make const_err a hard error

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -258,6 +258,9 @@ impl<'tcx> fmt::Display for FrameInfo<'tcx> {
             {
                 write!(f, "inside closure")?;
             } else {
+                // Note: this triggers a `good_path_bug` state, which means that if we ever get here
+                // we must emit a diagnostic. We should never display a `FrameInfo` unless we
+                // actually want to emit a warning or error to the user.
                 write!(f, "inside `{}`", self.instance)?;
             }
             if !self.span.is_dummy() {

compiler/rustc_const_eval/src/interpret/intern.rs

@@ -332,8 +332,6 @@ pub enum InternKind {
 ///
 /// This *cannot raise an interpreter error*.  Doing so is left to validation, which
 /// tracks where in the value we are and thus can show much better error messages.
-/// Any errors here would anyway be turned into `const_err` lints, whereas validation failures
-/// are hard errors.
 #[instrument(level = "debug", skip(ecx))]
 pub fn intern_const_alloc_recursive<
     'mir,