Don't bind hidden types when searching for matching impls

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

@@ -2163,6 +2163,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let InferOk { obligations, .. } = self
             .infcx
             .at(&cause, obligation.param_env)
+            .define_opaque_types(false)
             .eq(placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
         nested_obligations.extend(obligations);

src/test/ui/impl-trait/nested_impl_trait.rs

@@ -4,7 +4,7 @@ fn fine(x: impl Into<u32>) -> impl Into<u32> { x }
 
 fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
 //~^ ERROR nested `impl Trait` is not allowed
-//~| ERROR `impl Into<u32>` doesn't implement `Debug`
+//~| ERROR the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
 
 fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
 //~^ ERROR nested `impl Trait` is not allowed
@@ -17,7 +17,7 @@ struct X;
 impl X {
     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
     //~^ ERROR nested `impl Trait` is not allowed
-    //~| ERROR `impl Into<u32>` doesn't implement `Debug`
+    //~| ERROR the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
 }
 
 fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {

src/test/ui/impl-trait/nested_impl_trait.stderr

@@ -46,27 +46,21 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
 LL | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
    |                                          ^^^^^^^^^^^^^^
 
-error[E0277]: `impl Into<u32>` doesn't implement `Debug`
+error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
   --> $DIR/nested_impl_trait.rs:5:70
    |
 LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                                                      ^ `impl Into<u32>` cannot be formatted using `{:?}` because it doesn't implement `Debug`
+   |                                                                      ^ the trait `From<impl Into<u32>>` is not implemented for `impl Debug`
    |
-help: consider further restricting this bound
+   = note: required because of the requirements on the impl of `Into<impl Debug>` for `impl Into<u32>`
-   |
-LL | fn bad_in_ret_position(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
-   |                                          +++++++++++++++++
 
-error[E0277]: `impl Into<u32>` doesn't implement `Debug`
+error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
   --> $DIR/nested_impl_trait.rs:18:58
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                                          ^ `impl Into<u32>` cannot be formatted using `{:?}` because it doesn't implement `Debug`
+   |                                                          ^ the trait `From<impl Into<u32>>` is not implemented for `impl Debug`
    |
-help: consider further restricting this bound
+   = note: required because of the requirements on the impl of `Into<impl Debug>` for `impl Into<u32>`
-   |
-LL |     fn bad(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
-   |                              +++++++++++++++++
 
 error: aborting due to 8 previous errors
 

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs

@@ -0,0 +1,17 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo = impl PartialEq<(Foo, i32)>;
+
+struct Bar;
+
+impl PartialEq<(Foo, i32)> for Bar {
+    fn eq(&self, _other: &(Foo, i32)) -> bool {
+        true
+    }
+}
+
+fn foo() -> Foo {
+    Bar //~ ERROR can't compare `Bar` with `(Bar, i32)`
+}
+
+fn main() {}

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr

@@ -0,0 +1,11 @@
+error[E0277]: can't compare `Bar` with `(Bar, i32)`
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:14:5
+   |
+LL |     Bar
+   |     ^^^ no implementation for `Bar == (Bar, i32)`
+   |
+   = help: the trait `PartialEq<(Bar, i32)>` is not implemented for `Bar`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration.rs

@@ -2,36 +2,18 @@
 
 #![feature(type_alias_impl_trait)]
 
-mod direct {
+type Foo = impl PartialEq<(Foo, i32)>;
-    type Foo = impl PartialEq<(Foo, i32)>;
 
-    struct Bar;
+struct Bar;
 
-    impl PartialEq<(Foo, i32)> for Bar {
+impl PartialEq<(Bar, i32)> for Bar {
-        fn eq(&self, _other: &(Foo, i32)) -> bool {
+    fn eq(&self, _other: &(Bar, i32)) -> bool {
-            true
+        true
-        }
-    }
-
-    fn foo() -> Foo {
-        Bar
     }
 }
 
-mod indirect {
+fn foo() -> Foo {
-    type Foo = impl PartialEq<(Foo, i32)>;
+    Bar
-
-    struct Bar;
-
-    impl PartialEq<(Bar, i32)> for Bar {
-        fn eq(&self, _other: &(Bar, i32)) -> bool {
-            true
-        }
-    }
-
-    fn foo() -> Foo {
-        Bar
-    }
 }
 
 fn main() {}

src/test/ui/impl-trait/trait_resolution.rs

@@ -0,0 +1,30 @@
+// check-pass
+
+use std::fmt::Debug;
+
+pub struct EventStream<S> {
+    stream: S,
+}
+
+impl<S: Debug> EventStream<S> {
+    fn into_stream(self) -> impl Debug {
+        unimplemented!()
+    }
+
+    pub fn into_reader(self) -> impl Debug {
+        ReaderStream::from(self.into_stream())
+    }
+}
+
+#[derive(Debug)]
+pub struct ReaderStream<S> {
+    stream: S,
+}
+
+impl<S> From<S> for ReaderStream<S> {
+    fn from(stream: S) -> Self {
+        ReaderStream { stream }
+    }
+}
+
+fn main() {}