clean up issue-21950 (dyn trait cast without assoc type at the cast)

tests/ui/issues/issue-21950.rs

@@ -1,12 +0,0 @@
-trait Add<Rhs=Self> {
-    type Output;
-}
-
-impl Add for i32 {
-    type Output = i32;
-}
-
-fn main() {
-    let x = &10 as &dyn Add;
-    //~^ ERROR E0191
-}

tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.rs

@@ -0,0 +1,19 @@
+//! Tests that compiler yields error E0191 when value with existing trait implementation
+//! is cast as same `dyn` trait without specifying associated type at the cast.
+//!
+//! # Context
+//! Original issue: https://github.com/rust-lang/rust/issues/21950
+
+trait Add<Rhs=Self> {
+    type Output;
+}
+
+impl Add for i32 {
+    type Output = i32;
+}
+
+fn main() {
+    let x = &10 as &dyn Add<i32, Output = i32>; //OK
+    let x = &10 as &dyn Add;
+    //~^ ERROR E0191
+}

tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.stderr

@@ -1,5 +1,5 @@
 error[E0191]: the value of the associated type `Output` in `Add` must be specified
-  --> $DIR/issue-21950.rs:10:25
+  --> $DIR/cast-as-dyn-trait-wo-assoc-type-issue-21950.rs:17:25
    |
 LL |     type Output;
    |     ----------- `Output` defined here