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

This reverts commit ad00868.

Author: oli-obk

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

@@ -2563,7 +2563,7 @@ 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 Debug`, 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 Debug`, 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-bound-eval.current.stderr

@@ -0,0 +1,42 @@
+error[E0282]: type annotations needed
+  --> $DIR/recursive-bound-eval.rs:19:28
+   |
+LL |     move || recursive_fn().parse()
+   |                            ^^^^^ cannot infer type
+
+error[E0599]: no method named `parse` found for opaque type `impl Parser<_>` in the current scope
+  --> $DIR/recursive-bound-eval.rs:19:28
+   |
+LL |     move || recursive_fn().parse()
+   |                            ^^^^^ method not found in `impl Parser<_>`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+help: trait `Parser` which provides `parse` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use Parser;
+   |
+
+error[E0391]: cycle detected when computing type of opaque `recursive_fn::{opaque#0}`
+  --> $DIR/recursive-bound-eval.rs:17:29
+   |
+LL | pub fn recursive_fn<E>() -> impl Parser<E> {
+   |                             ^^^^^^^^^^^^^^
+   |
+note: ...which requires type-checking `recursive_fn`...
+  --> $DIR/recursive-bound-eval.rs:19:13
+   |
+LL |     move || recursive_fn().parse()
+   |             ^^^^^^^^^^^^^^
+   = note: ...which requires evaluating trait selection obligation `recursive_fn::{opaque#0}: core::marker::Unpin`...
+   = note: ...which again requires computing type of opaque `recursive_fn::{opaque#0}`, completing the cycle
+note: cycle used when computing type of `recursive_fn::{opaque#0}`
+  --> $DIR/recursive-bound-eval.rs:17:29
+   |
+LL | pub fn recursive_fn<E>() -> impl Parser<E> {
+   |                             ^^^^^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0282, E0391, E0599.
+For more information about an error, try `rustc --explain E0282`.

tests/ui/impl-trait/recursive-bound-eval.next.stderr

@@ -1,5 +1,5 @@
 error[E0282]: type annotations needed
-  --> $DIR/recursive-bound-eval.rs:20:13
+  --> $DIR/recursive-bound-eval.rs:19:13
    |
 LL |     move || recursive_fn().parse()
    |             ^^^^^^^^^^^^^^ cannot infer type

tests/ui/impl-trait/recursive-bound-eval.rs

@@ -4,8 +4,6 @@
 //@revisions: next current
 //@[next] compile-flags: -Znext-solver
 
-//@[current] check-pass
-
 pub trait Parser<E> {
     fn parse(&self) -> E;
 }
@@ -17,8 +15,10 @@ impl<E, T: Fn() -> E> Parser<E> for T {
 }
 
 pub fn recursive_fn<E>() -> impl Parser<E> {
+    //[current]~^ ERROR: cycle detected
     move || recursive_fn().parse()
-    //[next]~^ ERROR: type annotations needed
+    //~^ ERROR: type annotations needed
+    //[current]~^^ ERROR: no method named `parse` found for opaque type
 }
 
 fn main() {}

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`.