Fix #54822 and associated faulty tests

Type checking associated constants can require trait bounds, but an empty
parameter environment was provided to the trait solver.  Providing an
appropriate parameter environment seems to fix #54822 and also make one of the
cases in src/test/ui/nll/trait-associated-constant.rs that should compile
successfully do so.  It also (slightly) improves the error message in
src/test/ui/associated-const/associated-const-generic-obligations.rs

6 files changed, 33 insertions, 28 deletions

diff --git a/src/librustc_typeck/check/compare_method.rs b/src/librustc_typeck/check/compare_method.rs
index 1e5f5d244e9..f79bf4e999d 100644
--- a/src/librustc_typeck/check/compare_method.rs
+++ b/src/librustc_typeck/check/compare_method.rs
@@ -917,7 +917,7 @@ pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 
     tcx.infer_ctxt().enter(|infcx| {
-        let param_env = ty::ParamEnv::empty();
+        let param_env = tcx.param_env(impl_c.def_id);
         let inh = Inherited::new(infcx, impl_c.def_id);
         let infcx = &inh.infcx;
 
diff --git a/src/test/ui/associated-const/associated-const-generic-obligations.rs b/src/test/ui/associated-const/associated-const-generic-obligations.rs
index e0b502edaa1..498e315b5c8 100644
--- a/src/test/ui/associated-const/associated-const-generic-obligations.rs
+++ b/src/test/ui/associated-const/associated-const-generic-obligations.rs
@@ -12,7 +12,7 @@ trait Bar: Foo {
 
 impl<T: Foo> Bar for T {
     const FROM: &'static str = "foo";
-    //~^ ERROR the trait bound `T: Foo` is not satisfied [E0277]
+    //~^ ERROR implemented const `FROM` has an incompatible type for trait [E0326]
 }
 
 fn main() {}
diff --git a/src/test/ui/associated-const/associated-const-generic-obligations.stderr b/src/test/ui/associated-const/associated-const-generic-obligations.stderr
index e4b86d84caf..eeee26a7567 100644
--- a/src/test/ui/associated-const/associated-const-generic-obligations.stderr
+++ b/src/test/ui/associated-const/associated-const-generic-obligations.stderr
@@ -1,11 +1,15 @@
-error[E0277]: the trait bound `T: Foo` is not satisfied
-  --> $DIR/associated-const-generic-obligations.rs:14:5
+error[E0326]: implemented const `FROM` has an incompatible type for trait
+  --> $DIR/associated-const-generic-obligations.rs:14:17
    |
+LL |     const FROM: Self::Out;
+   |                 --------- type in trait
+...
 LL |     const FROM: &'static str = "foo";
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |                 ^^^^^^^^^^^^ expected associated type, found reference
    |
-   = help: consider adding a `where T: Foo` bound
+   = note: expected type `<T as Foo>::Out`
+              found type `&'static str`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0326`.
diff --git a/src/test/ui/associated-const/associated-const-trait-bound.rs b/src/test/ui/associated-const/associated-const-trait-bound.rs
new file mode 100644
index 00000000000..0ce46d312af
--- /dev/null
+++ b/src/test/ui/associated-const/associated-const-trait-bound.rs
@@ -0,0 +1,21 @@
+// compile-pass
+
+trait ConstDefault {
+    const DEFAULT: Self;
+}
+
+trait Foo: Sized {}
+
+trait FooExt: Foo {
+    type T: ConstDefault;
+}
+
+trait Bar<F: FooExt> {
+    const T: F::T;
+}
+
+impl<F: FooExt> Bar<F> for () {
+    const T: F::T = <F::T as ConstDefault>::DEFAULT;
+}
+
+fn main() {}
diff --git a/src/test/ui/nll/trait-associated-constant.rs b/src/test/ui/nll/trait-associated-constant.rs
index 9d3e1a690f0..2ba65e10ea7 100644
--- a/src/test/ui/nll/trait-associated-constant.rs
+++ b/src/test/ui/nll/trait-associated-constant.rs
@@ -26,7 +26,6 @@ struct FailStruct2 { }
 
 impl<'a: 'b, 'b> Anything<'a, 'b> for FailStruct2 {
     const AC: Option<&'a str> = None;
-    //~^ ERROR: mismatched types
 }
 
 fn main() {}
diff --git a/src/test/ui/nll/trait-associated-constant.stderr b/src/test/ui/nll/trait-associated-constant.stderr
index 78ef513f3ee..786ca8e19e4 100644
--- a/src/test/ui/nll/trait-associated-constant.stderr
+++ b/src/test/ui/nll/trait-associated-constant.stderr
@@ -17,25 +17,6 @@ note: ...does not necessarily outlive the lifetime 'b as defined on the impl at
 LL | impl<'a: 'b, 'b, 'c> Anything<'a, 'b> for FailStruct1 {
    |              ^^
 
-error[E0308]: mismatched types
-  --> $DIR/trait-associated-constant.rs:28:5
-   |
-LL |     const AC: Option<&'a str> = None;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
-   |
-   = note: expected type `std::option::Option<&'b str>`
-              found type `std::option::Option<&'a str>`
-note: the lifetime 'a as defined on the impl at 27:6...
-  --> $DIR/trait-associated-constant.rs:27:6
-   |
-LL | impl<'a: 'b, 'b> Anything<'a, 'b> for FailStruct2 {
-   |      ^^
-note: ...does not necessarily outlive the lifetime 'b as defined on the impl at 27:14
-  --> $DIR/trait-associated-constant.rs:27:14
-   |
-LL | impl<'a: 'b, 'b> Anything<'a, 'b> for FailStruct2 {
-   |              ^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0308`.