Rollup merge of #99576 - compiler-errors:foreign-fundamental-drop-is-bad, r=TaKO8Ki

Do not allow `Drop` impl on foreign fundamental types

`Drop` should not be implemented on `Pin<T>` even if `T` is local.

This does not trigger regular orphan rules is because `Pin` is `#[fundamental]`... but we don't allow specialized `Drop` impls anyways, so these rules are not sufficient to prevent this impl on stable. Let's just choose even stricter rules, since we shouldn't be implementing `Drop` on a foreign ADT ever.

Fixes #99575

Author: Dylan-DPC

compiler/rustc_error_messages/locales/en-US/typeck.ftl

@@ -24,8 +24,8 @@ typeck_lifetimes_or_bounds_mismatch_on_trait =
     .generics_label = lifetimes in impl do not match this {$item_kind} in trait
 
 typeck_drop_impl_on_wrong_item =
-    the `Drop` trait may only be implemented for structs, enums, and unions
-    .label = must be a struct, enum, or union
+    the `Drop` trait may only be implemented for local structs, enums, and unions
+    .label = must be a struct, enum, or union in the current crate
 
 typeck_field_already_declared =
     field `{$field_name}` is already declared

compiler/rustc_typeck/src/coherence/builtin.rs

@@ -47,9 +47,11 @@ impl<'tcx> Checker<'tcx> {
 }
 
 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
-    // Destructors only work on nominal types.
-    if let ty::Adt(..) | ty::Error(_) = tcx.type_of(impl_did).kind() {
-        return;
+    // Destructors only work on local ADT types.
+    match tcx.type_of(impl_did).kind() {
+        ty::Adt(def, _) if def.did().is_local() => return,
+        ty::Error(_) => return,
+        _ => {}
     }
 
     let sp = match tcx.hir().expect_item(impl_did).kind {

src/test/ui/drop/drop-foreign-fundamental.rs

@@ -0,0 +1,23 @@
+use std::ops::Deref;
+use std::pin::Pin;
+
+struct Whatever<T>(T);
+
+impl<T> Deref for Whatever<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
+struct A;
+
+impl Drop for Pin<Whatever<A>> {
+    //~^ ERROR  the `Drop` trait may only be implemented for local structs, enums, and unions
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let x = Pin::new(Whatever(1.0f32));
+}

src/test/ui/drop/drop-foreign-fundamental.stderr

@@ -0,0 +1,9 @@
+error[E0120]: the `Drop` trait may only be implemented for local structs, enums, and unions
+  --> $DIR/drop-foreign-fundamental.rs:16:15
+   |
+LL | impl Drop for Pin<Whatever<A>> {
+   |               ^^^^^^^^^^^^^^^^ must be a struct, enum, or union in the current crate
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0120`.

src/test/ui/dropck/drop-on-non-struct.rs

@@ -1,5 +1,5 @@
 impl<'a> Drop for &'a mut isize {
-    //~^ ERROR the `Drop` trait may only be implemented for structs, enums, and unions
+    //~^ ERROR the `Drop` trait may only be implemented for local structs, enums, and unions
     //~^^ ERROR E0117
     fn drop(&mut self) {
         println!("kaboom");
@@ -8,8 +8,7 @@ impl<'a> Drop for &'a mut isize {
 
 impl Drop for Nonexistent {
     //~^ ERROR cannot find type `Nonexistent`
-    fn drop(&mut self) { }
+    fn drop(&mut self) {}
 }
 
-fn main() {
-}
+fn main() {}

src/test/ui/dropck/drop-on-non-struct.stderr

@@ -15,11 +15,11 @@ LL | impl<'a> Drop for &'a mut isize {
    |
    = note: define and implement a trait or new type instead
 
-error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
+error[E0120]: the `Drop` trait may only be implemented for local structs, enums, and unions
   --> $DIR/drop-on-non-struct.rs:1:19
    |
 LL | impl<'a> Drop for &'a mut isize {
-   |                   ^^^^^^^^^^^^^ must be a struct, enum, or union
+   |                   ^^^^^^^^^^^^^ must be a struct, enum, or union in the current crate
 
 error: aborting due to 3 previous errors
 

src/test/ui/error-codes/E0117.rs

@@ -1,4 +1,4 @@
 impl Drop for u32 {} //~ ERROR E0117
-//~| ERROR the `Drop` trait may only be implemented for structs, enums, and unions
+//~| ERROR the `Drop` trait may only be implemented for local structs, enums, and unions
 
 fn main() {}

src/test/ui/error-codes/E0117.stderr

@@ -9,11 +9,11 @@ LL | impl Drop for u32 {}
    |
    = note: define and implement a trait or new type instead
 
-error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
+error[E0120]: the `Drop` trait may only be implemented for local structs, enums, and unions
   --> $DIR/E0117.rs:1:15
    |
 LL | impl Drop for u32 {}
-   |               ^^^ must be a struct, enum, or union
+   |               ^^^ must be a struct, enum, or union in the current crate
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0120.stderr

@@ -1,8 +1,8 @@
-error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
+error[E0120]: the `Drop` trait may only be implemented for local structs, enums, and unions
   --> $DIR/E0120.rs:3:15
    |
 LL | impl Drop for dyn MyTrait {
-   |               ^^^^^^^^^^^ must be a struct, enum, or union
+   |               ^^^^^^^^^^^ must be a struct, enum, or union in the current crate
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-41974.stderr

@@ -7,11 +7,11 @@ LL | impl<T> Drop for T where T: A {
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
    = note: only traits defined in the current crate can be implemented for a type parameter
 
-error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
+error[E0120]: the `Drop` trait may only be implemented for local structs, enums, and unions
   --> $DIR/issue-41974.rs:7:18
    |
 LL | impl<T> Drop for T where T: A {
-   |                  ^ must be a struct, enum, or union
+   |                  ^ must be a struct, enum, or union in the current crate
 
 error: aborting due to 2 previous errors