Avoid inference constraints from opaque blanket impls

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -1177,13 +1177,19 @@ where
                     if ecx.shallow_resolve(self_ty).is_ty_var() {
                         // We force the certainty of impl candidates to be `Maybe`.
                         let certainty = certainty.and(Certainty::AMBIGUOUS);
-                        ecx.evaluate_added_goals_and_make_canonical_response(certainty)
+                        let response =
+                            ecx.evaluate_added_goals_and_make_canonical_response(certainty)?;
+
+                        let Certainty::Maybe(maybe) = response.value.certainty else {
+                            unreachable!();
+                        };
+
+                        // Blanket impls here only guide inference while the opaque hidden type
+                        // is unknown. Do not let constraints from matching the impl itself
+                        // constrain other inference variables.
+                        Ok(ecx.make_ambiguous_response_no_constraints(maybe))
                     } else {
                         // We don't want to use impls if they constrain the opaque.
-                        //
-                        // FIXME(trait-system-refactor-initiative#229): This isn't
-                        // perfect yet as it still allows us to incorrectly constrain
-                        // other inference variables.
                         Err(NoSolution.into())
                     }
                 })

tests/ui/impl-trait/non-defining-uses/avoid-inference-constraints-from-blanket-2.rs

@@ -1,15 +1,11 @@
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
 //@ ignore-compare-mode-next-solver (explicit revisions)
-//@[current] check-pass
+//@ check-pass
 
-// Regression test for trait-system-refactor-initiative#205. Avoid
+// Regression test for trait-system-refactor-initiative#205 and #229. Avoid
 // constraining other impl arguments when applying blanket impls.
 
-// FIXME(-Znext-solver): This currently incompletely constrains the
-// argument of `opaque: Trait<?x>` using the blanket impl of trait.
-// Ideally we don't do that.
-
 trait Trait<T> {}
 
 impl<T> Trait<u64> for T {}
@@ -25,7 +21,6 @@ fn foo() -> impl Sized {
         return impls_trait::<_, _>(x);
     }
     let _: u32 = x;
-    //[next]~^ ERROR mismatched types
     1u64
 }
 fn main() {}

tests/ui/impl-trait/non-defining-uses/avoid-inference-constraints-from-blanket-3.rs

@@ -1,16 +1,13 @@
 //@ compile-flags: -Znext-solver
 //@ ignore-compare-mode-next-solver (explicit revisions)
+//@ check-pass
 #![allow(unconditional_recursion)]
 
-// Regression test for trait-system-refactor-initiative#205. Avoid
+// Regression test for trait-system-refactor-initiative#205 and #229. Avoid
 // constraining other impl arguments when applying blanket impls,
 // especially if the nested where-bounds of the blanket impl don't
 // actually apply for the opaque.
 
-// FIXME(-Znext-solver): This currently incompletely constrains the
-// argument of `opaque: Trait<?x>` using the blanket impl of trait.
-// Ideally we don't do that.
-
 trait Trait<T> {}
 
 impl<T: Copy> Trait<u32> for T {}
@@ -19,7 +16,7 @@ fn impls_trait<T: Trait<U>, U>(_: T) {}
 
 fn test() -> impl Sized {
     let x = test();
-    impls_trait(x); //~ ERROR the trait bound `String: Trait<u32>` is not satisfied
+    impls_trait(x);
     String::new()
 }
 fn main() {}