Rollup merge of #124099 - voidc:disallow-ambiguous-expr-attrs, r=davidtwco

Disallow ambiguous attributes on expressions

This implements the suggestion in [#15701](rust-lang/rust#15701 (comment)) to disallow ambiguous outer attributes on expressions. This should resolve one of the concerns blocking the stabilization of `stmt_expr_attributes`.

Author: matthiaskrgr

tests/source/attrib.rs

@@ -214,8 +214,8 @@ type Os = NoSource;
 // #3313
 fn stmt_expr_attributes() {
     let foo ;
-    #[must_use]
-   foo = false ;
+    (#[must_use]
+   foo) = false ;
 }
 
 // #3509

tests/target/attrib.rs

@@ -248,8 +248,8 @@ type Os = NoSource;
 // #3313
 fn stmt_expr_attributes() {
     let foo;
-    #[must_use]
-    foo = false;
+    (#[must_use]
+    foo) = false;
 }
 
 // #3509