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).

2 files changed, 32 insertions, 8 deletions

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`.