Rollup merge of #125293 - dingxiangfei2009:tail-expr-temp-lifetime, r=estebank,davidtwco

Place tail expression behind terminating scope

This PR implements #123739 so that we can do further experiments in nightly.

A little rewrite has been applied to `for await` lowering. It was previously `unsafe { Pin::unchecked_new(into_async_iter(..)) }`. Under the edition 2024 rule, however, `into_async_iter` gets dropped at the end of the `unsafe` block. This presumably the first Edition 2024 migration rule goes by hoisting `into_async_iter(..)` into `match` one level above, so it now looks like the following.
```rust
match into_async_iter($iter_expr) {
  ref mut iter => match unsafe { Pin::unchecked_new(iter) } {
    ...
  }
}
```

1 files changed, 2 insertions, 0 deletions

diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 066a5a8f046..5cfbcdcbbbe 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -588,6 +588,8 @@ declare_features! (
     (incomplete, return_type_notation, "1.70.0", Some(109417)),
     /// Allows `extern "rust-cold"`.
     (unstable, rust_cold_cc, "1.63.0", Some(97544)),
+    /// Shortern the tail expression lifetime
+    (unstable, shorter_tail_lifetimes, "1.79.0", Some(123739)),
     /// Allows the use of SIMD types in functions declared in `extern` blocks.
     (unstable, simd_ffi, "1.0.0", Some(27731)),
     /// Allows specialization of implementations (RFC 1210).