Add test for Implementors of exported traits

Author: tesuji

src/test/rustdoc/auxiliary/all_unstable.rs

@@ -0,0 +1,12 @@
+#![crate_name = "unstabled"]
+#![feature(staged_api)]
+#![unstable(feature = "extremely_unstable", issue = "none")]
+
+#[unstable(feature = "extremely_unstable_foo", issue = "none")]
+pub struct Foo {}
+
+#[unstable(feature = "extremely_unstable_foo", issue = "none")]
+pub trait Join {}
+
+#[unstable(feature = "extremely_unstable_foo", issue = "none")]
+impl Join for Foo {}

src/test/rustdoc/issue-75588.rs

@@ -0,0 +1,11 @@
+// aux-build:all_unstable.rs
+
+// Ensure unstably exported traits have their Implementors sections.
+
+#![crate_name = "foo"]
+#![feature(extremely_unstable_foo)]
+
+extern crate unstabled;
+
+// @has foo/trait.Join.html '//*[@id="impl-Join-for-Foo"]//code' 'impl Join for Foo'
+pub use unstabled::Join;