update coherence docs, fix generator + opaque type ICE

compiler/rustc_middle/src/traits/mod.rs

@@ -60,6 +60,9 @@ pub enum Reveal {
     ///     let <() as Assoc>::Output = true;
     /// }
     /// ```
+    ///
+    /// We also do not reveal the hidden type of opaque types during
+    /// type-checking.
     UserFacing,
 
     /// At codegen time, all monomorphic projections will succeed.

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -645,7 +645,7 @@ fn orphan_check_trait_ref<'tcx>(
                 .substs
                 .types()
                 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
-                .find(|ty| ty_is_local_constructor(*ty, in_crate));
+                .find(|&ty| ty_is_local_constructor(tcx, ty, in_crate));
 
             debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
 
@@ -677,7 +677,7 @@ fn contained_non_local_types<'tcx>(
     ty: Ty<'tcx>,
     in_crate: InCrate,
 ) -> Vec<Ty<'tcx>> {
-    if ty_is_local_constructor(ty, in_crate) {
+    if ty_is_local_constructor(tcx, ty, in_crate) {
         Vec::new()
     } else {
         match fundamental_ty_inner_tys(tcx, ty) {
@@ -730,7 +730,7 @@ fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
     }
 }
 
-fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
+fn ty_is_local_constructor(tcx: TyCtxt<'_>, ty: Ty<'_>, in_crate: InCrate) -> bool {
     debug!("ty_is_local_constructor({:?})", ty);
 
     match *ty.kind() {
@@ -789,11 +789,6 @@ fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
             false
         }
 
-        ty::Closure(..) => {
-            // Similar to the `Opaque` case (#83613).
-            false
-        }
-
         ty::Dynamic(ref tt, ..) => {
             if let Some(principal) = tt.principal() {
                 def_id_is_local(principal.def_id(), in_crate)
@@ -804,8 +799,20 @@ fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
 
         ty::Error(_) => true,
 
-        ty::Generator(..) | ty::GeneratorWitness(..) => {
-            bug!("ty_is_local invoked on unexpected type: {:?}", ty)
+        // These variants should never appear during coherence checking because they
+        // cannot be named directly.
+        //
+        // They could be indirectly used through an opaque type. While using opaque types
+        // in impls causes an error, this path can still be hit afterwards.
+        //
+        // See `test/ui/coherence/coherence-with-closure.rs` for an example where this
+        // could happens.
+        ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(..) => {
+            tcx.sess.delay_span_bug(
+                DUMMY_SP,
+                format!("ty_is_local invoked on closure or generator: {:?}", ty),
+            );
+            true
         }
     }
 }

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -103,22 +103,31 @@ pub struct SelectionContext<'cx, 'tcx> {
     /// require themselves.
     freshener: TypeFreshener<'cx, 'tcx>,
 
-    /// If `true`, indicates that the evaluation should be conservative
-    /// and consider the possibility of types outside this crate.
+    /// During coherence we have to assume that other crates may add
+    /// additional impls which we currently don't know about.
+    ///
+    /// To deal with this evaluation should be conservative
+    /// and consider the possibility of impls from outside this crate.
     /// This comes up primarily when resolving ambiguity. Imagine
     /// there is some trait reference `$0: Bar` where `$0` is an
     /// inference variable. If `intercrate` is true, then we can never
     /// say for sure that this reference is not implemented, even if
     /// there are *no impls at all for `Bar`*, because `$0` could be
     /// bound to some type that in a downstream crate that implements
-    /// `Bar`. This is the suitable mode for coherence. Elsewhere,
-    /// though, we set this to false, because we are only interested
-    /// in types that the user could actually have written --- in
-    /// other words, we consider `$0: Bar` to be unimplemented if
+    /// `Bar`.
+    ///
+    /// Outside of coherence we set this to false because we are only
+    /// interested in types that the user could actually have written.
+    /// In other words, we consider `$0: Bar` to be unimplemented if
     /// there is no type that the user could *actually name* that
     /// would satisfy it. This avoids crippling inference, basically.
     intercrate: bool,
-
+    /// If `intercrate` is set, we remember predicates which were
+    /// considered ambiguous because of impls potentially added in other crates.
+    /// This is used in coherence to give improved diagnostics.
+    /// We don't do his until we detect a coherence error because it can
+    /// lead to false overflow results (#47139) and because always
+    /// computing it may negatively impact performance.
     intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
 
     /// The mode that trait queries run in, which informs our error handling
@@ -240,11 +249,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    /// Enables tracking of intercrate ambiguity causes. These are
-    /// used in coherence to give improved diagnostics. We don't do
-    /// this until we detect a coherence error because it can lead to
-    /// false overflow results (#47139) and because it costs
-    /// computation time.
+    /// Enables tracking of intercrate ambiguity causes. See
+    /// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
     pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
         assert!(self.intercrate);
         assert!(self.intercrate_ambiguity_causes.is_none());

src/test/ui/coherence/coherence-with-closure.rs

@@ -0,0 +1,15 @@
+// Test that encountering closures during coherence does not cause issues.
+#![feature(type_alias_impl_trait)]
+type OpaqueClosure = impl Sized;
+fn defining_use() -> OpaqueClosure {
+    || ()
+}
+
+struct Wrapper<T>(T);
+trait Trait {}
+impl Trait for Wrapper<OpaqueClosure> {}
+//~^ ERROR cannot implement trait on type alias impl trait
+impl<T: Sync> Trait for Wrapper<T> {}
+//~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
+
+fn main() {}

src/test/ui/coherence/coherence-with-closure.stderr

@@ -0,0 +1,24 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/coherence-with-closure.rs:10:24
+   |
+LL | impl Trait for Wrapper<OpaqueClosure> {}
+   |                        ^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/coherence-with-closure.rs:3:22
+   |
+LL | type OpaqueClosure = impl Sized;
+   |                      ^^^^^^^^^^
+
+error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
+  --> $DIR/coherence-with-closure.rs:12:1
+   |
+LL | impl Trait for Wrapper<OpaqueClosure> {}
+   | ------------------------------------- first implementation here
+LL |
+LL | impl<T: Sync> Trait for Wrapper<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueClosure>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0119`.

src/test/ui/coherence/coherence-with-generator.rs

@@ -0,0 +1,19 @@
+// Test that encountering closures during coherence does not cause issues.
+#![feature(type_alias_impl_trait, generators)]
+type OpaqueGenerator = impl Sized;
+fn defining_use() -> OpaqueGenerator {
+    || {
+        for i in 0..10 {
+            yield i;
+        }
+    }
+}
+
+struct Wrapper<T>(T);
+trait Trait {}
+impl Trait for Wrapper<OpaqueGenerator> {}
+//~^ ERROR cannot implement trait on type alias impl trait
+impl<T: Sync> Trait for Wrapper<T> {}
+//~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
+
+fn main() {}

src/test/ui/coherence/coherence-with-generator.stderr

@@ -0,0 +1,24 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/coherence-with-generator.rs:14:24
+   |
+LL | impl Trait for Wrapper<OpaqueGenerator> {}
+   |                        ^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/coherence-with-generator.rs:3:24
+   |
+LL | type OpaqueGenerator = impl Sized;
+   |                        ^^^^^^^^^^
+
+error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
+  --> $DIR/coherence-with-generator.rs:16:1
+   |
+LL | impl Trait for Wrapper<OpaqueGenerator> {}
+   | --------------------------------------- first implementation here
+LL |
+LL | impl<T: Sync> Trait for Wrapper<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueGenerator>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0119`.