Rollup merge of #88147 - FabianWolff:issue-88097, r=jackh726

Fix non-capturing closure return type coercion

Fixes #88097. For the example given there:
```rust
fn peculiar() -> impl Fn(u8) -> u8 {
    return |x| x + 1
}
```
which incorrectly reports an error, I noticed something weird in the debug log:
```
DEBUG rustc_typeck::check::coercion coercion::try_find_coercion_lub([closure@test.rs:2:12: 2:21], [closure@test.rs:2:12: 2:21], exprs=1 exprs)
```
Apparently, `try_find_coercion_lub()` thinks that the LUB for two closure types always has to be a function pointer (which explains the `expected closure, found fn pointer` error in #88097). There is one corner case where that isn't true, though — namely, when the two closure types are equal, in which case the trivial LUB is the type itself. This PR fixes this by inserting an explicit check for type equality in `try_find_coercion_lub()`.

1 files changed, 6 insertions, 0 deletions

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 3bfab9d513f..6fe96e4cc27 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -1003,6 +1003,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             exprs.len()
         );
 
+        // The following check fixes #88097, where the compiler erroneously
+        // attempted to coerce a closure type to itself via a function pointer.
+        if prev_ty == new_ty {
+            return Ok(prev_ty);
+        }
+
         // Special-case that coercion alone cannot handle:
         // Function items or non-capturing closures of differing IDs or InternalSubsts.
         let (a_sig, b_sig) = {