Put MIR checks for loops behind the feature flag

Author: ecstatic-morse

src/librustc_mir/transform/check_consts/ops.rs

@@ -170,6 +170,10 @@ impl NonConstOp for LiveDrop {
 #[derive(Debug)]
 pub struct Loop;
 impl NonConstOp for Loop {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_loop)
+    }
+
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
         // This should be caught by the HIR const-checker.
         item.tcx.sess.delay_span_bug(

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -390,8 +390,10 @@ fn check_terminator(
             cleanup: _,
         } => check_operand(tcx, cond, span, def_id, body),
 
-        TerminatorKind::FalseUnwind { .. } => {
+        TerminatorKind::FalseUnwind { .. } if !tcx.features().const_loop => {
             Err((span, "loops are not allowed in const fn".into()))
         },
+
+        TerminatorKind::FalseUnwind { .. } => Ok(()),
     }
 }