test(const_eval): add test cases for #114994

tests/ui/consts/const-eval/issue-114994-fail.rs

@@ -0,0 +1,10 @@
+// This checks that function pointer signatures that are referenced mutably
+// but contain a &mut T parameter still fail in a constant context: see issue #114994.
+//
+// check-fail
+
+const fn use_mut_const_fn(_f: &mut fn(&mut String)) { //~ ERROR mutable references are not allowed in constant functions
+    ()
+}
+
+fn main() {}

tests/ui/consts/const-eval/issue-114994-fail.stderr

@@ -0,0 +1,12 @@
+error[E0658]: mutable references are not allowed in constant functions
+  --> $DIR/issue-114994-fail.rs:6:27
+   |
+LL | const fn use_mut_const_fn(_f: &mut fn(&mut String)) {
+   |                           ^^
+   |
+   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/consts/const-eval/issue-114994.rs

@@ -0,0 +1,18 @@
+// This checks that function pointer signatures containing &mut T types
+// work in a constant context: see issue #114994.
+//
+// check-pass
+
+const fn use_const_fn(_f: fn(&mut String)) {
+    ()
+}
+
+const fn get_some_fn() -> fn(&mut String) {
+    String::clear
+}
+
+const fn some_const_fn() {
+    let _f: fn(&mut String) = String::clear;
+}
+
+fn main() {}