $left {
            $( $pattern )|+ $( if $guard )? => {}
            ref left_val => {
                panic!("assertion `left matches right` failed");
            }
        }
    },
}
/// Selects code at compile-time based on `cfg` predicates.
///
/// This macro evaluates, at compile-time, a series of `cfg` predicates,
/// selects the first that is true, and emits the code guarded by that
/// predicate. The code guarded by other predicates is not emitted.
///
/// An optional trailing `_` wildcard can be used to specify a fallback. If
/// none of the predicates are true, a [`compile_error`] is emitted.
///
/// # Example
///
/// ```
/// #![feature(cfg_select)]
///
/// cfg_select! {
///     unix => {
///         fn foo() { /* unix specific functionality */ }
///     }
///     target_pointer_width = "32" => {
///         fn foo() { /* non-unix, 32-bit functionality */ }
///     }
///     _ => {
///         fn foo() { /* fallback implementation */ }
///     }
/// }
/// ```
///
/// The `cfg_select!` macro can also be used in expression position, with or without braces on the
/// right-hand side:
///
/// ```
/// #![feature(cfg_select)]
///
/// let _some_string = cfg_select! {
///     unix => "With great power comes great electricity bills",
///     _ => { "Behind every successful diet is an unwatched pizza" }
/// };
/// ```
#[unstable(feature = "cfg_select", issue = "115585")]
#[rustc_diagnostic_item = "cfg_select"]
#[rustc_builtin_macro]
pub macro cfg_select($($tt:tt)*) {
    /* compiler built-in */
}

/// Asserts that a boolean expression is `true` at runtime.
///
/// This will invoke the [`panic!`] macro if the provided expression cannot be
/// evaluated to `true` at runtime.
///
/// Like [`assert!`], this macro also has a second version, where a custom panic
/// message can be provided.
///
/// # Uses
///
/// Unlike [`assert!`], `debug_assert!` statements are only enabled in non
/// optimized builds by default. An optimized build will not execute
/// `debug_assert!` statements unless `-C debug-assertions` is passed to the
/// compiler. This makes `debug_assert!` useful for checks that are too
/// expensive to be present in a release build but may be helpful during
/// development. The result of expanding `debug_assert!` is always type checked.
///
/// An unchecked assertion allows a program in an inconsistent state to keep
/// running, which might have unexpected consequences but does not introduce
/// unsafety as long as this only happens in safe code. The performance cost
/// of assertions, however, is not measurable in general. Replacing [`assert!`]
/// with `debug_assert!` is thus only encouraged after thorough profiling, and
/// more importantly, only in safe code!
///
/// # Examples
///
/// ```
/// // the panic message for these assertions is the stringified value of the
/// // expression given.
/// debug_assert!(true);
///
/// fn some_expensive_computation() -> bool {
///     // Some expensive computation here
///     true
/// }
/// debug_assert!(some_expensive_computation());
///
/// // assert with a custom message
/// let x = true;
/// debug_assert!(x, "x wasn't true!");
///
/// let a = 3; let b = 27;
/// debug_assert!(a + b == 30, "a = {}, b = {}", a, b);
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "debug_assert_macro"]
#[allow_internal_unstable(edition_panic)]
macro_rules! debug_assert {
    ($($arg:tt)*) => {
        if $crate::cfg!(debug_assertions) {
            $crate::assert!($($arg)*);
        }
    };
}

/// Asserts that two expressions are equal to each other.
///
/// On panic, this macro will print the values of the expressions with their
/// debug representations.
///
/// Unlike [`assert_eq!`], `debug_assert_eq!` statements are only enabled in non
/// optimized builds by default. An optimized build will not execute
/// `debug_assert_eq!` statements unless `-C debug-assertions` is passed to the
/// compiler. This makes `debug_assert_eq!` useful for checks that are too
/// expensive to be present in a release build but may be helpful during
/// development. The result of expanding `debug_assert_eq!` is always type checked.
///
/// # Examples
///
/// ```
/// let a = 3;
/// let b = 1 + 2;
/// debug_assert_eq!(a, b);
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "debug_assert_eq_macro"]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! debug_assert_eq {
    ($($arg:tt)*) => {
        if $crate::cfg!(debug_assertions) {
            $crate::assert_eq!($($arg)*);
        }
    };
}

/// Asserts that two expressions are not equal to each other.
///
/// On panic, this macro will print the values of the expressions with their
/// debug representations.
///
/// Unlike [`assert_ne!`], `debug_assert_ne!` statements are only enabled in non
/// optimized builds by default. An optimized build will not execute
/// `debug_assert_ne!` statements unless `-C debug-assertions` is passed to the
/// compiler. This makes `debug_assert_ne!` useful for checks that are too
/// expensive to be present in a release build but may be helpful during
/// development. The result of expanding `debug_assert_ne!` is always type checked.
///
/// # Examples
///
/// ```
/// let a = 3;
/// let b = 2;
/// debug_assert_ne!(a, b);
/// ```
#[macro_export]
#[stable(feature = "assert_ne", since = "1.13.0")]
#[rustc_diagnostic_item = "debug_assert_ne_macro"]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! debug_assert_ne {
    ($($arg:tt)*) => {
        if $crate::cfg!(debug_assertions) {
            $crate::assert_ne!($($arg)*);
        }
    };
}

/// Asserts that an expression matches the provided pattern.
///
/// This macro is generally preferable to `debug_assert!(matches!(value, pattern))`, because it can
/// print the debug representation of the actual value shape that did not meet expectations. In
/// contrast, using [`debug_assert!`] will only print that expectations were not met, but not why.
///
/// The pattern syntax is exactly the same as found in a match arm and the `matches!` macro. The
/// optional if guard can be used to add additional checks that must be true for the matched value,
/// otherwise this macro will panic.
///
/// On panic, this macro will print the value of the expression with its debug representation.
///
/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
///
/// Unlike [`assert_matches!`], `debug_assert_matches!` statements are only enabled in non optimized
/// builds by default. An optimized build will not execute `debug_assert_matches!` statements unless
/// `-C debug-assertions` is passed to the compiler. This makes `debug_assert_matches!` useful for
/// checks that are too expensive to be present in a release build but may be helpful during
/// development. The result of expanding `debug_assert_matches!` is always type checked.
///
/// # Examples
///
/// ```
/// #![feature(assert_matches)]
///
/// use std::assert_matches::debug_assert_matches;
///
/// let a = Some(345);
/// let b = Some(56);
/// debug_assert_matches!(a, Some(_));
/// debug_assert_matches!(b, Some(_));
///
/// debug_assert_matches!(a, Some(345));
/// debug_assert_matches!(a, Some(345) | None);
///
/// // debug_assert_matches!(a, None); // panics
/// // debug_assert_matches!(b, Some(345)); // panics
/// // debug_assert_matches!(b, Some(345) | None); // panics
///
/// debug_assert_matches!(a, Some(x) if x > 100);
/// // debug_assert_matches!(a, Some(x) if x < 100); // panics
/// ```
#[unstable(feature = "assert_matches", issue = "82775")]
#[allow_internal_unstable(assert_matches)]
#[rustc_macro_transparency = "semitransparent"]
#[cfg(not(feature = "ferrocene_subset"))]
pub macro debug_assert_matches($($arg:tt)*) {
    if $crate::cfg!(debug_assertions) {
        $crate::assert_matches::assert_matches!($($arg)*);
    }
}

/// Returns whether the given expression matches the provided pattern.
///
/// The pattern syntax is exactly the same as found in a match arm. The optional if guard can be
/// used to add additional checks that must be true for the matched value, otherwise this macro will
/// return `false`.
///
/// When testing that a value matches a pattern, it's generally preferable to use
/// [`assert_matches!`] as it will print the debug representation of the value if the assertion
/// fails.
///
/// # Examples
///
/// ```
/// let foo = 'f';
/// assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
///
/// let bar = Some(4);
/// assert!(matches!(bar, Some(x) if x > 2));
/// ```
#[macro_export]
#[stable(feature = "matches_macro", since = "1.42.0")]
#[rustc_diagnostic_item = "matches_macro"]
#[allow_internal_unstable(non_exhaustive_omitted_patterns_lint, stmt_expr_attributes)]
macro_rules! matches {
    ($expression:expr, $pattern:pat $(if $guard:expr)? $(,)?) => {
        #[allow(non_exhaustive_omitted_patterns)]
        match $expression {
            $pattern $(if $guard)? => true,
            _ => false
        }
    };
}

/// Unwraps a result or propagates its error.
///
/// The [`?` operator][propagating-errors] was added to replace `try!`
/// and should be used instead. Furthermore, `try` is a reserved word
/// in Rust 2018, so if you must use it, you will need to use the
/// [raw-identifier syntax][ris]: `r#try`.
///
/// [propagating-errors]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
/// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
///
/// `try!` matches the given [`Result`]. In case of the `Ok` variant, the
/// expression has the value of the wrapped value.
///
/// In case of the `Err` variant, it retrieves the inner error. `try!` then
/// performs conversion using `From`. This provides automatic conversion
/// between specialized errors and more general ones. The resulting
/// error is then immediately returned.
///
/// Because of the early return, `try!` can only be used in functions that
/// return [`Result`].
///
/// # Examples
///
/// ```
/// use std::io;
/// use std::fs::File;
/// use std::io::prelude::*;
///
/// enum MyError {
///     FileWriteError
/// }
///
/// impl From<io::Error> for MyError {
///     fn from(e: io::Error) -> MyError {
///         MyError::FileWriteError
///     }
/// }
///
/// // The preferred method of quick returning Errors
/// fn write_to_file_question() -> Result<(), MyError> {
///     let mut file = File::create("my_best_friends.txt")?;
///     file.write_all(b"This is a list of my best friends.")?;
///     Ok(())
/// }
///
/// // The previous method of quick returning Errors
/// fn write_to_file_using_try() -> Result<(), MyError> {
///     let mut file = r#try!(File::create("my_best_friends.txt"));
///     r#try!(file.write_all(b"This is a list of my best friends."));
///     Ok(())
/// }
///
/// // This is equivalent to:
/// fn write_to_file_using_match() -> Result<(), MyError> {
///     let mut file = r#try!(File::create("my_best_friends.txt"));
///     match file.write_all(b"This is a list of my best friends.") {
///         Ok(v) => v,
///         Err(e) => return Err(From::from(e)),
///     }
///     Ok(())
/// }
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(since = "1.39.0", note = "use the `?` operator instead")]
#[doc(alias = "?")]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! r#try {
    ($expr:expr $(,)?) => {
        match $expr {
            $crate::result::Result::Ok(val) => val,
            $crate::result::Result::Err(err) => {
                return $crate::result::Result::Err($crate::convert::From::from(err));
            }
        }
    };
}

/// Writes formatted data into a buffer.
///
/// This macro accepts a 'writer', a format string, and a list of arguments. Arguments will be
/// formatted according to the specified format string and the result will be passed to the writer.
/// The writer may be any value with a `write_fmt` method; generally this comes from an
/// implementation of either the [`fmt::Write`] or the [`io::Write`] trait. The macro
/// returns whatever the `write_fmt` method returns; commonly a [`fmt::Result`], or an
/// [`io::Result`].
///
/// See [`std::fmt`] for more information on the format string syntax.
///
/// [`std::fmt`]: ../std/fmt/index.html
/// [`fmt::Write`]: crate::fmt::Write
/// [`io::Write`]: ../std/io/trait.Write.html
/// [`fmt::Result`]: crate::fmt::Result
/// [`io::Result`]: ../std/io/type.Result.html
///
/// # Examples
///
/// ```
/// use std::io::Write;
///
/// fn main() -> std::io::Result<()> {
///     let mut w = Vec::new();
///     write!(&mut w, "test")?;
///     write!(&mut w, "formatted {}", "arguments")?;
///
///     assert_eq!(w, b"testformatted arguments");
///     Ok(())
/// }
/// ```
///
/// A module can import both `std::fmt::Write` and `std::io::Write` and call `write!` on objects
/// implementing either, as objects do not typically implement both. However, the module must
/// avoid conflict between the trait names, such as by importing them as `_` or otherwise renaming
/// them:
///
/// ```
/// use std::fmt::Write as _;
/// use std::io::Write as _;
///
/// fn main() -> Result<(), Box<dyn std::error::Error>> {
///     let mut s = String::new();
///     let mut v = Vec::new();
///
///     write!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
///     write!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
///     assert_eq!(v, b"s = \"abc 123\"");
///     Ok(())
/// }
/// ```
///
/// If you also need the trait names themselves, such as to implement one or both on your types,
/// import the containing module and then name them with a prefix:
///
/// ```
/// # #![allow(unused_imports)]
/// use std::fmt::{self, Write as _};
/// use std::io::{self, Write as _};
///
/// struct Example;
///
/// impl fmt::Write for Example {
///     fn write_str(&mut self, _s: &str) -> core::fmt::Result {
///          unimplemented!();
///     }
/// }
/// ```
///
/// Note: This macro can be used in `no_std` setups as well.
/// In a `no_std` setup you are responsible for the implementation details of the components.
///
/// ```no_run
/// use core::fmt::Write;
///
/// struct Example;
///
/// impl Write for Example {
///     fn write_str(&mut self, _s: &str) -> core::fmt::Result {
///          unimplemented!();
///     }
/// }
///
/// let mut m = Example{};
/// write!(&mut m, "Hello World").expect("Not written");
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "write_macro"]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! write {
    ($dst:expr, $($arg:tt)*) => {
        $dst.write_fmt($crate::format_args!($($arg)*))
    };
}

/// Writes formatted data into a buffer, with a newline appended.
///
/// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
/// (no additional CARRIAGE RETURN (`\r`/`U+000D`).
///
/// For more information, see [`write!`]. For information on the format string syntax, see
/// [`std::fmt`].
///
/// [`std::fmt`]: ../std/fmt/index.html
///
/// # Examples
///
/// ```
/// use std::io::{Write, Result};
///
/// fn main() -> Result<()> {
///     let mut w = Vec::new();
///     writeln!(&mut w)?;
///     writeln!(&mut w, "test")?;
///     writeln!(&mut w, "formatted {}", "arguments")?;
///
///     assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());
///     Ok(())
/// }
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "writeln_macro"]
#[allow_internal_unstable(format_args_nl)]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! writeln {
    ($dst:expr $(,)?) => {
        $crate::write!($dst, "\n")
    };
    ($dst:expr, $($arg:tt)*) => {
        $dst.write_fmt($crate::format_args_nl!($($arg)*))
    };
}

/// Indicates unreachable code.
///
/// This is useful any time that the compiler can't determine that some code is unreachable. For
/// example:
///
/// * Match arms with guard conditions.
/// * Loops that dynamically terminate.
/// * Iterators that dynamically terminate.
///
/// If the determination that the code is unreachable proves incorrect, the
/// program immediately terminates with a [`panic!`].
///
/// The unsafe counterpart of this macro is the [`unreachable_unchecked`] function, which
/// will cause undefined behavior if the code is reached.
///
/// [`unreachable_unchecked`]: crate::hint::unreachable_unchecked
///
/// # Panics
///
/// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
/// fixed, specific message.
///
/// Like `panic!`, this macro has a second form for displaying custom values.
///
/// # Examples
///
/// Match arms:
///
/// ```
/// # #[allow(dead_code)]
/// fn foo(x: Option<i32>) {
///     match x {
///         Some(n) if n >= 0 => println!("Some(Non-negative)"),
///         Some(n) if n <  0 => println!("Some(Negative)"),
///         Some(_)           => unreachable!(), // compile error if commented out
///         None              => println!("None")
///     }
/// }
/// ```
///
/// Iterators:
///
/// ```
/// # #[allow(dead_code)]
/// fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
///     for i in 0.. {
///         if 3*i < i { panic!("u32 overflow"); }
///         if x < 3*i { return i-1; }
///     }
///     unreachable!("The loop should always return");
/// }
/// ```
#[macro_export]
#[rustc_builtin_macro(unreachable)]
#[allow_internal_unstable(edition_panic)]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "unreachable_macro"]
macro_rules! unreachable {
    // Expands to either `$crate::panic::unreachable_2015` or `$crate::panic::unreachable_2021`
    // depending on the edition of the caller.
    ($($arg:tt)*) => {
        /* compiler built-in */
    };
}

/// Indicates unimplemented code by panicking with a message of "not implemented".
///
/// This allows your code to type-check, which is useful if you are prototyping or
/// implementing a trait that requires multiple methods which you don't plan to use all of.
///
/// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
/// conveys an intent of implementing the functionality later and the message is "not yet
/// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
///
/// Also, some IDEs will mark `todo!`s.
///
/// # Panics
///
/// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
/// fixed, specific message.
///
/// Like `panic!`, this macro has a second form for displaying custom values.
///
/// [`todo!`]: crate::todo
///
/// # Examples
///
/// Say we have a trait `Foo`:
///
/// ```
/// trait Foo {
///     fn bar(&self) -> u8;
///     fn baz(&self);
///     fn qux(&self) -> Result<u64, ()>;
/// }
/// ```
///
/// We want to implement `Foo` for 'MyStruct', but for some reason it only makes sense
/// to implement the `bar()` function. `baz()` and `qux()` will still need to be defined
/// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
/// to allow our code to compile.
///
/// We still want to have our program stop running if the unimplemented methods are
/// reached.
///
/// ```
/// # trait Foo {
/// #     fn bar(&self) -> u8;
/// #     fn baz(&self);
/// #     fn qux(&self) -> Result<u64, ()>;
/// # }
/// struct MyStruct;
///
/// impl Foo for MyStruct {
///     fn bar(&self) -> u8 {
///         1 + 1
///     }
///
///     fn baz(&self) {
///         // It makes no sense to `baz` a `MyStruct`, so we have no logic here
///         // at all.
///         // This will display "thread 'main' panicked at 'not implemented'".
///         unimplemented!();
///     }
///
///     fn qux(&self) -> Result<u64, ()> {
///         // We have some logic here,
///         // We can add a message to unimplemented! to display our omission.
///         // This will display:
///         // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
///         unimplemented!("MyStruct isn't quxable");
///     }
/// }
///
/// fn main() {
///     let s = MyStruct;
///     s.bar();
/// }
/// ```
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "unimplemented_macro"]
#[allow_internal_unstable(panic_internals)]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! unimplemented {
    () => {
        $crate::panicking::panic("not implemented")
    };
    ($($arg:tt)+) => {
        $crate::panic!("not implemented: {}", $crate::format_args!($($arg)+))
    };
}

/// Indicates unfinished code.
///
/// This can be useful if you are prototyping and just
/// want a placeholder to let your code pass type analysis.
///
/// The difference between [`unimplemented!`] and `todo!` is that while `todo!` conveys
/// an intent of implementing the functionality later and the message is "not yet
/// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
///
/// Also, some IDEs will mark `todo!`s.
///
/// # Panics
///
/// This will always [`panic!`] because `todo!` is just a shorthand for `panic!` with a
/// fixed, specific message.
///
/// Like `panic!`, this macro has a second form for displaying custom values.
///
/// # Examples
///
/// Here's an example of some in-progress code. We have a trait `Foo`:
///
/// ```
/// trait Foo {
///     fn bar(&self) -> u8;
///     fn baz(&self);
///     fn qux(&self) -> Result<u64, ()>;
/// }
/// ```
///
/// We want to implement `Foo` on one of our types, but we also want to work on
/// just `bar()` first. In order for our code to compile, we need to implement
/// `baz()` and `qux()`, so we can use `todo!`:
///
/// ```
/// # trait Foo {
/// #     fn bar(&self) -> u8;
/// #     fn baz(&self);
/// #     fn qux(&self) -> Result<u64, ()>;
/// # }
/// struct MyStruct;
///
/// impl Foo for MyStruct {
///     fn bar(&self) -> u8 {
///         1 + 1
///     }
///
///     fn baz(&self) {
///         // Let's not worry about implementing baz() for now
///         todo!();
///     }
///
///     fn qux(&self) -> Result<u64, ()> {
///         // We can add a message to todo! to display our omission.
///         // This will display:
///         // "thread 'main' panicked at 'not yet implemented: MyStruct is not yet quxable'".
///         todo!("MyStruct is not yet quxable");
///     }
/// }
///
/// fn main() {
///     let s = MyStruct;
///     s.bar();
///
///     // We aren't even using baz() or qux(), so this is fine.
/// }
/// ```
#[macro_export]
#[stable(feature = "todo_macro", since = "1.40.0")]
#[rustc_diagnostic_item = "todo_macro"]
#[allow_internal_unstable(panic_internals)]
#[cfg(not(feature = "ferrocene_subset"))]
macro_rules! todo {
    () => {
        $crate::panicking::panic("not yet implemented")
    };
    ($($arg:tt)+) => {
        $crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+))
    };
}

/// Definitions of built-in macros.
///
/// Most of the macro properties (stability, visibility, etc.) are taken from the source code here,
/// with exception of expansion functions transforming macro inputs into outputs,
/// those functions are provided by the compiler.
pub(crate) mod builtin {

    /// Causes compilation to fail with the given error message when encountered.
    ///
    /// This macro should be used when a crate uses a conditional compilation strategy to provide
    /// better error messages for erroneous conditions. It's the compiler-level form of [`panic!`],
    /// but emits an error during *compilation* rather than at *runtime*.
    ///
    /// # Examples
    ///
    /// Two such examples are macros and `#[cfg]` environments.
    ///
    /// Emit a better compiler error if a macro is passed invalid values. Without the final branch,
    /// the compiler would still emit an error, but the error's message would not mention the two
    /// valid values.
    ///
    /// ```compile_fail
    /// macro_rules! give_me_foo_or_bar {
    ///     (foo) => {};
    ///     (bar) => {};
    ///     ($x:ident) => {
    ///         compile_error!("This macro only accepts `foo` or `bar`");
    ///     }
    /// }
    ///
    /// give_me_foo_or_bar!(neither);
    /// // ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"
    /// ```
    ///
    /// Emit a compiler error if one of a number of features isn't available.
    ///
    /// ```compile_fail
    /// #[cfg(not(any(feature = "foo", feature = "bar")))]
    /// compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");
    /// ```
    #[stable(feature = "compile_error_macro", since = "1.20.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! compile_error {
        ($msg:expr $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Constructs parameters for the other string-formatting macros.
    ///
    /// This macro functions by taking a formatting string literal containing
    /// `{}` for each additional argument passed. `format_args!` prepares the
    /// additional parameters to ensure the output can be interpreted as a string
    /// and canonicalizes the arguments into a single type. Any value that implements
    /// the [`Display`] trait can be passed to `format_args!`, as can any
    /// [`Debug`] implementation be passed to a `{:?}` within the formatting string.
    ///
    /// This macro produces a value of type [`fmt::Arguments`]. This value can be
    /// passed to the macros within [`std::fmt`] for performing useful redirection.
    /// All other formatting macros ([`format!`], [`write!`], [`println!`], etc) are
    /// proxied through this one. `format_args!`, unlike its derived macros, avoids
    /// heap allocations.
    ///
    /// You can use the [`fmt::Arguments`] value that `format_args!` returns
    /// in `Debug` and `Display` contexts as seen below. The example also shows
    /// that `Debug` and `Display` format to the same thing: the interpolated
    /// format string in `format_args!`.
    ///
    /// ```rust
    /// let args = format_args!("{} foo {:?}", 1, 2);
    /// let debug = format!("{args:?}");
    /// let display = format!("{args}");
    /// assert_eq!("1 foo 2", display);
    /// assert_eq!(display, debug);
    /// ```
    ///
    /// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
    /// for details of the macro argument syntax, and further information.
    ///
    /// [`Display`]: crate::fmt::Display
    /// [`Debug`]: crate::fmt::Debug
    /// [`fmt::Arguments`]: crate::fmt::Arguments
    /// [`std::fmt`]: ../std/fmt/index.html
    /// [`format!`]: ../std/macro.format.html
    /// [`println!`]: ../std/macro.println.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::fmt;
    ///
    /// let s = fmt::format(format_args!("hello {}", "world"));
    /// assert_eq!(s, format!("hello {}", "world"));
    /// ```
    ///
    /// # Argument lifetimes
    ///
    /// Except when no formatting arguments are used,
    /// the produced `fmt::Arguments` value borrows temporary values.
    /// To allow it to be stored for later use, the arguments' lifetimes, as well as those of
    /// temporaries they borrow, may be [extended] when `format_args!` appears in the initializer
    /// expression of a `let` statement. The syntactic rules used to determine when temporaries'
    /// lifetimes are extended are documented in the [Reference].
    ///
    /// [extended]: ../reference/destructors.html#temporary-lifetime-extension
    /// [Reference]: ../reference/destructors.html#extending-based-on-expressions
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_diagnostic_item = "format_args_macro"]
    #[allow_internal_unsafe]
    #[allow_internal_unstable(fmt_internals, fmt_arguments_from_str)]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! format_args {
        ($fmt:expr) => {{ /* compiler built-in */ }};
        ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
    }

    /// Same as [`format_args`], but can be used in some const contexts.
    ///
    /// This macro is used by the panic macros for the `const_panic` feature.
    ///
    /// This macro will be removed once `format_args` is allowed in const contexts.
    #[unstable(feature = "const_format_args", issue = "none")]
    #[allow_internal_unstable(fmt_internals, fmt_arguments_from_str)]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! const_format_args {
        ($fmt:expr) => {{ /* compiler built-in */ }};
        ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
    }

    /// Same as [`format_args`], but adds a newline in the end.
    #[unstable(
        feature = "format_args_nl",
        issue = "none",
        reason = "`format_args_nl` is only for internal \
                  language use and is subject to change"
    )]
    #[allow_internal_unstable(fmt_internals, fmt_arguments_from_str)]
    #[rustc_builtin_macro]
    #[doc(hidden)]
    #[macro_export]
    macro_rules! format_args_nl {
        ($fmt:expr) => {{ /* compiler built-in */ }};
        ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
    }

    /// Inspects an environment variable at compile time.
    ///
    /// This macro will expand to the value of the named environment variable at
    /// compile time, yielding an expression of type `&'static str`. Use
    /// [`std::env::var`] instead if you want to read the value at runtime.
    ///
    /// [`std::env::var`]: ../std/env/fn.var.html
    ///
    /// If the environment variable is not defined, then a compilation error
    /// will be emitted. To not emit a compile error, use the [`option_env!`]
    /// macro instead. A compilation error will also be emitted if the
    /// environment variable is not a valid Unicode string.
    ///
    /// # Examples
    ///
    /// ```
    /// let path: &'static str = env!("PATH");
    /// println!("the $PATH variable at the time of compiling was: {path}");
    /// ```
    ///
    /// You can customize the error message by passing a string as the second
    /// parameter:
    ///
    /// ```compile_fail
    /// let doc: &'static str = env!("documentation", "what's that?!");
    /// ```
    ///
    /// If the `documentation` environment variable is not defined, you'll get
    /// the following error:
    ///
    /// ```text
    /// error: what's that?!
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    #[rustc_diagnostic_item = "env_macro"] // useful for external lints
    macro_rules! env {
        ($name:expr $(,)?) => {{ /* compiler built-in */ }};
        ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Optionally inspects an environment variable at compile time.
    ///
    /// If the named environment variable is present at compile time, this will
    /// expand into an expression of type `Option<&'static str>` whose value is
    /// `Some` of the value of the environment variable (a compilation error
    /// will be emitted if the environment variable is not a valid Unicode
    /// string). If the environment variable is not present, then this will
    /// expand to `None`. See [`Option<T>`][Option] for more information on this
    /// type.  Use [`std::env::var`] instead if you want to read the value at
    /// runtime.
    ///
    /// [`std::env::var`]: ../std/env/fn.var.html
    ///
    /// A compile time error is only emitted when using this macro if the
    /// environment variable exists and is not a valid Unicode string. To also
    /// emit a compile error if the environment variable is not present, use the
    /// [`env!`] macro instead.
    ///
    /// # Examples
    ///
    /// ```
    /// let key: Option<&'static str> = option_env!("SECRET_KEY");
    /// println!("the secret key might be: {key:?}");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    #[rustc_diagnostic_item = "option_env_macro"] // useful for external lints
    macro_rules! option_env {
        ($name:expr $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Concatenates literals into a byte slice.
    ///
    /// This macro takes any number of comma-separated literals, and concatenates them all into
    /// one, yielding an expression of type `&[u8; _]`, which represents all of the literals
    /// concatenated left-to-right. The literals passed can be any combination of:
    ///
    /// - byte literals (`b'r'`)
    /// - byte strings (`b"Rust"`)
    /// - arrays of bytes/numbers (`[b'A', 66, b'C']`)
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(concat_bytes)]
    ///
    /// # fn main() {
    /// let s: &[u8; 6] = concat_bytes!(b'A', b"BC", [68, b'E', 70]);
    /// assert_eq!(s, b"ABCDEF");
    /// # }
    /// ```
    #[unstable(feature = "concat_bytes", issue = "87555")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! concat_bytes {
        ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Concatenates literals into a static string slice.
    ///
    /// This macro takes any number of comma-separated literals, yielding an
    /// expression of type `&'static str` which represents all of the literals
    /// concatenated left-to-right.
    ///
    /// Integer and floating point literals are [stringified](core::stringify) in order to be
    /// concatenated.
    ///
    /// # Examples
    ///
    /// ```
    /// let s = concat!("test", 10, 'b', true);
    /// assert_eq!(s, "test10btrue");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[rustc_diagnostic_item = "macro_concat"]
    #[macro_export]
    macro_rules! concat {
        ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Expands to the line number on which it was invoked.
    ///
    /// With [`column!`] and [`file!`], these macros provide debugging information for
    /// developers about the location within the source.
    ///
    /// The expanded expression has type `u32` and is 1-based, so the first line
    /// in each file evaluates to 1, the second to 2, etc. This is consistent
    /// with error messages by common compilers or popular editors.
    /// The returned line is *not necessarily* the line of the `line!` invocation itself,
    /// but rather the first macro invocation leading up to the invocation
    /// of the `line!` macro.
    ///
    /// # Examples
    ///
    /// ```
    /// let current_line = line!();
    /// println!("defined on line: {current_line}");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! line {
        () => {
            /* compiler built-in */
        };
    }

    /// Expands to the column number at which it was invoked.
    ///
    /// With [`line!`] and [`file!`], these macros provide debugging information for
    /// developers about the location within the source.
    ///
    /// The expanded expression has type `u32` and is 1-based, so the first column
    /// in each line evaluates to 1, the second to 2, etc. This is consistent
    /// with error messages by common compilers or popular editors.
    /// The returned column is *not necessarily* the line of the `column!` invocation itself,
    /// but rather the first macro invocation leading up to the invocation
    /// of the `column!` macro.
    ///
    /// # Examples
    ///
    /// ```
    /// let current_col = column!();
    /// println!("defined on column: {current_col}");
    /// ```
    ///
    /// `column!` counts Unicode code points, not bytes or graphemes. As a result, the first two
    /// invocations return the same value, but the third does not.
    ///
    /// ```
    /// let a = ("foobar", column!()).1;
    /// let b = ("人之初性本善", column!()).1;
    /// let c = ("f̅o̅o̅b̅a̅r̅", column!()).1; // Uses combining overline (U+0305)
    ///
    /// assert_eq!(a, b);
    /// assert_ne!(b, c);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! column {
        () => {
            /* compiler built-in */
        };
    }

    /// Expands to the file name in which it was invoked.
    ///
    /// With [`line!`] and [`column!`], these macros provide debugging information for
    /// developers about the location within the source.
    ///
    /// The expanded expression has type `&'static str`, and the returned file
    /// is not the invocation of the `file!` macro itself, but rather the
    /// first macro invocation leading up to the invocation of the `file!`
    /// macro.
    ///
    /// The file name is derived from the crate root's source path passed to the Rust compiler
    /// and the sequence the compiler takes to get from the crate root to the
    /// module containing `file!`, modified by any flags passed to the Rust compiler (e.g.
    /// `--remap-path-prefix`).  If the crate's source path is relative, the initial base
    /// directory will be the working directory of the Rust compiler.  For example, if the source
    /// path passed to the compiler is `./src/lib.rs` which has a `mod foo;` with a source path of
    /// `src/foo/mod.rs`, then calling `file!` inside `mod foo;` will return `./src/foo/mod.rs`.
    ///
    /// Future compiler options might make further changes to the behavior of `file!`,
    /// including potentially making it entirely empty. Code (e.g. test libraries)
    /// relying on `file!` producing an openable file path would be incompatible
    /// with such options, and might wish to recommend not using those options.
    ///
    /// # Examples
    ///
    /// ```
    /// let this_file = file!();
    /// println!("defined in file: {this_file}");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! file {
        () => {
            /* compiler built-in */
        };
    }

    /// Stringifies its arguments.
    ///
    /// This macro will yield an expression of type `&'static str` which is the
    /// stringification of all the tokens passed to the macro. No restrictions
    /// are placed on the syntax of the macro invocation itself.
    ///
    /// Note that the expanded results of the input tokens may change in the
    /// future. You should be careful if you rely on the output.
    ///
    /// # Examples
    ///
    /// ```
    /// let one_plus_one = stringify!(1 + 1);
    /// assert_eq!(one_plus_one, "1 + 1");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! stringify {
        ($($t:tt)*) => {
            /* compiler built-in */
        };
    }

    /// Includes a UTF-8 encoded file as a string.
    ///
    /// The file is located relative to the current file (similarly to how
    /// modules are found). The provided path is interpreted in a platform-specific
    /// way at compile time. So, for instance, an invocation with a Windows path
    /// containing backslashes `\` would not compile correctly on Unix.
    ///
    /// This macro will yield an expression of type `&'static str` which is the
    /// contents of the file.
    ///
    /// # Examples
    ///
    /// Assume there are two files in the same directory with the following
    /// contents:
    ///
    /// File 'spanish.in':
    ///
    /// ```text
    /// adiós
    /// ```
    ///
    /// File 'main.rs':
    ///
    /// ```ignore (cannot-doctest-external-file-dependency)
    /// fn main() {
    ///     let my_str = include_str!("spanish.in");
    ///     assert_eq!(my_str, "adiós\n");
    ///     print!("{my_str}");
    /// }
    /// ```
    ///
    /// Compiling 'main.rs' and running the resulting binary will print "adiós".
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    #[rustc_diagnostic_item = "include_str_macro"]
    macro_rules! include_str {
        ($file:expr $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Includes a file as a reference to a byte array.
    ///
    /// The file is located relative to the current file (similarly to how
    /// modules are found). The provided path is interpreted in a platform-specific
    /// way at compile time. So, for instance, an invocation with a Windows path
    /// containing backslashes `\` would not compile correctly on Unix.
    ///
    /// This macro will yield an expression of type `&'static [u8; N]` which is
    /// the contents of the file.
    ///
    /// # Examples
    ///
    /// Assume there are two files in the same directory with the following
    /// contents:
    ///
    /// File 'spanish.in':
    ///
    /// ```text
    /// adiós
    /// ```
    ///
    /// File 'main.rs':
    ///
    /// ```ignore (cannot-doctest-external-file-dependency)
    /// fn main() {
    ///     let bytes = include_bytes!("spanish.in");
    ///     assert_eq!(bytes, b"adi\xc3\xb3s\n");
    ///     print!("{}", String::from_utf8_lossy(bytes));
    /// }
    /// ```
    ///
    /// Compiling 'main.rs' and running the resulting binary will print "adiós".
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    #[rustc_diagnostic_item = "include_bytes_macro"]
    macro_rules! include_bytes {
        ($file:expr $(,)?) => {{ /* compiler built-in */ }};
    }

    /// Expands to a string that represents the current module path.
    ///
    /// The current module path can be thought of as the hierarchy of modules
    /// leading back up to the crate root. The first component of the path
    /// returned is the name of the crate currently being compiled.
    ///
    /// # Examples
    ///
    /// ```
    /// mod test {
    ///     pub fn foo() {
    ///         assert!(module_path!().ends_with("test"));
    ///     }
    /// }
    ///
    /// test::foo();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! module_path {
        () => {
            /* compiler built-in */
        };
    }

    /// Evaluates boolean combinations of configuration flags at compile-time.
    ///
    /// In addition to the `#[cfg]` attribute, this macro is provided to allow
    /// boolean expression evaluation of configuration flags. This frequently
    /// leads to less duplicated code.
    ///
    /// The syntax given to this macro is the same syntax as the [`cfg`]
    /// attribute.
    ///
    /// `cfg!`, unlike `#[cfg]`, does not remove any code and only evaluates to true or false. For
    /// example, all blocks in an if/else expression need to be valid when `cfg!` is used for
    /// the condition, regardless of what `cfg!` is evaluating.
    ///
    /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
    ///
    /// # Examples
    ///
    /// ```
    /// let my_directory = if cfg!(windows) {
    ///     "windows-specific-directory"
    /// } else {
    ///     "unix-directory"
    /// };
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! cfg {
        ($($cfg:tt)*) => {
            /* compiler built-in */
        };
    }

    /// Parses a file as an expression or an item according to the context.
    ///
    /// **Warning**: For multi-file Rust projects, the `include!` macro is probably not what you
    /// are looking for. Usually, multi-file Rust projects use
    /// [modules](https://doc.rust-lang.org/reference/items/modules.html). Multi-file projects and
    /// modules are explained in the Rust-by-Example book
    /// [here](https://doc.rust-lang.org/rust-by-example/mod/split.html) and the module system is
    /// explained in the Rust Book
    /// [here](https://doc.rust-lang.org/book/ch07-02-defining-modules-to-control-scope-and-privacy.html).
    ///
    /// The included file is placed in the surrounding code
    /// [unhygienically](https://doc.rust-lang.org/reference/macros-by-example.html#hygiene). If
    /// the included file is parsed as an expression and variables or functions share names across
    /// both files, it could result in variables or functions being different from what the
    /// included file expected.
    ///
    /// The included file is located relative to the current file (similarly to how modules are
    /// found). The provided path is interpreted in a platform-specific way at compile time. So,
    /// for instance, an invocation with a Windows path containing backslashes `\` would not
    /// compile correctly on Unix.
    ///
    /// # Uses
    ///
    /// The `include!` macro is primarily used for two purposes. It is used to include
    /// documentation that is written in a separate file and it is used to include [build artifacts
    /// usually as a result from the `build.rs`
    /// script](https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script).
    ///
    /// When using the `include` macro to include stretches of documentation, remember that the
    /// included file still needs to be a valid Rust syntax. It is also possible to
    /// use the [`include_str`] macro as `#![doc = include_str!("...")]` (at the module level) or
    /// `#[doc = include_str!("...")]` (at the item level) to include documentation from a plain
    /// text or markdown file.
    ///
    /// # Examples
    ///
    /// Assume there are two files in the same directory with the following contents:
    ///
    /// File 'monkeys.in':
    ///
    /// ```ignore (only-for-syntax-highlight)
    /// ['🙈', '🙊', '🙉']
    ///     .iter()
    ///     .cycle()
    ///     .take(6)
    ///     .collect::<String>()
    /// ```
    ///
    /// File 'main.rs':
    ///
    /// ```ignore (cannot-doctest-external-file-dependency)
    /// fn main() {
    ///     let my_string = include!("monkeys.in");
    ///     assert_eq!("🙈🙊🙉🙈🙊🙉", my_string);
    ///     println!("{my_string}");
    /// }
    /// ```
    ///
    /// Compiling 'main.rs' and running the resulting binary will print
    /// "🙈🙊🙉🙈🙊🙉".
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    #[rustc_diagnostic_item = "include_macro"] // useful for external lints
    macro_rules! include {
        ($file:expr $(,)?) => {{ /* compiler built-in */ }};
    }

    /// This macro uses forward-mode automatic differentiation to generate a new function.
    /// It may only be applied to a function. The new function will compute the derivative
    /// of the function to which the macro was applied.
    ///
    /// The expected usage syntax is:
    /// `#[autodiff_forward(NAME, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
    ///
    /// - `NAME`: A string that represents a valid function name.
    /// - `INPUT_ACTIVITIES`: Specifies one valid activity for each input parameter.
    /// - `OUTPUT_ACTIVITY`: Must not be set if the function implicitly returns nothing
    ///   (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
    ///
    /// ACTIVITIES might either be `Dual` or `Const`, more options will be exposed later.
    ///
    /// `Const` should be used on non-float arguments, or float-based arguments as an optimization
    /// if we are not interested in computing the derivatives with respect to this argument.
    ///
    /// `Dual` can be used for float scalar values or for references, raw pointers, or other
    /// indirect input arguments. It can also be used on a scalar float return value.
    /// If used on a return value, the generated function will return a tuple of two float scalars.
    /// If used on an input argument, a new shadow argument of the same type will be created,
    /// directly following the original argument.
    ///
    /// ### Usage examples:
    ///
    /// ```rust,ignore (autodiff requires a -Z flag as well as fat-lto for testing)
    /// #![feature(autodiff)]
    /// use std::autodiff::*;
    /// #[autodiff_forward(rb_fwd1, Dual, Const, Dual)]
    /// #[autodiff_forward(rb_fwd2, Const, Dual, Dual)]
    /// #[autodiff_forward(rb_fwd3, Dual, Dual, Dual)]
    /// fn rosenbrock(x: f64, y: f64) -> f64 {
    ///     (1.0 - x).powi(2) + 100.0 * (y - x.powi(2)).powi(2)
    /// }
    /// #[autodiff_forward(rb_inp_fwd, Dual, Dual, Dual)]
    /// fn rosenbrock_inp(x: f64, y: f64, out: &mut f64) {
    ///     *out = (1.0 - x).powi(2) + 100.0 * (y - x.powi(2)).powi(2);
    /// }
    ///
    /// fn main() {
    ///   let x0 = rosenbrock(1.0, 3.0); // 400.0
    ///   let (x1, dx1) = rb_fwd1(1.0, 1.0, 3.0); // (400.0, -800.0)
    ///   let (x2, dy1) = rb_fwd2(1.0, 3.0, 1.0); // (400.0, 400.0)
    ///   // When seeding both arguments at once the tangent return is the sum of both.
    ///   let (x3, dxy) = rb_fwd3(1.0, 1.0, 3.0, 1.0); // (400.0, -400.0)
    ///
    ///   let mut out = 0.0;
    ///   let mut dout = 0.0;
    ///   rb_inp_fwd(1.0, 1.0, 3.0, 1.0, &mut out, &mut dout);
    ///   // (out, dout) == (400.0, -400.0)
    /// }
    /// ```
    ///
    /// We might want to track how one input float affects one or more output floats. In this case,
    /// the shadow of one input should be initialized to `1.0`, while the shadows of the other
    /// inputs should be initialized to `0.0`. The shadow of the output(s) should be initialized to
    /// `0.0`. After calling the generated function, the shadow of the input will be zeroed,
    /// while the shadow(s) of the output(s) will contain the derivatives. Forward mode is generally
    /// more efficient if we have more output floats marked as `Dual` than input floats.
    /// Related information can also be found under the term "Vector-Jacobian product" (VJP).
    #[unstable(feature = "autodiff", issue = "124509")]
    #[allow_internal_unstable(rustc_attrs)]
    #[allow_internal_unstable(core_intrinsics)]
    #[rustc_builtin_macro]
    pub macro autodiff_forward($item:item) {
        /* compiler built-in */
    }

    /// This macro uses reverse-mode automatic differentiation to generate a new function.
    /// It may only be applied to a function. The new function will compute the derivative
    /// of the function to which the macro was applied.
    ///
    /// The expected usage syntax is:
    /// `#[autodiff_reverse(NAME, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
    ///
    /// - `NAME`: A string that represents a valid function name.
    /// - `INPUT_ACTIVITIES`: Specifies one valid activity for each input parameter.
    /// - `OUTPUT_ACTIVITY`: Must not be set if the function implicitly returns nothing
    ///   (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
    ///
    /// ACTIVITIES might either be `Active`, `Duplicated` or `Const`, more options will be exposed later.
    ///
    /// `Active` can be used for float scalar values.
    /// If used on an input, a new float will be appended to the return tuple of the generated
    /// function. If the function returns a float scalar, `Active` can be used for the return as
    /// well. In this case a float scalar will be appended to the argument list, it works as seed.
    ///
    /// `Duplicated` can be used on references, raw pointers, or other indirect input
    /// arguments. It creates a new shadow argument of the same type, following the original argument.
    /// A const reference or pointer argument will receive a mutable reference or pointer as shadow.
    ///
    /// `Const` should be used on non-float arguments, or float-based arguments as an optimization
    /// if we are not interested in computing the derivatives with respect to this argument.
    ///
    /// ### Usage examples:
    ///
    /// ```rust,ignore (autodiff requires a -Z flag as well as fat-lto for testing)
    /// #![feature(autodiff)]
    /// use std::autodiff::*;
    /// #[autodiff_reverse(rb_rev, Active, Active, Active)]
    /// fn rosenbrock(x: f64, y: f64) -> f64 {
    ///     (1.0 - x).powi(2) + 100.0 * (y - x.powi(2)).powi(2)
    /// }
    /// #[autodiff_reverse(rb_inp_rev, Active, Active, Duplicated)]
    /// fn rosenbrock_inp(x: f64, y: f64, out: &mut f64) {
    ///     *out = (1.0 - x).powi(2) + 100.0 * (y - x.powi(2)).powi(2);
    /// }
    ///
    /// fn main() {
    ///     let (output1, dx1, dy1) = rb_rev(1.0, 3.0, 1.0);
    ///     dbg!(output1, dx1, dy1); // (400.0, -800.0, 400.0)
    ///     let mut output2 = 0.0;
    ///     let mut seed = 1.0;
    ///     let (dx2, dy2) = rb_inp_rev(1.0, 3.0, &mut output2, &mut seed);
    ///     // (dx2, dy2, output2, seed) == (-800.0, 400.0, 400.0, 0.0)
    /// }
    /// ```
    ///
    ///
    /// We often want to track how one or more input floats affect one output float. This output can
    /// be a scalar return value, or a mutable reference or pointer argument. In the latter case, the
    /// mutable input should be marked as duplicated and its shadow initialized to `0.0`. The shadow of
    /// the output should be marked as active or duplicated and initialized to `1.0`. After calling
    /// the generated function, the shadow(s) of the input(s) will contain the derivatives. The
    /// shadow of the outputs ("seed") will be reset to zero.
    /// If the function has more than one output float marked as active or duplicated, users might want to
    /// set one of them to `1.0` and the others to `0.0` to compute partial derivatives.
    /// Unlike forward-mode, a call to the generated function does not reset the shadow of the
    /// inputs.
    /// Reverse mode is generally more efficient if we have more active/duplicated input than
    /// output floats.
    ///
    /// Related information can also be found under the term "Jacobian-Vector Product" (JVP).
    #[unstable(feature = "autodiff", issue = "124509")]
    #[allow_internal_unstable(rustc_attrs)]
    #[allow_internal_unstable(core_intrinsics)]
    #[rustc_builtin_macro]
    pub macro autodiff_reverse($item:item) {
        /* compiler built-in */
    }

    /// Asserts that a boolean expression is `true` at runtime.
    ///
    /// This will invoke the [`panic!`] macro if the provided expression cannot be
    /// evaluated to `true` at runtime.
    ///
    /// # Uses
    ///
    /// Assertions are always checked in both debug and release builds, and cannot
    /// be disabled. See [`debug_assert!`] for assertions that are not enabled in
    /// release builds by default.
    ///
    /// Unsafe code may rely on `assert!` to enforce run-time invariants that, if
    /// violated could lead to unsafety.
    ///
    /// Other use-cases of `assert!` include testing and enforcing run-time
    /// invariants in safe code (whose violation cannot result in unsafety).
    ///
    /// # Custom Messages
    ///
    /// This macro has a second form, where a custom panic message can
    /// be provided with or without arguments for formatting. See [`std::fmt`]
    /// for syntax for this form. Expressions used as format arguments will only
    /// be evaluated if the assertion fails.
    ///
    /// [`std::fmt`]: ../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// ```
    /// // the panic message for these assertions is the stringified value of the
    /// // expression given.
    /// assert!(true);
    ///
    /// fn some_computation() -> bool {
    ///     // Some expensive computation here
    ///     true
    /// }
    ///
    /// assert!(some_computation());
    ///
    /// // assert with a custom message
    /// let x = true;
    /// assert!(x, "x wasn't true!");
    ///
    /// let a = 3; let b = 27;
    /// assert!(a + b == 30, "a = {}, b = {}", a, b);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    #[macro_export]
    #[rustc_diagnostic_item = "assert_macro"]
    #[allow_internal_unstable(
        core_intrinsics,
        panic_internals,
        edition_panic,
        generic_assert_internals
    )]
    macro_rules! assert {
        ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
        ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
    }

    /// Prints passed tokens into the standard output.
    #[unstable(
        feature = "log_syntax",
        issue = "29598",
        reason = "`log_syntax!` is not stable enough for use and is subject to change"
    )]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! log_syntax {
        ($($arg:tt)*) => {
            /* compiler built-in */
        };
    }

    /// Enables or disables tracing functionality used for debugging other macros.
    #[unstable(
        feature = "trace_macros",
        issue = "29598",
        reason = "`trace_macros` is not stable enough for use and is subject to change"
    )]
    #[rustc_builtin_macro]
    #[macro_export]
    macro_rules! trace_macros {
        (true) => {{ /* compiler built-in */ }};
        (false) => {{ /* compiler built-in */ }};
    }

    /// Attribute macro used to apply derive macros.
    ///
    /// See [the reference] for more info.
    ///
    /// [the reference]: ../../../reference/attributes/derive.html
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_builtin_macro]
    pub macro derive($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro used to apply derive macros for implementing traits
    /// in a const context.
    ///
    /// See [the reference] for more info.
    ///
    /// [the reference]: ../../../reference/attributes/derive.html
    #[unstable(feature = "derive_const", issue = "118304")]
    #[rustc_builtin_macro]
    pub macro derive_const($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro applied to a function to turn it into a unit test.
    ///
    /// See [the reference] for more info.
    ///
    /// [the reference]: ../../../reference/attributes/testing.html#the-test-attribute
    #[stable(feature = "rust1", since = "1.0.0")]
    #[allow_internal_unstable(test, rustc_attrs, coverage_attribute)]
    #[rustc_builtin_macro]
    pub macro test($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro applied to a function to turn it into a benchmark test.
    #[unstable(
        feature = "test",
        issue = "50297",
        reason = "`bench` is a part of custom test frameworks which are unstable"
    )]
    #[allow_internal_unstable(test, rustc_attrs, coverage_attribute)]
    #[rustc_builtin_macro]
    pub macro bench($item:item) {
        /* compiler built-in */
    }

    /// An implementation detail of the `#[test]` and `#[bench]` macros.
    #[unstable(
        feature = "custom_test_frameworks",
        issue = "50297",
        reason = "custom test frameworks are an unstable feature"
    )]
    #[allow_internal_unstable(test, rustc_attrs)]
    #[rustc_builtin_macro]
    pub macro test_case($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro applied to a static to register it as a global allocator.
    ///
    /// See also [`std::alloc::GlobalAlloc`](../../../std/alloc/trait.GlobalAlloc.html).
    #[stable(feature = "global_allocator", since = "1.28.0")]
    #[allow_internal_unstable(rustc_attrs)]
    #[rustc_builtin_macro]
    pub macro global_allocator($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro applied to a function to give it a post-condition.
    ///
    /// The attribute carries an argument token-tree which is
    /// eventually parsed as a unary closure expression that is
    /// invoked on a reference to the return value.
    #[unstable(feature = "contracts", issue = "128044")]
    #[allow_internal_unstable(contracts_internals)]
    #[rustc_builtin_macro]
    pub macro contracts_ensures($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro applied to a function to give it a precondition.
    ///
    /// The attribute carries an argument token-tree which is
    /// eventually parsed as an boolean expression with access to the
    /// function's formal parameters
    #[unstable(feature = "contracts", issue = "128044")]
    #[allow_internal_unstable(contracts_internals)]
    #[rustc_builtin_macro]
    pub macro contracts_requires($item:item) {
        /* compiler built-in */
    }

    /// Attribute macro applied to a function to register it as a handler for allocation failure.
    ///
    /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).
    #[unstable(feature = "alloc_error_handler", issue = "51540")]
    #[allow_internal_unstable(rustc_attrs)]
    #[rustc_builtin_macro]
    pub macro alloc_error_handler($item:item) {
        /* compiler built-in */
    }

    /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
    #[unstable(
        feature = "cfg_accessible",
        issue = "64797",
        reason = "`cfg_accessible` is not fully implemented"
    )]
    #[rustc_builtin_macro]
    pub macro cfg_accessible($item:item) {
        /* compiler built-in */
    }

    /// Expands all `#[cfg]` and `#[cfg_attr]` attributes in the code fragment it's applied to.
    #[unstable(
        feature = "cfg_eval",
        issue = "82679",
        reason = "`cfg_eval` is a recently implemented feature"
    )]
    #[rustc_builtin_macro]
    pub macro cfg_eval($($tt:tt)*) {
        /* compiler built-in */
    }

    /// Provide a list of type aliases and other opaque-type-containing type definitions
    /// to an item with a body. This list will be used in that body to define opaque
    /// types' hidden types.
    /// Can only be applied to things that have bodies.
    #[unstable(
        feature = "type_alias_impl_trait",
        issue = "63063",
        reason = "`type_alias_impl_trait` has open design concerns"
    )]
    #[rustc_builtin_macro]
    pub macro define_opaque($($tt:tt)*) {
        /* compiler built-in */
    }

    /// Unstable placeholder for type ascription.
    #[allow_internal_unstable(builtin_syntax)]
    #[unstable(
        feature = "type_ascription",
        issue = "23416",
        reason = "placeholder syntax for type ascription"
    )]
    #[rustfmt::skip]
    pub macro type_ascribe($expr:expr, $ty:ty) {
        builtin # type_ascribe($expr, $ty)
    }

    /// Unstable placeholder for deref patterns.
    #[allow_internal_unstable(builtin_syntax)]
    #[unstable(
        feature = "deref_patterns",
        issue = "87121",
        reason = "placeholder syntax for deref patterns"
    )]
    pub macro deref($pat:pat) {
        builtin # deref($pat)
    }

    /// Derive macro generating an impl of the trait `From`.
    /// Currently, it can only be used on single-field structs.
    // Note that the macro is in a different module than the `From` trait,
    // to avoid triggering an unstable feature being used if someone imports
    // `std::convert::From`.
    #[rustc_builtin_macro]
    #[unstable(feature = "derive_from", issue = "144889")]
    pub macro From($item: item) {
        /* compiler built-in */
    }
}