Replace the ICEing on const fn loops with an error

Author: oli-obk

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -365,10 +365,8 @@ fn check_terminator(
             cleanup: _,
         } => check_operand(tcx, mir, cond, span),
 
-        | TerminatorKind::FalseUnwind { .. } => span_bug!(
-            terminator.source_info.span,
-            "min_const_fn encountered `{:#?}`",
-            terminator
-        ),
+        TerminatorKind::FalseUnwind { .. } => {
+            Err((span, "loops are not allowed in const fn".into()))
+        },
     }
 }

src/test/ui/consts/min_const_fn/loop_ice.rs

@@ -0,0 +1,5 @@
+const fn foo() {
+    loop {} //~ ERROR loops are not allowed in const fn
+}
+
+fn main() {}

src/test/ui/consts/min_const_fn/loop_ice.stderr

@@ -0,0 +1,8 @@
+error: loops are not allowed in const fn
+  --> $DIR/loop_ice.rs:2:5
+   |
+LL |     loop {} //~ ERROR loops are not allowed in const fn
+   |     ^^^^^^^
+
+error: aborting due to previous error
+