Fix rust-lang#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 rust-lang#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

Author: kyren

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;
 

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() {}

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`.

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() {}

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() {}

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`.