When checking whether an impl applies, constrain hidden types of opaque types.

We already handle this case this way on the coherence side, and it matches the new solver's behaviour. While there is some breakage around type-alias-impl-trait (see new "type annotations needed" in tests/ui/type-alias-impl-trait/issue-84660-unsoundness.rs), no stable code breaks, and no new stable code is accepted.

Author: oli-obk

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

@@ -2539,7 +2539,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         let InferOk { obligations, .. } = self
             .infcx
             .at(&cause, obligation.param_env)
-            .eq(DefineOpaqueTypes::No, placeholder_obligation_trait_ref, impl_trait_ref)
+            .eq(DefineOpaqueTypes::Yes, placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| {
                 debug!("match_impl: failed eq_trait_refs due to `{}`", e.to_string(self.tcx()))
             })?;

tests/ui/impl-trait/equality.rs

@@ -22,7 +22,7 @@ fn sum_to(n: u32) -> impl Foo {
         0
     } else {
         n + sum_to(n - 1)
-        //~^ ERROR cannot add `impl Foo` to `u32`
+        //~^ ERROR cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
     }
 }
 
@@ -32,12 +32,15 @@ trait Leak: Sized {
 }
 impl<T> Leak for T {
     default type T = ();
-    default fn leak(self) -> Self::T { panic!() }
+    default fn leak(self) -> Self::T {
+        panic!()
+    }
 }
 impl Leak for i32 {
     type T = i32;
-    fn leak(self) -> i32 { self }
+    fn leak(self) -> i32 {
+        self
+    }
 }
 
-fn main() {
-}
+fn main() {}

tests/ui/impl-trait/equality.stderr

@@ -22,20 +22,13 @@ help: change the type of the numeric literal from `u32` to `i32`
 LL |     0_i32
    |       ~~~
 
-error[E0277]: cannot add `impl Foo` to `u32`
+error[E0284]: type annotations needed: cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
   --> $DIR/equality.rs:24:11
    |
 LL |         n + sum_to(n - 1)
-   |           ^ no implementation for `u32 + impl Foo`
-   |
-   = help: the trait `Add<impl Foo>` is not implemented for `u32`
-   = help: the following other types implement trait `Add<Rhs>`:
-             <&'a u32 as Add<u32>>
-             <&u32 as Add<&u32>>
-             <u32 as Add<&u32>>
-             <u32 as Add>
+   |           ^ cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
 
 error: aborting due to 2 previous errors; 1 warning emitted
 
-Some errors have detailed explanations: E0277, E0308.
-For more information about an error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0284, E0308.
+For more information about an error, try `rustc --explain E0284`.

tests/ui/impl-trait/nested_impl_trait.stderr

@@ -46,19 +46,23 @@ error[E0277]: the trait bound `impl Into<u32>: Into<impl Debug>` is not satisfie
   --> $DIR/nested_impl_trait.rs:6:46
    |
 LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Into<u32>`, which is required by `impl Into<u32>: Into<impl Debug>`
+   |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `impl Into<u32>`
    |
-   = help: the trait `Into<U>` is implemented for `T`
-   = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
+help: consider further restricting this bound
+   |
+LL | fn bad_in_ret_position(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
+   |                                          +++++++++++++++++
 
 error[E0277]: the trait bound `impl Into<u32>: Into<impl Debug>` is not satisfied
   --> $DIR/nested_impl_trait.rs:19:34
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Into<u32>`, which is required by `impl Into<u32>: Into<impl Debug>`
+   |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `impl Into<u32>`
+   |
+help: consider further restricting this bound
    |
-   = help: the trait `Into<U>` is implemented for `T`
-   = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
+LL |     fn bad(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
+   |                              +++++++++++++++++
 
 error: aborting due to 7 previous errors
 

tests/ui/impl-trait/recursive-type-alias-impl-trait-declaration.rs

@@ -11,7 +11,7 @@ impl PartialEq<(Bar, i32)> for Bar {
 }
 
 fn foo() -> Foo {
-    //~^ ERROR can't compare `Bar` with `(Foo, i32)`
+    //~^ ERROR overflow evaluating the requirement `Bar: PartialEq<(Foo, i32)>`
     Bar
 }
 

tests/ui/impl-trait/recursive-type-alias-impl-trait-declaration.stderr

@@ -1,15 +1,9 @@
-error[E0277]: can't compare `Bar` with `(Foo, i32)`
+error[E0275]: overflow evaluating the requirement `Bar: PartialEq<(Foo, i32)>`
   --> $DIR/recursive-type-alias-impl-trait-declaration.rs:13:13
    |
 LL | fn foo() -> Foo {
-   |             ^^^ no implementation for `Bar == (Foo, i32)`
-LL |
-LL |     Bar
-   |     --- return type was inferred to be `Bar` here
-   |
-   = help: the trait `PartialEq<(Foo, i32)>` is not implemented for `Bar`
-   = help: the trait `PartialEq<(Bar, i32)>` is implemented for `Bar`
+   |             ^^^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0275`.

tests/ui/type-alias-impl-trait/constrain_in_projection.current.stderr

@@ -4,7 +4,7 @@
 //@ revisions: current next
 //@ ignore-compare-mode-next-solver (explicit revisions)
 //@[next] compile-flags: -Znext-solver
-//@[next]check-pass
+//@check-pass
 
 #![feature(type_alias_impl_trait)]
 
@@ -22,7 +22,6 @@ impl Trait<()> for Foo {
 
 fn bop(_: Bar) {
     let x = <Foo as Trait<Bar>>::Assoc::default();
-    //[current]~^ `Foo: Trait<Bar>` is not satisfied
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/constrain_in_projection.rs

@@ -1,13 +1,19 @@
-error[E0277]: the trait bound `Foo: Trait<Bar>` is not satisfied
+error[E0283]: type annotations needed: cannot satisfy `Foo: Trait<Bar>`
   --> $DIR/constrain_in_projection2.rs:27:14
    |
 LL |     let x = <Foo as Trait<Bar>>::Assoc::default();
-   |              ^^^ the trait `Trait<Bar>` is not implemented for `Foo`
+   |              ^^^ help: use the fully qualified path to an implementation: `<Type as Trait>::Assoc`
    |
-   = help: the following other types implement trait `Trait<T>`:
-             <Foo as Trait<()>>
-             <Foo as Trait<u32>>
+note: multiple `impl`s satisfying `Foo: Trait<Bar>` found
+  --> $DIR/constrain_in_projection2.rs:18:1
+   |
+LL | impl Trait<()> for Foo {
+   | ^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | impl Trait<u32> for Foo {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: associated types cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0283`.

tests/ui/type-alias-impl-trait/constrain_in_projection2.current.stderr

@@ -25,8 +25,7 @@ impl Trait<u32> for Foo {
 
 fn bop(_: Bar) {
     let x = <Foo as Trait<Bar>>::Assoc::default();
-    //[next]~^ ERROR: cannot satisfy `Foo: Trait<Bar>`
-    //[current]~^^ ERROR: `Foo: Trait<Bar>` is not satisfied
+    //~^ ERROR: cannot satisfy `Foo: Trait<Bar>`
 }
 
 fn main() {}