borrow_interior_mutable_const ICE into FN

Convert the ICE reported in #12979 into a false negative.
We prefer a false negative to a ICE (because the ICE could
still affect the user even when not activating the lint).

Author: blyxyas

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 (&ty::ValTree::Leaf(variant_index), fields) = val.split_first().unwrap() 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)),

tests/ui/borrow_interior_mutable_const/others.rs

@@ -47,6 +47,17 @@ impl<T> std::ops::Deref for StaticRef<T> {
     }
 }
 
+// ICE regression test
+mod issue12979 {
+    use std::cell::UnsafeCell;
+
+    const ATOMIC_TUPLE: (Vec<UnsafeCell<u8>>, ()) = (Vec::new(), ());
+
+    fn main() {
+        let _x = &ATOMIC_TUPLE.0;
+    }
+}
+
 // use a tuple to make sure referencing a field behind a pointer isn't linted.
 const CELL_REF: StaticRef<(UnsafeCell<u32>,)> = unsafe { StaticRef::new(std::ptr::null()) };
 

tests/ui/borrow_interior_mutable_const/others.stderr

@@ -1,5 +1,5 @@
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:54:5
+  --> tests/ui/borrow_interior_mutable_const/others.rs:65:5
    |
 LL |     ATOMIC.store(1, Ordering::SeqCst);
    |     ^^^^^^
@@ -12,103 +12,103 @@ LL | #![deny(clippy::borrow_interior_mutable_const)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:55:16
+  --> tests/ui/borrow_interior_mutable_const/others.rs:66:16
    |
 LL |     assert_eq!(ATOMIC.load(Ordering::SeqCst), 5);
    |                ^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:58:22
+  --> tests/ui/borrow_interior_mutable_const/others.rs:69:22
    |
 LL |     let _once_ref = &ONCE_INIT;
    |                      ^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:59:25
+  --> tests/ui/borrow_interior_mutable_const/others.rs:70:25
    |
 LL |     let _once_ref_2 = &&ONCE_INIT;
    |                         ^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:60:27
+  --> tests/ui/borrow_interior_mutable_const/others.rs:71:27
    |
 LL |     let _once_ref_4 = &&&&ONCE_INIT;
    |                           ^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:61:26
+  --> tests/ui/borrow_interior_mutable_const/others.rs:72:26
    |
 LL |     let _once_mut = &mut ONCE_INIT;
    |                          ^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:72:14
+  --> tests/ui/borrow_interior_mutable_const/others.rs:83:14
    |
 LL |     let _ = &ATOMIC_TUPLE;
    |              ^^^^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:73:14
+  --> tests/ui/borrow_interior_mutable_const/others.rs:84:14
    |
 LL |     let _ = &ATOMIC_TUPLE.0;
    |              ^^^^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:74:19
+  --> tests/ui/borrow_interior_mutable_const/others.rs:85:19
    |
 LL |     let _ = &(&&&&ATOMIC_TUPLE).0;
    |                   ^^^^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:75:14
+  --> tests/ui/borrow_interior_mutable_const/others.rs:86:14
    |
 LL |     let _ = &ATOMIC_TUPLE.0[0];
    |              ^^^^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:76:13
+  --> tests/ui/borrow_interior_mutable_const/others.rs:87:13
    |
 LL |     let _ = ATOMIC_TUPLE.0[0].load(Ordering::SeqCst);
    |             ^^^^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:81:13
+  --> tests/ui/borrow_interior_mutable_const/others.rs:92:13
    |
 LL |     let _ = ATOMIC_TUPLE.0[0];
    |             ^^^^^^^^^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:86:5
+  --> tests/ui/borrow_interior_mutable_const/others.rs:97:5
    |
 LL |     CELL.set(2);
    |     ^^^^
    |
    = help: assign this const to a local or static variable, and use the variable here
 
 error: a `const` item with interior mutability should not be borrowed
-  --> tests/ui/borrow_interior_mutable_const/others.rs:87:16
+  --> tests/ui/borrow_interior_mutable_const/others.rs:98:16
    |
 LL |     assert_eq!(CELL.get(), 6);
    |                ^^^^