Fix significant drop detection

Author: lrh2000

clippy_lints/src/matches/significant_drop_in_scrutinee.rs

@@ -119,41 +119,49 @@ impl<'a, 'tcx> SigDropChecker<'a, 'tcx> {
         self.cx.typeck_results().expr_ty(ex)
     }
 
-    fn has_seen_type(&mut self, ty: Ty<'tcx>) -> bool {
-        !self.seen_types.insert(ty)
+    fn has_sig_drop_attr(&mut self, cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+        self.seen_types.clear();
+        self.has_sig_drop_attr_impl(cx, ty)
     }
 
-    fn has_sig_drop_attr(&mut self, cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+    fn has_sig_drop_attr_impl(&mut self, cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
         if let Some(adt) = ty.ty_adt_def() {
             if get_attr(cx.sess(), cx.tcx.get_attrs_unchecked(adt.did()), "has_significant_drop").count() > 0 {
                 return true;
             }
         }
 
-        match ty.kind() {
-            rustc_middle::ty::Adt(a, b) => {
-                for f in a.all_fields() {
-                    let ty = f.ty(cx.tcx, b);
-                    if !self.has_seen_type(ty) && self.has_sig_drop_attr(cx, ty) {
-                        return true;
-                    }
-                }
+        if !self.seen_types.insert(ty) {
+            return false;
+        }
 
-                for generic_arg in *b {
-                    if let GenericArgKind::Type(ty) = generic_arg.unpack() {
-                        if self.has_sig_drop_attr(cx, ty) {
-                            return true;
-                        }
-                    }
-                }
-                false
+        let result = match ty.kind() {
+            rustc_middle::ty::Adt(adt, args) => {
+                // if some field has significant drop,
+                adt.all_fields()
+                    .map(|field| field.ty(cx.tcx, args))
+                    .any(|ty| self.has_sig_drop_attr(cx, ty))
+                    // or if there is no generic lifetime and..
+                    // (to avoid false positive on `Ref<'a, MutexGuard<Foo>>`)
+                    || (args
+                        .iter()
+                        .all(|arg| !matches!(arg.unpack(), GenericArgKind::Lifetime(_)))
+                        // some generic parameter has significant drop
+                        // (to avoid false negative on `Box<MutexGuard<Foo>>`)
+                        && args
+                            .iter()
+                            .filter_map(|arg| match arg.unpack() {
+                                GenericArgKind::Type(ty) => Some(ty),
+                                _ => None,
+                            })
+                            .any(|ty| self.has_sig_drop_attr(cx, ty)))
             },
-            rustc_middle::ty::Array(ty, _)
-            | rustc_middle::ty::RawPtr(ty, _)
-            | rustc_middle::ty::Ref(_, ty, _)
-            | rustc_middle::ty::Slice(ty) => self.has_sig_drop_attr(cx, *ty),
+            rustc_middle::ty::Tuple(tys) => tys.iter().any(|ty| self.has_sig_drop_attr(cx, ty)),
+            rustc_middle::ty::Array(ty, _) | rustc_middle::ty::Slice(ty) => self.has_sig_drop_attr(cx, *ty),
             _ => false,
-        }
+        };
+
+        result
     }
 }
 

tests/ui/significant_drop_in_scrutinee.stderr

@@ -28,6 +28,9 @@ LL |     match s.lock_m().get_the_value() {
 LL |             println!("{}", s.lock_m().get_the_value());
    |                            ---------- another value with significant `Drop` created here
 ...
+LL |             println!("{}", s.lock_m().get_the_value());
+   |                            ---------- another value with significant `Drop` created here
+...
 LL |     }
    |      - temporary lives until here
    |
@@ -47,6 +50,9 @@ LL |     match s.lock_m_m().get_the_value() {
 LL |             println!("{}", s.lock_m().get_the_value());
    |                            ---------- another value with significant `Drop` created here
 ...
+LL |             println!("{}", s.lock_m().get_the_value());
+   |                            ---------- another value with significant `Drop` created here
+...
 LL |     }
    |      - temporary lives until here
    |
@@ -360,7 +366,7 @@ LL |     match s.lock().deref().deref() {
    |           ^^^^^^^^^^^^^^^^^^^^^^^^
 ...
 LL |         _ => println!("Value is {}", s.lock().deref()),
-   |                                      ---------------- another value with significant `Drop` created here
+   |                                      -------- another value with significant `Drop` created here
 LL |     };
    |      - temporary lives until here
    |
@@ -378,7 +384,7 @@ LL |     match s.lock().deref().deref() {
    |           ^^^^^^^^^^^^^^^^^^^^^^^^
 ...
 LL |         matcher => println!("Value is {}", s.lock().deref()),
-   |                                            ---------------- another value with significant `Drop` created here
+   |                                            -------- another value with significant `Drop` created here
 LL |         _ => println!("Value was not a match"),
 LL |     };
    |      - temporary lives until here