Make wording verbose

Author: JohnTitor

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -326,7 +326,12 @@ fn check_terminator(
                     abi::Abi::Rust if tcx.is_min_const_fn(def_id) => {},
                     abi::Abi::Rust => return Err((
                         span,
-                        "can only call other `const` within a `const`".into(),
+                        format!(
+                            "can only call other `const fn` within a `const fn`, \
+                             but `{:?}` is not stable as `const fn`",
+                            func,
+                        )
+                        .into(),
                     )),
                     abi => return Err((
                         span,