Add tests for mutable borrows in const fns

src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs

@@ -0,0 +1,7 @@
+fn main() {
+    foo(&mut 5);
+}
+
+const fn foo(x: &mut i32) -> i32 { //~ ERROR mutable references in const fn are unstable
+    *x + 1
+}

src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr

@@ -0,0 +1,12 @@
+error[E0723]: mutable references in const fn are unstable
+  --> $DIR/feature-gate-const_fn_mut_refs.rs:5:14
+   |
+LL | const fn foo(x: &mut i32) -> i32 {
+   |              ^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/const_fn_mut_refs.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(const_fn_mut_refs)]
+
+struct Foo {
+    x: i32
+}
+
+const fn bar(foo: &mut Foo) -> i32 {
+    foo.x + 1
+}
+
+fn main() {
+    assert_eq!(bar(&mut Foo{x: 0}), 1);
+}