Add test for legacy-const-generic arguments

Author: GuillaumeGomez

src/test/rustdoc/legacy-const-generic.rs

@@ -0,0 +1,16 @@
+#![crate_name = "foo"]
+#![feature(rustc_attrs)]
+
+// @has 'foo/fn.foo.html'
+// @has - '//*[@class="rust fn"]' 'fn foo(x: usize, const Y: usize, z: usize) -> [usize; 3]'
+#[rustc_legacy_const_generics(1)]
+pub fn foo<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
+    [x, Y, z]
+}
+
+// @has 'foo/fn.bar.html'
+// @has - '//*[@class="rust fn"]' 'fn bar(x: usize, const Y: usize, const Z: usize) -> [usize; 3]'
+#[rustc_legacy_const_generics(1, 2)]
+pub fn bar<const Y: usize, const Z: usize>(x: usize) -> [usize; 3] {
+    [x, Y, z]
+}