Only include dyn Trait<Assoc = ...> associated type bounds for Self: Sized associated types if they are provided

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -172,7 +172,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let principal_trait = regular_traits.into_iter().next();
 
-        let mut needed_associated_types = vec![];
+        // A stable ordering of associated types from the principal trait and all its
+        // supertraits. We use this to ensure that different substitutions of a trait
+        // don't result in `dyn Trait` types with different projections lists, which
+        // can be unsound: <https://github.com/rust-lang/rust/pull/136458>.
+        // We achieve a stable ordering by walking over the unsubstituted principal
+        // trait ref.
+        let mut ordered_associated_types = vec![];
+
         if let Some((principal_trait, ref spans)) = principal_trait {
             let principal_trait = principal_trait.map_bound(|trait_pred| {
                 assert_eq!(trait_pred.polarity, ty::PredicatePolarity::Positive);
@@ -197,16 +204,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         // FIXME(negative_bounds): Handle this correctly...
                         let trait_ref =
                             tcx.anonymize_bound_vars(bound_predicate.rebind(pred.trait_ref));
-                        needed_associated_types.extend(
+                        ordered_associated_types.extend(
                             tcx.associated_items(pred.trait_ref.def_id)
                                 .in_definition_order()
                                 // We only care about associated types.
                                 .filter(|item| item.is_type())
                                 // No RPITITs -- they're not dyn-compatible for now.
                                 .filter(|item| !item.is_impl_trait_in_trait())
-                                // If the associated type has a `where Self: Sized` bound,
-                                // we do not need to constrain the associated type.
-                                .filter(|item| !tcx.generics_require_sized_self(item.def_id))
                                 .map(|item| (item.def_id, trait_ref)),
                         );
                     }
@@ -278,14 +282,26 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
         }
 
+        // We compute the list of projection bounds taking the ordered associated types,
+        // and check if there was an entry in the collected `projection_bounds`. Those
+        // are computed by first taking the user-written associated types, then elaborating
+        // the principal trait ref, and only using those if there was no user-written.
+        // See note below about how we handle missing associated types with `Self: Sized`,
+        // which are not required to be provided, but are still used if they are provided.
         let mut missing_assoc_types = FxIndexSet::default();
-        let projection_bounds: Vec<_> = needed_associated_types
+        let projection_bounds: Vec<_> = ordered_associated_types
             .into_iter()
             .filter_map(|key| {
                 if let Some(assoc) = projection_bounds.get(&key) {
                     Some(*assoc)
                 } else {
-                    missing_assoc_types.insert(key);
+                    // If the associated type has a `where Self: Sized` bound, then
+                    // we do not need to provide the associated type. This results in
+                    // a `dyn Trait` type that has a different number of projection
+                    // bounds, which may lead to type mismatches.
+                    if !tcx.generics_require_sized_self(key.0) {
+                        missing_assoc_types.insert(key);
+                    }
                     None
                 }
             })

compiler/rustc_middle/src/ty/sty.rs

@@ -723,7 +723,10 @@ impl<'tcx> Ty<'tcx> {
         repr: DynKind,
     ) -> Ty<'tcx> {
         if cfg!(debug_assertions) {
-            let projection_count = obj.projection_bounds().count();
+            let projection_count = obj
+                .projection_bounds()
+                .filter(|item| !tcx.generics_require_sized_self(item.item_def_id()))
+                .count();
             let expected_count: usize = obj
                 .principal_def_id()
                 .into_iter()

tests/ui/traits/object/constrain-via-unnecessary-bound.rs

@@ -0,0 +1,24 @@
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/rust/issues/140645>.
+// Test that we lower impossible-to-satisfy associated type bounds, which
+// may for example constrain impl parameters.
+
+pub trait Other {}
+
+pub trait Trait {
+    type Assoc
+    where
+        Self: Sized;
+}
+
+impl Other for dyn Trait {}
+// `dyn Trait<Assoc = ()>` is a different "nominal type" than `dyn Trait`.
+impl Other for dyn Trait<Assoc = ()> {}
+//~^ WARN unnecessary associated type bound for dyn-incompatible associated type
+
+// I hope it's clear that `dyn Trait` (w/o `Assoc`) wouldn't match this impl.
+impl<T> dyn Trait<Assoc = T> {}
+//~^ WARN unnecessary associated type bound for dyn-incompatible associated type
+
+fn main() {}

tests/ui/traits/object/constrain-via-unnecessary-bound.stderr

@@ -0,0 +1,19 @@
+warning: unnecessary associated type bound for dyn-incompatible associated type
+  --> $DIR/constrain-via-unnecessary-bound.rs:17:26
+   |
+LL | impl Other for dyn Trait<Assoc = ()> {}
+   |                          ^^^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound, and while the associated type can be specified, it cannot be used because trait objects are never `Sized`
+   = note: `#[warn(unused_associated_type_bounds)]` on by default
+
+warning: unnecessary associated type bound for dyn-incompatible associated type
+  --> $DIR/constrain-via-unnecessary-bound.rs:21:19
+   |
+LL | impl<T> dyn Trait<Assoc = T> {}
+   |                   ^^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound, and while the associated type can be specified, it cannot be used because trait objects are never `Sized`
+
+warning: 2 warnings emitted
+

tests/ui/traits/object/pretty.stderr

@@ -154,12 +154,12 @@ error[E0308]: mismatched types
   --> $DIR/pretty.rs:41:56
    |
 LL | fn dyn_has_gat(x: &dyn HasGat<u8, Assoc<bool> = ()>) { x }
-   |                                                     -  ^ expected `()`, found `&dyn HasGat<u8>`
+   |                                                     -  ^ expected `()`, found `&dyn HasGat<u8, Assoc<bool> = ()>`
    |                                                     |
-   |                                                     help: try adding a return type: `-> &dyn HasGat<u8>`
+   |                                                     help: try adding a return type: `-> &dyn HasGat<u8, Assoc<bool> = ()>`
    |
    = note: expected unit type `()`
-              found reference `&dyn HasGat<u8>`
+              found reference `&dyn HasGat<u8, Assoc<bool> = ()>`
 
 error: aborting due to 14 previous errors; 1 warning emitted