Add specific error for unstable const fn features

Author: varkor

src/librustc_mir/diagnostics.rs

@@ -2370,6 +2370,37 @@ let value = (&foo(), &foo());
 ```
 "##,
 
+E0723: r##"
+An feature unstable in `const` contexts was used.
+
+Erroneous code example:
+
+```compile_fail,E0723
+trait T {}
+
+impl T for () {}
+
+const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
+    ()
+}
+```
+
+To enable this feature on a nightly version of rustc, add the `const_fn`
+feature flag:
+
+```compile_fail,E0723
+#![feature(const_fn)]
+
+trait T {}
+
+impl T for () {}
+
+const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
+    ()
+}
+```
+"##,
+
 }
 
 register_diagnostics! {

src/librustc_mir/transform/qualify_consts.rs

@@ -1179,7 +1179,17 @@ impl MirPass for QualifyAndPromoteConstants {
                         // enforce `min_const_fn` for stable const fns
                         use super::qualify_min_const_fn::is_min_const_fn;
                         if let Err((span, err)) = is_min_const_fn(tcx, def_id, mir) {
-                            tcx.sess.span_err(span, &err);
+                            let mut diag = struct_span_err!(
+                                tcx.sess,
+                                span,
+                                E0723,
+                                "{} (see issue #57563)",
+                                err,
+                            );
+                            diag.help(
+                                "add #![feature(const_fn)] to the crate attributes to enable",
+                            );
+                            diag.emit();
                         } else {
                             // this should not produce any errors, but better safe than sorry
                             // FIXME(#53819)