Rollup merge of rust-lang#146536 - omskscream:rust-lang#133895/clean-ui-tests, r=davidtwco

clean up several trait related UI tests

Part of rust-lang#133895
Cleaned up several `issue-xxxx` trait related tests from `/tests/ui/issues`, one commit per issue for review. Will squash them once approved.

Related issues:
rust-lang#19479
rust-lang#2284
rust-lang#18088
rust-lang#21950

Author: jdonszelmann

tests/ui/issues/issue-18088.rs

@@ -1,5 +1,11 @@
 //@ check-pass
 
+//! Tests that it's possible to define an associated type in a trait
+//! using an associated type from type parameter bound trait in a blanket implementation.
+//!
+//! # Context
+//! Original issue: https://github.com/rust-lang/rust/issues/19479
+
 trait Base {
     fn dummy(&self) { }
 }

tests/ui/issues/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/issues/issue-2284.rs

@@ -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

tests/ui/issues/issue-19479.rs renamed to tests/ui/traits/associated_type_bound/assoc-type-via-another-trait-issue-19479.rs

@@ -0,0 +1,21 @@
+//@ run-pass
+#![allow(dead_code)]
+
+//! Tests that user-defined trait is prioritized in compile time over
+//! the core::marker trait with the same name, allowing shadowing core traits.
+//!
+//! # Context
+//! Original issue: https://github.com/rust-lang/rust/issues/2284
+//! Original fix pull request: https://github.com/rust-lang/rust/pull/3792
+
+
+trait Send {
+    fn f(&self);
+}
+
+fn f<T:Send>(t: T) {
+    t.f();
+}
+
+pub fn main() {
+}

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

@@ -0,0 +1,13 @@
+//@ check-pass
+
+//! Tests that operators from supertrait are available directly on `self` for an inheritor trait.
+//!
+//! # Context
+//! Original issue: https://github.com/rust-lang/rust/issues/18088
+
+pub trait Indexable<T>: std::ops::Index<usize, Output = T> {
+    fn index2(&self, i: usize) -> &T {
+        &self[i]
+    }
+}
+fn main() {}