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

Disallow ambiguous attributes on expressions

This implements the suggestion in [rust-lang#15701](rust-lang#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/ui/cfg_attr_rustfmt.fixed

@@ -16,7 +16,7 @@ fn foo(
 
 fn skip_on_statements() {
     #[rustfmt::skip]
-    5+3;
+    { 5+3; }
 }
 
 #[rustfmt::skip]
@@ -33,11 +33,11 @@ mod foo {
 #[clippy::msrv = "1.29"]
 fn msrv_1_29() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    1+29;
+    { 1+29; }
 }
 
 #[clippy::msrv = "1.30"]
 fn msrv_1_30() {
     #[rustfmt::skip]
-    1+30;
+    { 1+30; }
 }

tests/ui/cfg_attr_rustfmt.rs

@@ -16,7 +16,7 @@ fn foo(
 
 fn skip_on_statements() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    5+3;
+    { 5+3; }
 }
 
 #[cfg_attr(rustfmt, rustfmt_skip)]
@@ -33,11 +33,11 @@ mod foo {
 #[clippy::msrv = "1.29"]
 fn msrv_1_29() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    1+29;
+    { 1+29; }
 }
 
 #[clippy::msrv = "1.30"]
 fn msrv_1_30() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    1+30;
+    { 1+30; }
 }