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

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

4 files changed, 10 insertions, 2 deletions

diff --git a/src/tools/clippy/tests/ui/manual_async_fn.fixed b/src/tools/clippy/tests/ui/manual_async_fn.fixed
index 136cc96be70..e9ca66f125d 100644
--- a/src/tools/clippy/tests/ui/manual_async_fn.fixed
+++ b/src/tools/clippy/tests/ui/manual_async_fn.fixed
@@ -80,6 +80,7 @@ fn elided_not_bound(_: &i32) -> impl Future<Output = i32> {
     async { 42 }
 }
 
+#[allow(clippy::needless_lifetimes)]
 async fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> i32 { 42 }
 
 // should be ignored
diff --git a/src/tools/clippy/tests/ui/manual_async_fn.rs b/src/tools/clippy/tests/ui/manual_async_fn.rs
index ddc453ffdb7..c3fa846485b 100644
--- a/src/tools/clippy/tests/ui/manual_async_fn.rs
+++ b/src/tools/clippy/tests/ui/manual_async_fn.rs
@@ -98,6 +98,7 @@ fn elided_not_bound(_: &i32) -> impl Future<Output = i32> {
     async { 42 }
 }
 
+#[allow(clippy::needless_lifetimes)]
 fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> + 'a + 'b {
     async { 42 }
 }
diff --git a/src/tools/clippy/tests/ui/manual_async_fn.stderr b/src/tools/clippy/tests/ui/manual_async_fn.stderr
index 7435f46074c..b83abfccd4e 100644
--- a/src/tools/clippy/tests/ui/manual_async_fn.stderr
+++ b/src/tools/clippy/tests/ui/manual_async_fn.stderr
@@ -140,7 +140,7 @@ LL | fn elided(_: &i32) -> impl Future<Output = i32> + '_ { 42 }
    |                                                      ~~~~~~
 
 error: this function can be simplified using the `async fn` syntax
-  --> $DIR/manual_async_fn.rs:101:1
+  --> $DIR/manual_async_fn.rs:102:1
    |
 LL | fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> + 'a + 'b {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/src/tools/clippy/tests/ui/needless_lifetimes.stderr b/src/tools/clippy/tests/ui/needless_lifetimes.stderr
index ffa152427a9..8df50d79ca5 100644
--- a/src/tools/clippy/tests/ui/needless_lifetimes.stderr
+++ b/src/tools/clippy/tests/ui/needless_lifetimes.stderr
@@ -19,6 +19,12 @@ LL | fn in_and_out<'a>(x: &'a u8, _y: u8) -> &'a u8 {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided (or replaced with `'_` if needed by type declaration)
+  --> $DIR/needless_lifetimes.rs:37:1
+   |
+LL | async fn func<'a>(args: &[&'a str]) -> Option<&'a str> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: explicit lifetimes given in parameter types where they could be elided (or replaced with `'_` if needed by type declaration)
   --> $DIR/needless_lifetimes.rs:56:1
    |
 LL | fn deep_reference_3<'a>(x: &'a u8, _y: u8) -> Result<&'a u8, ()> {
@@ -192,5 +198,5 @@ error: explicit lifetimes given in parameter types where they could be elided (o
 LL |         fn lifetime_elsewhere_provided<'a>(self: Box<Self>, here: &'a ()) -> &'a () {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 32 previous errors
+error: aborting due to 33 previous errors