Add the full issue reference to equality constraints in where clauses

src/librustc_passes/ast_validation.rs

@@ -737,8 +737,14 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         for predicate in &generics.where_clause.predicates {
             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
                 self.err_handler()
-                    .span_err(predicate.span, "equality constraints are not yet \
-                                               supported in where clauses (see #20041)");
+                    .struct_span_err(
+                        predicate.span,
+                        "equality constraints are not yet supported in `where` clauses",
+                    )
+                    .note(
+                        "for more information, see https://github.com/rust-lang/rust/issues/20041",
+                    )
+                    .emit();
             }
         }
 

src/test/ui/where-clauses/where-equality-constraints.rs

@@ -1,6 +1,6 @@
 fn f() where u8 = u16 {}
-//~^ ERROR equality constraints are not yet supported in where clauses
+//~^ ERROR equality constraints are not yet supported in `where` clauses
 fn g() where for<'a> &'static (u8,) == u16, {}
-//~^ ERROR equality constraints are not yet supported in where clauses
+//~^ ERROR equality constraints are not yet supported in `where` clauses
 
 fn main() {}

src/test/ui/where-clauses/where-equality-constraints.stderr

@@ -1,14 +1,18 @@
-error: equality constraints are not yet supported in where clauses (see #20041)
+error: equality constraints are not yet supported in `where` clauses
   --> $DIR/where-equality-constraints.rs:1:14
    |
 LL | fn f() where u8 = u16 {}
    |              ^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/20041
 
-error: equality constraints are not yet supported in where clauses (see #20041)
+error: equality constraints are not yet supported in `where` clauses
   --> $DIR/where-equality-constraints.rs:3:14
    |
 LL | fn g() where for<'a> &'static (u8,) == u16, {}
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/20041
 
 error: aborting due to 2 previous errors