Do not look for significant drop inside .await expansion (rust-lang#13985)

Temporaries created inside the expansion of `.await` will be dropped and
need no checking. Looking inside the expansion will trigger false
positives.

changelog: [`significant_drop_in_scrutinee`]: do not falsely warn for
temporaries created by `.await` expansion

Fix rust-lang#13927

Author: dswij

clippy_lints/src/matches/significant_drop_in_scrutinee.rs

@@ -441,8 +441,9 @@ impl<'tcx> Visitor<'tcx> for SigDropHelper<'_, 'tcx> {
         let parent_expr_before = self.parent_expr.replace(ex);
 
         match ex.kind {
-            // Skip blocks because values in blocks will be dropped as usual.
-            ExprKind::Block(..) => (),
+            // Skip blocks because values in blocks will be dropped as usual, and await
+            // desugaring because temporary insides the future will have been dropped.
+            ExprKind::Block(..) | ExprKind::Match(_, _, MatchSource::AwaitDesugar) => (),
             _ => walk_expr(self, ex),
         }
 

tests/ui/significant_drop_in_scrutinee.rs

@@ -832,4 +832,22 @@ fn should_trigger_lint_in_while_let() {
     }
 }
 
+async fn foo_async(mutex: &Mutex<i32>) -> Option<MutexGuard<'_, i32>> {
+    Some(mutex.lock().unwrap())
+}
+
+async fn should_trigger_lint_for_async(mutex: Mutex<i32>) -> i32 {
+    match *foo_async(&mutex).await.unwrap() {
+        n if n < 10 => n,
+        _ => 10,
+    }
+}
+
+async fn should_not_trigger_lint_in_async_expansion(mutex: Mutex<i32>) -> i32 {
+    match foo_async(&mutex).await {
+        Some(guard) => *guard,
+        _ => 0,
+    }
+}
+
 fn main() {}

tests/ui/significant_drop_in_scrutinee.stderr

@@ -568,5 +568,21 @@ LL |     }
    |
    = note: this might lead to deadlocks or other unexpected behavior
 
-error: aborting due to 29 previous errors
+error: temporary with significant `Drop` in `match` scrutinee will live until the end of the `match` expression
+  --> tests/ui/significant_drop_in_scrutinee.rs:840:11
+   |
+LL |     match *foo_async(&mutex).await.unwrap() {
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     }
+   |      - temporary lives until here
+   |
+   = note: this might lead to deadlocks or other unexpected behavior
+help: try moving the temporary above the match
+   |
+LL ~     let value = *foo_async(&mutex).await.unwrap();
+LL ~     match value {
+   |
+
+error: aborting due to 30 previous errors