const validation: properly ignore zero-sized UnsafeCell

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -1086,8 +1086,13 @@ fn visit_union(
     ) -> InterpResult<'tcx> {
         // Special check for CTFE validation, preventing `UnsafeCell` inside unions in immutable memory.
         if self.ctfe_mode.is_some_and(|c| !c.allow_immutable_unsafe_cell()) {
-            if !val.layout.is_zst() && !val.layout.ty.is_freeze(*self.ecx.tcx, self.ecx.typing_env)
-            {
+            // Unsized unions are currently not a thing, but let's keep this code consistent with
+            // the check in `visit_value`.
+            let zst = self
+                .ecx
+                .size_and_align_of(&val.meta(), &val.layout)?
+                .is_some_and(|(s, _a)| s.bytes() == 0);
+            if !zst && !val.layout.ty.is_freeze(*self.ecx.tcx, self.ecx.typing_env) {
                 if !self.in_mutable_memory(val) {
                     throw_validation_failure!(self.path, UnsafeCellInImmutable);
                 }
@@ -1131,7 +1136,13 @@ fn visit_value(&mut self, val: &PlaceTy<'tcx, M::Provenance>) -> InterpResult<'t
 
         // Special check preventing `UnsafeCell` in the inner part of constants
         if self.ctfe_mode.is_some_and(|c| !c.allow_immutable_unsafe_cell()) {
-            if !val.layout.is_zst()
+            // Exclude ZST values. We need to compute the dynamic size/align to properly
+            // handle slices and trait objects.
+            let zst = self
+                .ecx
+                .size_and_align_of(&val.meta(), &val.layout)?
+                .is_some_and(|(s, _a)| s.bytes() == 0);
+            if !zst
                 && let Some(def) = val.layout.ty.ty_adt_def()
                 && def.is_unsafe_cell()
             {

tests/ui/consts/unsafe_cell_in_const.rs

@@ -0,0 +1,15 @@
+//! Ensure we do not complain about zero-sized `UnsafeCell` in a const in any form.
+//! See <https://github.com/rust-lang/rust/issues/142948>.
+
+//@ check-pass
+use std::cell::UnsafeCell;
+
+const X1: &mut UnsafeCell<[i32; 0]> = UnsafeCell::from_mut(&mut []);
+
+const X2: &mut UnsafeCell<[i32]> = UnsafeCell::from_mut(&mut []);
+
+trait Trait {}
+impl Trait for [i32; 0] {}
+const X3: &mut UnsafeCell<dyn Trait> = UnsafeCell::from_mut(&mut []);
+
+fn main() {}