Add regression test for #81289

Author: LeSeulArtichaut

src/test/rustdoc/mut-params.rs

@@ -0,0 +1,18 @@
+// Rustdoc shouldn't display `mut` in function arguments, which are
+// implementation details. Regression test for #81289.
+
+#![crate_name = "foo"]
+
+pub struct Foo;
+
+// @count foo/struct.Foo.html '//*[@class="impl-items"]//*[@class="method"]' 2
+// @!has - '//*[@class="impl-items"]//*[@class="method"]' 'mut'
+impl Foo {
+    pub fn foo(mut self) {}
+
+    pub fn bar(mut bar: ()) {}
+}
+
+// @count foo/fn.baz.html '//*[@class="rust fn"]' 1
+// @!has - '//*[@class="rust fn"]' 'mut'
+pub fn baz(mut foo: Foo) {}