Move /src/test to /tests

tests/ui/impl-header-lifetime-elision/trait-underscore.rs

@@ -0,0 +1,35 @@
+// Test that `impl MyTrait<'_> for &i32` is equivalent to `impl<'a,
+// 'b> MyTrait<'a> for &'b i32`.
+//
+// run-pass
+
+#![allow(warnings)]
+
+trait MyTrait<'a> { }
+
+// This is equivalent to `MyTrait<'a> for &'b i32`, which is proven by
+// the code below.
+impl MyTrait<'_> for &i32 {
+}
+
+// When called, T will be `&'x i32` for some `'x`, so since we can
+// prove that `&'x i32: for<'a> MyTrait<'a>, then we know that the
+// lifetime parameter above is disconnected.
+fn impls_my_trait<T: for<'a> MyTrait<'a>>() { }
+
+fn impls_my_trait_val<T: for<'a> MyTrait<'a>>(_: T) {
+    impls_my_trait::<T>();
+}
+
+fn random_where_clause()
+where for<'a, 'b> &'a i32: MyTrait<'b> { }
+
+fn main() {
+    let x = 22;
+    let f = &x;
+    impls_my_trait_val(f);
+
+    impls_my_trait::<&'static i32>();
+
+    random_where_clause();
+}