rustdoc: add test case for "variadic tuple" search notation

src/test/rustdoc/primitive-tuple-variadic.rs

@@ -0,0 +1,18 @@
+// compile-flags: --crate-type lib --edition 2018
+
+#![crate_name = "foo"]
+#![feature(rustdoc_internals)]
+
+pub trait Foo {}
+
+// @has foo/trait.Foo.html
+// @has - '//section[@id="impl-Foo-for-(T%2C)"]/h3' 'impl<T> Foo for (T₁, T₂, …, Tₙ)'
+#[doc(tuple_variadic)]
+impl<T> Foo for (T,) {}
+
+pub trait Bar {}
+
+// @has foo/trait.Bar.html
+// @has - '//section[@id="impl-Bar-for-(U%2C)"]/h3' 'impl<U: Foo> Bar for (U₁, U₂, …, Uₙ)'
+#[doc(tuple_variadic)]
+impl<U: Foo> Bar for (U,) {}