`borrow_interior_mutable_const` ICE into FN (#13877)

This PR just makes sure that we exit the function before getting an ICE
and adds a regression test. Related to #12979. We would need to keep
researching the issue, but as the ICEing code isn't that complicated,
getting a hotfix into nightly is urgent.

changelog:[`borrow_interior_mutable_const`] Fix ICE #12979

1 files changed, 6 insertions, 5 deletions

diff --git a/clippy_lints/src/non_copy_const.rs b/clippy_lints/src/non_copy_const.rs
index ebd301d5156..8409d179b0f 100644
--- a/clippy_lints/src/non_copy_const.rs
+++ b/clippy_lints/src/non_copy_const.rs
@@ -189,6 +189,8 @@ impl<'tcx> NonCopyConst<'tcx> {
     }
 
     fn is_value_unfrozen_raw_inner(cx: &LateContext<'tcx>, val: ty::ValTree<'tcx>, ty: Ty<'tcx>) -> bool {
+        // No branch that we check (yet) should continue if val isn't a ValTree::Branch
+        let ty::ValTree::Branch(val) = val else { return false };
         match *ty.kind() {
             // the fact that we have to dig into every structs to search enums
             // leads us to the point checking `UnsafeCell` directly is the only option.
@@ -197,12 +199,13 @@ impl<'tcx> NonCopyConst<'tcx> {
             // contained value.
             ty::Adt(def, ..) if def.is_union() => false,
             ty::Array(ty, _) => val
-                .unwrap_branch()
                 .iter()
                 .any(|field| Self::is_value_unfrozen_raw_inner(cx, *field, ty)),
             ty::Adt(def, args) if def.is_enum() => {
-                let (&variant_index, fields) = val.unwrap_branch().split_first().unwrap();
-                let variant_index = VariantIdx::from_u32(variant_index.unwrap_leaf().to_u32());
+                let Some((&ty::ValTree::Leaf(variant_index), fields)) = val.split_first() else {
+                    return false;
+                };
+                let variant_index = VariantIdx::from_u32(variant_index.to_u32());
                 fields
                     .iter()
                     .copied()
@@ -215,12 +218,10 @@ impl<'tcx> NonCopyConst<'tcx> {
                     .any(|(field, ty)| Self::is_value_unfrozen_raw_inner(cx, field, ty))
             },
             ty::Adt(def, args) => val
-                .unwrap_branch()
                 .iter()
                 .zip(def.non_enum_variant().fields.iter().map(|field| field.ty(cx.tcx, args)))
                 .any(|(field, ty)| Self::is_value_unfrozen_raw_inner(cx, *field, ty)),
             ty::Tuple(tys) => val
-                .unwrap_branch()
                 .iter()
                 .zip(tys)
                 .any(|(field, ty)| Self::is_value_unfrozen_raw_inner(cx, *field, ty)),