Add GAT related tests

tests/source/issue_4257.rs

@@ -0,0 +1,13 @@
+#![feature(generic_associated_types)]
+#![allow(incomplete_features)]
+
+trait Trait<T> {
+    type Type<'a> where T: 'a;
+    fn foo(x: &T) -> Self::Type<'_>;
+}
+impl<T> Trait<T> for () {
+    type Type<'a> where T: 'a = &'a T;
+    fn foo(x: &T) -> Self::Type<'_> {
+        x
+    }
+}

tests/source/issue_4322.rs

@@ -0,0 +1,3 @@
+trait Bar {
+  type X<'a> where Self: 'a;
+}

tests/source/issue_4943.rs

@@ -0,0 +1,9 @@
+#![feature(generic_associated_types)]
+
+impl SomeStruct {
+    fn process<T>(v: T) -> <Self as GAT>::R<T>
+    where Self: GAT<R<T> = T>
+    {
+        SomeStruct::do_something(v)
+    }
+}

tests/source/issue_4954.rs

@@ -0,0 +1,5 @@
+trait Foo {
+  type Arg<'a>;
+}
+
+struct Bar<T>(T) where for<'a> T: Foo<Arg<'a> = ()>;

tests/target/issue_4257.rs

@@ -0,0 +1,18 @@
+#![feature(generic_associated_types)]
+#![allow(incomplete_features)]
+
+trait Trait<T> {
+    type Type<'a>
+    where
+        T: 'a;
+    fn foo(x: &T) -> Self::Type<'_>;
+}
+impl<T> Trait<T> for () {
+    type Type<'a>
+    where
+        T: 'a,
+    = &'a T;
+    fn foo(x: &T) -> Self::Type<'_> {
+        x
+    }
+}

tests/target/issue_4322.rs

@@ -0,0 +1,5 @@
+trait Bar {
+    type X<'a>
+    where
+        Self: 'a;
+}

tests/target/issue_4943.rs

@@ -1,3 +1,5 @@
+#![feature(generic_associated_types)]
+
 impl SomeStruct {
     fn process<T>(v: T) -> <Self as GAT>::R<T>
     where

tests/target/issue_4954.rs

@@ -0,0 +1,7 @@
+trait Foo {
+    type Arg<'a>;
+}
+
+struct Bar<T>(T)
+where
+    for<'a> T: Foo<Arg<'a> = ()>;