Stop generating inference vars for nested impl trait and let type equality handle it.

This means we stop supporting the case where a locally defined trait has only a single impl so we can always use that impl (see nested-tait-inference.rs).

9 files changed, 70 insertions, 50 deletions

diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index dc0ebf23058..ff636616db4 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -1,4 +1,3 @@
-use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{InferCtxt, InferOk};
 use crate::traits::{self, PredicateObligation, PredicateObligations};
 use hir::def_id::{DefId, LocalDefId};
@@ -604,20 +603,6 @@ struct Instantiator<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
-    #[instrument(level = "trace", skip(self))]
-    fn instantiate_opaque_types(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        if let Some(ty) = self.fold_opaque_ty_new(ty, |infcx, span| {
-            infcx.next_ty_var(TypeVariableOrigin {
-                kind: TypeVariableOriginKind::TypeInference,
-                span,
-            })
-        }) {
-            return ty;
-        }
-
-        ty
-    }
-
     fn fold_opaque_ty_new(
         &mut self,
         ty: Ty<'tcx>,
@@ -720,7 +705,6 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
                     ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
                         ty_var
                     }
-                    ty::Opaque(..) => self.instantiate_opaque_types(ty),
                     _ => ty,
                 },
                 lt_op: |lt| lt,
diff --git a/src/test/ui/impl-trait/nested_impl_trait.rs b/src/test/ui/impl-trait/nested_impl_trait.rs
index be2c21a7743..6eac2dece1f 100644
--- a/src/test/ui/impl-trait/nested_impl_trait.rs
+++ b/src/test/ui/impl-trait/nested_impl_trait.rs
@@ -4,6 +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`
 
 fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
 //~^ ERROR nested `impl Trait` is not allowed
@@ -16,6 +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`
 }
 
 fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
diff --git a/src/test/ui/impl-trait/nested_impl_trait.stderr b/src/test/ui/impl-trait/nested_impl_trait.stderr
index 59c7e4d5f4e..87ff4ffc4fb 100644
--- a/src/test/ui/impl-trait/nested_impl_trait.stderr
+++ b/src/test/ui/impl-trait/nested_impl_trait.stderr
@@ -8,7 +8,7 @@ LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                              outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:8:42
+  --> $DIR/nested_impl_trait.rs:9:42
    |
 LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                ----------^^^^^^^^^^-
@@ -17,7 +17,7 @@ LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:12:37
+  --> $DIR/nested_impl_trait.rs:13:37
    |
 LL | fn bad_in_arg_position(_: impl Into<impl Debug>) { }
    |                           ----------^^^^^^^^^^-
@@ -26,7 +26,7 @@ LL | fn bad_in_arg_position(_: impl Into<impl Debug>) { }
    |                           outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:17:44
+  --> $DIR/nested_impl_trait.rs:18:44
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  ----------^^^^^^^^^^-
@@ -35,18 +35,40 @@ LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  outer `impl Trait`
 
 error[E0562]: `impl Trait` not allowed outside of function and method return types
-  --> $DIR/nested_impl_trait.rs:8:32
+  --> $DIR/nested_impl_trait.rs:9:32
    |
 LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                ^^^^^^^^^^^^^^^^^^^^^
 
 error[E0562]: `impl Trait` not allowed outside of function and method return types
-  --> $DIR/nested_impl_trait.rs:25:42
+  --> $DIR/nested_impl_trait.rs:27:42
    |
 LL | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
    |                                          ^^^^^^^^^^^^^^
 
-error: aborting due to 6 previous errors
+error[E0277]: `impl Into<u32>` doesn't implement `Debug`
+  --> $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`
+   |
+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]: `impl Into<u32>` doesn't implement `Debug`
+  --> $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`
+   |
+help: consider further restricting this bound
+   |
+LL |     fn bad(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
+   |                              +++++++++++++++++
+
+error: aborting due to 8 previous errors
 
-Some errors have detailed explanations: E0562, E0666.
-For more information about an error, try `rustc --explain E0562`.
+Some errors have detailed explanations: E0277, E0562, E0666.
+For more information about an error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/bound_reduction2.rs b/src/test/ui/type-alias-impl-trait/bound_reduction2.rs
index 2177bf3b1c4..ba780e301ed 100644
--- a/src/test/ui/type-alias-impl-trait/bound_reduction2.rs
+++ b/src/test/ui/type-alias-impl-trait/bound_reduction2.rs
@@ -15,5 +15,5 @@ impl<W> Trait<W> for () {}
 
 fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {
     ()
-    //~^ ERROR type annotations needed
+    //~^ ERROR non-defining opaque type use
 }
diff --git a/src/test/ui/type-alias-impl-trait/bound_reduction2.stderr b/src/test/ui/type-alias-impl-trait/bound_reduction2.stderr
index 4ff14333c19..21e79ee36a6 100644
--- a/src/test/ui/type-alias-impl-trait/bound_reduction2.stderr
+++ b/src/test/ui/type-alias-impl-trait/bound_reduction2.stderr
@@ -1,8 +1,14 @@
-error[E0282]: type annotations needed
+error: non-defining opaque type use in defining scope
   --> $DIR/bound_reduction2.rs:17:5
    |
 LL |     ()
-   |     ^^ cannot infer type
+   |     ^^
+   |
+note: used non-generic type `<T as TraitWithAssoc>::Assoc` for generic parameter
+  --> $DIR/bound_reduction2.rs:9:10
+   |
+LL | type Foo<V> = impl Trait<V>;
+   |          ^
 
 error: could not find defining uses
   --> $DIR/bound_reduction2.rs:9:15
@@ -12,4 +18,3 @@ LL | type Foo<V> = impl Trait<V>;
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0282`.
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference.rs b/src/test/ui/type-alias-impl-trait/nested-tait-inference.rs
index 075235ec15d..78a665d97a0 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference.rs
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference.rs
@@ -1,18 +1,19 @@
 #![feature(type_alias_impl_trait)]
 #![allow(dead_code)]
 
-// check-pass
-
 use std::fmt::Debug;
 
 type FooX = impl Debug;
+//~^ ERROR could not find defining uses
 
 trait Foo<A> { }
 
 impl Foo<()> for () { }
 
 fn foo() -> impl Foo<FooX> {
+    // FIXME(type-alias-impl-trait): We could probably make this work.
     ()
+    //~^ ERROR: the trait bound `(): Foo<impl Debug>` is not satisfied
 }
 
 fn main() { }
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference.stderr b/src/test/ui/type-alias-impl-trait/nested-tait-inference.stderr
new file mode 100644
index 00000000000..d9585621d2d
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference.stderr
@@ -0,0 +1,18 @@
+error[E0277]: the trait bound `(): Foo<impl Debug>` is not satisfied
+  --> $DIR/nested-tait-inference.rs:15:5
+   |
+LL |     ()
+   |     ^^ the trait `Foo<impl Debug>` is not implemented for `()`
+   |
+   = help: the following implementations were found:
+             <() as Foo<()>>
+
+error: could not find defining uses
+  --> $DIR/nested-tait-inference.rs:6:13
+   |
+LL | type FooX = impl Debug;
+   |             ^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference2.rs b/src/test/ui/type-alias-impl-trait/nested-tait-inference2.rs
index 365f10f9475..4cbbe01ed51 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference2.rs
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference2.rs
@@ -13,8 +13,7 @@ impl Foo<u32> for () {}
 
 fn foo() -> impl Foo<FooX> {
     ()
-    //~^ ERROR: type annotations needed
-    //~| ERROR: type annotations needed
+    //~^ ERROR: the trait bound `(): Foo<impl Debug>` is not satisfied
 }
 
 fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference2.stderr b/src/test/ui/type-alias-impl-trait/nested-tait-inference2.stderr
index 294cd07675b..e08488287f5 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference2.stderr
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference2.stderr
@@ -1,22 +1,12 @@
-error[E0282]: type annotations needed
+error[E0277]: the trait bound `(): Foo<impl Debug>` is not satisfied
   --> $DIR/nested-tait-inference2.rs:15:5
    |
 LL |     ()
-   |     ^^ cannot infer type
-
-error[E0283]: type annotations needed
-  --> $DIR/nested-tait-inference2.rs:15:5
-   |
-LL |     ()
-   |     ^^ cannot infer type
-   |
-note: multiple `impl`s satisfying `(): Foo<_>` found
-  --> $DIR/nested-tait-inference2.rs:11:1
+   |     ^^ the trait `Foo<impl Debug>` is not implemented for `()`
    |
-LL | impl Foo<()> for () {}
-   | ^^^^^^^^^^^^^^^^^^^
-LL | impl Foo<u32> for () {}
-   | ^^^^^^^^^^^^^^^^^^^^
+   = help: the following implementations were found:
+             <() as Foo<()>>
+             <() as Foo<u32>>
 
 error: could not find defining uses
   --> $DIR/nested-tait-inference2.rs:6:13
@@ -24,7 +14,6 @@ error: could not find defining uses
 LL | type FooX = impl Debug;
    |             ^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0282, E0283.
-For more information about an error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0277`.