Auto merge of rust-lang#116706 - compiler-errors:pin-negate, r=<try>

WIP

WIP fix for rust-lang#85099, will write up a description w/ `@lcnr` when I'm at my computer

r? lcnr

compiler/rustc_feature/src/builtin_attrs.rs

@@ -714,6 +714,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_coinductive, AttributeType::Normal, template!(Word), WarnFollowing, @only_local: true,
         "#![rustc_coinductive] changes a trait to be coinductive, allowing cycles in the trait solver."
     ),
+    rustc_attr!(
+        rustc_no_implicit_negative_coherence, AttributeType::Normal, template!(Word), WarnFollowing, @only_local: false,
+        "#![rustc_no_implicit_negative_coherence] makes the implementation overlap \
+        with other implementations even if where clauses are known to never hold"
+    ),
     rustc_attr!(
         rustc_allow_incoherent_impl, AttributeType::Normal, template!(Word), ErrorFollowing, @only_local: true,
         "#[rustc_allow_incoherent_impl] has to be added to all impl items of an incoherent inherent impl."

compiler/rustc_span/src/symbol.rs

@@ -1383,6 +1383,7 @@ symbols! {
         rustc_mir,
         rustc_must_implement_one_of,
         rustc_never_returns_null_ptr,
+        rustc_no_implicit_negative_coherence,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,
         rustc_object_lifetime_default,

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -220,7 +220,10 @@ fn overlap<'tcx>(
         ),
     );
 
-    if overlap_mode.use_implicit_negative() {
+    if overlap_mode.use_implicit_negative()
+        && !tcx.has_attr(impl1_def_id, sym::rustc_no_implicit_negative_coherence)
+        && !tcx.has_attr(impl2_def_id, sym::rustc_no_implicit_negative_coherence)
+    {
         for mode in [TreatInductiveCycleAs::Ambig, TreatInductiveCycleAs::Recur] {
             if let Some(failing_obligation) = selcx.with_treat_inductive_cycle_as(mode, |selcx| {
                 impl_intersection_has_impossible_obligation(selcx, &obligations)

library/core/src/pin.rs

@@ -406,7 +406,7 @@ use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
 #[lang = "pin"]
 #[fundamental]
 #[repr(transparent)]
-#[derive(Copy, Clone)]
+#[derive(Copy)]
 pub struct Pin<P> {
     // FIXME(#93176): this field is made `#[unstable] #[doc(hidden)] pub` to:
     //   - deter downstream users from accessing it (which would be unsound!),
@@ -418,6 +418,15 @@ pub struct Pin<P> {
     pub pointer: P,
 }
 
+// N.B. manual implementation exists to add `rustc_no_implicit_negative_coherence`
+#[stable(feature = "pin", since = "1.33.0")]
+#[cfg_attr(not(bootstrap), rustc_no_implicit_negative_coherence)]
+impl<P: Clone> Clone for Pin<P> {
+    fn clone(&self) -> Self {
+        Pin { pointer: self.pointer.clone() }
+    }
+}
+
 // The following implementations aren't derived in order to avoid soundness
 // issues. `&self.pointer` should not be accessible to untrusted trait
 // implementations.
@@ -967,6 +976,7 @@ impl<P: Deref> Deref for Pin<P> {
 }
 
 #[stable(feature = "pin", since = "1.33.0")]
+#[cfg_attr(not(bootstrap), rustc_no_implicit_negative_coherence)]
 impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> {
     fn deref_mut(&mut self) -> &mut P::Target {
         Pin::get_mut(Pin::as_mut(self))

tests/ui/typeck/pin-unsound-issue-85099-derefmut.rs

@@ -1,6 +1,3 @@
-// check-pass
-// known-bug: #85099
-
 // Should fail. Can coerce `Pin<T>` into `Pin<U>` where
 // `T: Deref<Target: Unpin>` and `U: Deref<Target: !Unpin>`, using the
 // `CoerceUnsized` impl on `Pin` and an unorthodox `DerefMut` impl for
@@ -43,6 +40,7 @@ impl<'a, Fut: Future<Output = ()>> SomeTrait<'a, Fut> for Fut {
 }
 
 impl<'b, 'a, Fut> DerefMut for Pin<&'b dyn SomeTrait<'a, Fut>> {
+//~^ ERROR conflicting implementations of trait `DerefMut` for type `Pin<&dyn SomeTrait<'_, _>>`
     fn deref_mut<'c>(
         self: &'c mut Pin<&'b dyn SomeTrait<'a, Fut>>,
     ) -> &'c mut (dyn SomeTrait<'a, Fut> + 'b) {

tests/ui/typeck/pin-unsound-issue-85099-derefmut.stderr

@@ -0,0 +1,13 @@
+error[E0119]: conflicting implementations of trait `DerefMut` for type `Pin<&dyn SomeTrait<'_, _>>`
+  --> $DIR/pin-unsound-issue-85099-derefmut.rs:42:1
+   |
+LL | impl<'b, 'a, Fut> DerefMut for Pin<&'b dyn SomeTrait<'a, Fut>> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: conflicting implementation in crate `core`:
+           - impl<P> DerefMut for Pin<P>
+             where P: DerefMut, <P as Deref>::Target: Unpin;
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.