fix unnecessary_semicolon: FN on #[feature(stmt_expr_attributes)] (#15481)

inspired by #15476

changelog: [`unnecessary_semicolon`]: FN on
`#[feature(stmt_expr_attributes)]`

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### Summary Notes

- [Feature
freeze](#15481 (review))
by [blyxyas](https://github.com/blyxyas)

*Managed by `@rustbot`—see
[help](https://forge.rust-lang.org/triagebot/note.html) for details*

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Author: blyxyas

clippy_lints/src/unnecessary_semicolon.rs

@@ -88,7 +88,8 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessarySemicolon {
             )
             && cx.typeck_results().expr_ty(expr).is_unit()
             // if a stmt has attrs, then turning it into an expr will break the code, since attrs aren't allowed on exprs
-            && cx.tcx.hir_attrs(stmt.hir_id).is_empty()
+            // -- unless the corresponding feature is enabled
+            && (cx.tcx.hir_attrs(stmt.hir_id).is_empty() || cx.tcx.features().stmt_expr_attributes())
         {
             if let Some(block_is_unit) = self.is_last_in_block(stmt) {
                 if cx.tcx.sess.edition() <= Edition2021 && leaks_droppable_temporary_with_limited_lifetime(cx, expr) {

tests/ui/unnecessary_semicolon_feature_stmt_expr_attributes.fixed

@@ -0,0 +1,13 @@
+#![warn(clippy::unnecessary_semicolon)]
+#![feature(stmt_expr_attributes)]
+
+fn main() {
+    // removing the `;` would turn the stmt into an expr, but attrs aren't allowed on exprs
+    // -- unless the feature `stmt_expr_attributes` is enabled
+    #[rustfmt::skip]
+    match 0 {
+        0b00 => {}  0b01 => {}
+        0b11 => {}  _    => {}
+    }
+    //~^ unnecessary_semicolon
+}

tests/ui/unnecessary_semicolon_feature_stmt_expr_attributes.rs

@@ -0,0 +1,13 @@
+#![warn(clippy::unnecessary_semicolon)]
+#![feature(stmt_expr_attributes)]
+
+fn main() {
+    // removing the `;` would turn the stmt into an expr, but attrs aren't allowed on exprs
+    // -- unless the feature `stmt_expr_attributes` is enabled
+    #[rustfmt::skip]
+    match 0 {
+        0b00 => {}  0b01 => {}
+        0b11 => {}  _    => {}
+    };
+    //~^ unnecessary_semicolon
+}

tests/ui/unnecessary_semicolon_feature_stmt_expr_attributes.stderr

@@ -0,0 +1,11 @@
+error: unnecessary semicolon
+  --> tests/ui/unnecessary_semicolon_feature_stmt_expr_attributes.rs:11:6
+   |
+LL |     };
+   |      ^ help: remove
+   |
+   = note: `-D clippy::unnecessary-semicolon` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_semicolon)]`
+
+error: aborting due to 1 previous error
+