Add test for const deref methods display

src/test/rustdoc/deref-const-fn.rs

@@ -0,0 +1,38 @@
+// This test ensures that the const methods from Deref aren't shown as const.
+// For more information, see https://github.com/rust-lang/rust/issues/90855.
+
+#![crate_name = "foo"]
+
+#![feature(staged_api)]
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+// @has 'foo/struct.Bar.html'
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Bar;
+
+impl Bar {
+    // @has - '//*[@id="method.len"]' 'pub const fn len(&self) -> usize'
+    // @has - '//*[@id="method.len"]//span[@class="since"]' '1.0.0 (const: 1.0.0)'
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
+    pub const fn len(&self) -> usize { 0 }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Foo {
+    value: Bar,
+}
+
+// @has 'foo/struct.Foo.html'
+// @has - '//*[@id="method.len"]' 'pub fn len(&self) -> usize'
+// @!has - '//*[@id="method.len"]//span[@class="since"]' '1.0.0'
+// @!has - '//*[@id="method.len"]//span[@class="since"]' '(const: 1.0.0)'
+#[stable(feature = "rust1", since = "1.0.0")]
+impl std::ops::Deref for Foo {
+    type Target = Bar;
+
+    fn deref(&self) -> &Self::Target {
+        &self.value
+    }
+}