New upstream version 1.71.1+dfsg1

[rustc.git] / compiler / rustc_error_codes / src / error_codes / E0311.md

This error occurs when there is an unsatisfied outlives bound involving an
elided region and a generic type parameter or associated type.

Erroneous code example:

```compile_fail,E0311
fn no_restriction<T>(x: &()) -> &() {
    with_restriction::<T>(x)
}

fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
    x
}
```

Why doesn't this code compile? It helps to look at the lifetime bounds that are
automatically added by the compiler. For more details see the documentation for
[lifetime elision]( https://doc.rust-lang.org/reference/lifetime-elision.html).

The compiler elides the lifetime of `x` and the return type to some arbitrary
lifetime `'anon` in `no_restriction()`. The only information available to the
compiler is that `'anon` is valid for the duration of the function. When
calling `with_restriction()`, the compiler requires the completely unrelated
type parameter `T` to outlive `'anon` because of the `T: 'a` bound in
`with_restriction()`. This causes an error because `T` is not required to
outlive `'anon` in `no_restriction()`.

If `no_restriction()` were to use `&T` instead of `&()` as an argument, the
compiler would have added an implied bound, causing this to compile.

This error can be resolved by explicitly naming the elided lifetime for `x` and
then explicitly requiring that the generic parameter `T` outlives that lifetime:

```
fn no_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
    with_restriction::<T>(x)
}

fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
    x
}
```