Rollup merge of #127058 - compiler-errors:tighten-async-spans, r=oli-obk

Tighten `fn_decl_span` for async blocks

Tightens the span of `async {}` blocks in diagnostics, and subsequently async closures and async fns, by actually setting the `fn_decl_span` correctly. This is kinda a follow-up on #125078, but it fixes the problem in a more general way.

I think the diagnostics are significantly improved, since we no longer have a bunch of overlapping spans. I'll point out one caveat where I think the diagnostic may get a bit more confusing, but where I don't think it matters.

r? ````@estebank```` or ````@oli-obk```` or someone else on wg-diag or compiler i dont really care lol

1 files changed, 3 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index e0c70884fee..3cdc20b6c65 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -3432,8 +3432,9 @@ impl<'a> Parser<'a> {
             }
         }
         let capture_clause = self.parse_capture_clause()?;
+        let decl_span = lo.to(self.prev_token.span);
         let (attrs, body) = self.parse_inner_attrs_and_block()?;
-        let kind = ExprKind::Gen(capture_clause, body, kind);
+        let kind = ExprKind::Gen(capture_clause, body, kind, decl_span);
         Ok(self.mk_expr_with_attrs(lo.to(self.prev_token.span), kind, attrs))
     }
 
@@ -4022,7 +4023,7 @@ impl MutVisitor for CondChecker<'_> {
             | ExprKind::Match(_, _, _)
             | ExprKind::Closure(_)
             | ExprKind::Block(_, _)
-            | ExprKind::Gen(_, _, _)
+            | ExprKind::Gen(_, _, _, _)
             | ExprKind::TryBlock(_)
             | ExprKind::Underscore
             | ExprKind::Path(_, _)