let-else: fix attribute aliasing + add test for issue 89807

Author: cormacrelf

compiler/rustc_ast_lowering/src/block.rs

@@ -152,6 +152,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let block = self.lower_block(els, false);
             self.arena.alloc(self.expr_block(block, AttrVec::new()))
         };
+        self.alias_attrs(let_expr.hir_id, local_hir_id);
         self.alias_attrs(else_expr.hir_id, local_hir_id);
         let if_expr = self.arena.alloc(hir::Expr {
             hir_id: stmt_hir_id,

src/test/ui/let-else/let-else-allow-unused.rs

@@ -0,0 +1,14 @@
+// check-pass
+// issue #89807
+
+#![feature(let_else)]
+
+#[deny(unused_variables)]
+
+fn main() {
+    let value = Some(String::new());
+    #[allow(unused)]
+    let banana = 1;
+    #[allow(unused)]
+    let Some(chaenomeles) = value else { return }; // OK
+}