Error code E0229

An associated item constraint was written in an unexpected context.

Erroneous code example:

#![allow(unused)]
fn main() {
pub trait Foo {
    type A;
    fn boo(&self) -> <Self as Foo>::A;
}

struct Bar;

impl Foo for isize {
    type A = usize;
    fn boo(&self) -> usize { 42 }
}

fn baz<I>(x: &<I as Foo<A = Bar>>::A) {}
// error: associated item constraint are not allowed here
}
ⓘ

To solve this error, please move the associated item constraints to the type parameter declaration:

#![allow(unused)]
fn main() {
struct Bar;
trait Foo { type A; }
fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
}

Or into the where-clause:

#![allow(unused)]
fn main() {
struct Bar;
trait Foo { type A; }
fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
}