Rollup merge of #65593 - RalfJung:non-const-fn, r=oli-obk

add test for calling non-const fn

The good news is that there is an error. But I expected to see [this error](https://github.com/rust-lang/rust/blob/9578272d681c8691ca2ff3f5c4230b491bc1c694/src/librustc_mir/const_eval.rs#L346) surface. @oli-obk any idea why that message is not shown anywhere?

r? @oli-obk

src/test/ui/consts/miri_unleashed/non_const_fn.rs

@@ -0,0 +1,13 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+#![warn(const_err)]
+
+// A test demonstrating that we prevent calling non-const fn during CTFE.
+
+fn foo() {}
+
+const C: () = foo(); //~ WARN: skipping const checks
+//~^ WARN any use of this value will cause an error
+
+fn main() {
+    println!("{:?}", C); //~ ERROR: evaluation of constant expression failed
+}

src/test/ui/consts/miri_unleashed/non_const_fn.stderr

@@ -0,0 +1,29 @@
+warning: skipping const checks
+  --> $DIR/non_const_fn.rs:8:15
+   |
+LL | const C: () = foo();
+   |               ^^^^^
+
+warning: any use of this value will cause an error
+  --> $DIR/non_const_fn.rs:8:15
+   |
+LL | const C: () = foo();
+   | --------------^^^^^-
+   |               |
+   |               calling non-const function `foo`
+   |
+note: lint level defined here
+  --> $DIR/non_const_fn.rs:2:9
+   |
+LL | #![warn(const_err)]
+   |         ^^^^^^^^^
+
+error[E0080]: evaluation of constant expression failed
+  --> $DIR/non_const_fn.rs:12:22
+   |
+LL |     println!("{:?}", C);
+   |                      ^ referenced constant has errors
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.