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rustc --explain E0582 additional example #124746

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Jun 5, 2024
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rustc --explain E0582 additional example #124746

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ad7c2b0
Updated error code explanation
OliverKillane May 5, 2024
f3dcf65
fix whitespace
OliverKillane May 5, 2024
a5d0988
Update compiler/rustc_error_codes/src/error_codes/E0582.md
OliverKillane May 14, 2024
012288b
tidy fix from suggestion
OliverKillane May 15, 2024
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34 changes: 34 additions & 0 deletions compiler/rustc_error_codes/src/error_codes/E0582.md
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Expand Up @@ -27,6 +27,40 @@ fn bar<F, G>(t: F, u: G)
fn main() { }
```

This error also includes the use of associated types with lifetime parameters.
```compile_fail,E0582
trait Foo {
type Assoc<'a>;
}

struct Bar<X, F>
where
X: Foo,
F: for<'a> Fn(X::Assoc<'a>) -> &'a i32
{
x: X,
f: F
}
```
The latter scenario encounters this error because `Foo::Assoc<'a>` could be
implemented by a type that does not use the `'a` parameter, so there is no
guarentee that `X::Assoc<'a>` actually uses `'a`.

To fix this we can pass a dummy parameter:
```
# trait Foo {
# type Assoc<'a>;
# }
struct Bar<X, F>
where
X: Foo,
F: for<'a> Fn(X::Assoc<'a>, /* dummy */ &'a ()) -> &'a i32
{
x: X,
f: F
}
```

Note: The examples above used to be (erroneously) accepted by the
compiler, but this was since corrected. See [issue #33685] for more
details.
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