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A lifetime is only present in an associated-type binding, and not in the input
types to the trait.
Erroneous code example:
```compile_fail,E0582
fn bar<F>(t: F)
// No type can satisfy this requirement, since `'a` does not
// appear in any of the input types (here, `i32`):
where F: for<'a> Fn(i32) -> Option<&'a i32>
{
}
fn main() { }
```
To fix this issue, either use the lifetime in the inputs, or use
`'static`. Example:
```
fn bar<F, G>(t: F, u: G)
where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
G: Fn(i32) -> Option<&'static i32>,
{
}
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
guarantee 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.
[issue #33685]: https://github.com/rust-lang/rust/issues/33685
|
