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Auto merge of #91403 - cjgillot:inherit-async, r=oli-obk

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Inherit lifetimes for async fn instead of duplicating them.

The current desugaring of `async fn foo<'a>(&usize) -> &u8` is equivalent to
```rust
fn foo<'a, '0>(&'0 usize) -> foo<'static, 'static>::Opaque<'a, '0, '_>;
type foo<'_a, '_0>::Opaque<'a, '0, '1> = impl Future<Output = &'1 u8>;
```
following the RPIT model.

Duplicating all the inherited lifetime parameters and setting the inherited version to `'static` makes lowering more complex and causes issues like #61949. This PR removes the duplication of inherited lifetimes to directly use
```rust
fn foo<'a, '0>(&'0 usize) -> foo<'a, '0>::Opaque<'_>;
type foo<'a, '0>::Opaque<'1> = impl Future<Output = &'1 u8>;
```
following the TAIT model.

Fixes https://github.com/rust-lang/rust/issues/61949
This commit is contained in:
bors 2022-02-12 21:42:10 +00:00
commit 3cfa4def7c
34 changed files with 227 additions and 281 deletions
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6
compiler/rustc_error_codes/src/error_codes/E0760.md
View file

@ -1,4 +1,4 @@
`async fn`/`impl trait` return type cannot contain a projection
`impl trait` return type cannot contain a projection
or `Self` that references lifetimes from a parent scope.
Erroneous code example:
@ -7,7 +7,7 @@ Erroneous code example:
struct S<'a>(&'a i32);
impl<'a> S<'a> {
async fn new(i: &'a i32) -> Self {
fn new(i: &'a i32) -> impl Into<Self> {
S(&22)
}
}
@ -19,7 +19,7 @@ To fix this error we need to spell out `Self` to `S<'a>`:
struct S<'a>(&'a i32);
impl<'a> S<'a> {
async fn new(i: &'a i32) -> S<'a> {
fn new(i: &'a i32) -> impl Into<S<'a>> {
S(&22)
}
}