8. Statements¶
Syntax
Statement::=ExpressionStatement|Item|LetStatement|TerminatedMacroInvocation| ;
Legality Rules
8:1 An expression statement is an expression whose result is ignored.
8:2 An item statement is a statement that is expressed as an item.
8:3 An empty statement is a statement expressed as character 0x3B (semicolon).
8:4 A macro statement is a statement expressed as a terminated macro invocation.
Dynamic Semantics
8:5 Execution is the process by which a statement achieves its runtime effects.
8:6 The execution of an empty statement has no effect.
8.1. Let Statements¶
Syntax
LetStatement::=OuterAttributeOrDoc* letPatternWithoutAlternationTypeAscription?LetInitializer? ;LetInitializer::= =Expression(elseBlockExpression)?
Legality Rules
8.1:1 A let statement is a statement that introduces new bindings produced by its pattern-without-alternation that are optionally initialized to a value.
8.1:2 A let initializer is a construct that provides the value of the bindings of the let statement using an expression, or alternatively executes a block expression.
8.1:3 If a let statement lacks a block expression, then the pattern of the let statement shall be an irrefutable pattern.
8.1:4
If a let statement has a block expression, then the Expression of
the LetInitializer shall not be a LazyBooleanExpression or end with
token }.
8.1:5 The expected type of the pattern of the let statement is determined as follows:
8.1:6 If the let statement lacks a type ascription and a let initializer, then the expected type is the inferred type.
8.1:7 If the let statement lacks a type ascription, then the expected type is the type of the let initializer.
8.1:8 Otherwise the expected type is the type specified by the type ascription.
8.1:9 The type of a binding introduced by a let statement is determined as follows:
8.1:10 If the let statement appears with a type ascription, then the type is the type specified by the type ascription.
8.1:11 If the let statement lacks a type ascription, then the type is inferred using type inference.
8.1:12 The type of the block expression of a let statement shall be the never type.
8.1:13 The value of a binding introduced by a let statement is determined as follows:
8.1:14 If the let statement appears with a let initializer, then the value is the value of the expression of the let initializer.
8.1:15 Otherwise the binding is uninitialized.
Dynamic Semantics
8.1:16 The execution of a let statement with a let initializer proceeds as follows:
8.1:17 The expression of the let initializer is evaluated.
8.1:18 If the value of the expression is matched successfully against the pattern of the let statement, then the value is assigned to each binding introduced by the let statement.
8.1:19 Otherwise the block expression of the let initializer is evaluated.
Examples
let local = 0;
let local: u32;
let (a, b) = (0, 0);
let Some(value) = vector.pop() else {
panic!();
};
8.2. Expression Statements¶
Syntax
ExpressionStatement::=ExpressionWithBlock;? |ExpressionWithoutBlock;
Legality Rules
8.2:1 An expression statement is an expression whose result is ignored.
8.2:2 The expected type of an expression statement without character 0x3B (semicolon) is the unit type.
Dynamic Semantics
8.2:3 The execution of an expression statement proceeds as follows:
Examples
let mut values = vec![1, 2, 3];
8.2:6
The following expression statement ignores the result from pop().
values.pop();
8.2:7 The following expression statement does not require a semicolon.
if values.is_empty() {
values.push(42);
}
else {
values.remove(0);
}
8.2:8 The following expression statement is not an index expression.
[42];