Use delayed bug instead of panic in mir_const_qualif for invalid const context

The issue was that `mir_const_qualif` would panic when `const_kind`
returned `None`. This can happen when there are prior errors, such as
attempting to declare a const fn in a trait (which is not allowed).

The fix replaces the `span_bug!` with a `span_delayed_bug` and returns
a default value, allowing compilation to continue and report the actual
error instead of ICE-ing.

Author: yimiliya

compiler/rustc_mir_transform/src/lib.rs

@@ -363,10 +363,15 @@ fn mir_const_qualif(tcx: TyCtxt<'_>, def: LocalDefId) -> ConstQualifs {
     // No need to const-check a non-const `fn`.
     match ccx.const_kind {
         Some(ConstContext::Const { .. } | ConstContext::Static(_) | ConstContext::ConstFn) => {}
-        None => span_bug!(
-            tcx.def_span(def),
-            "`mir_const_qualif` should only be called on const fns and const items"
-        ),
+        None => {
+            // This can happen when there are prior errors (e.g., const fn in trait).
+            // Use a delayed bug instead of panicking to allow compilation to continue.
+            tcx.dcx().span_delayed_bug(
+                tcx.def_span(def),
+                "`mir_const_qualif` should only be called on const fns and const items",
+            );
+            return Default::default();
+        }
     }
 
     if body.return_ty().references_error() {

tests/ui/const-traits/const-closure-in-trait-ice-153891.rs

@@ -0,0 +1,10 @@
+// Test for issue #153891: ICE when using const closure in trait const fn
+#![feature(const_closures)]
+
+trait Tr {
+    const fn test() {
+        (const || {})()
+    }
+}
+
+fn main() {}