Rollup merge of #65389 - ecstatic-morse:zero-sized-array-no-drop, r=eddyb

Return `false` from `needs_drop` for all zero-sized arrays.

Resolves #65348.

This changes the result of the `needs_drop` query from `true` to `false` for types such as `[Box<i32>; 0]`. I believe this change to be sound because a zero-sized array can never actually hold a value. This is an elegant way of resolving #65348 and #64945, but obviously it has much broader implications.

Author: tmandry

src/librustc/ty/util.rs

@@ -1109,6 +1109,9 @@ fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>
 
         ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 
+        // Zero-length arrays never contain anything to drop.
+        ty::Array(_, len) if len.try_eval_usize(tcx, param_env) == Some(0) => false,
+
         // Structural recursion.
         ty::Array(ty, _) | ty::Slice(ty) => needs_drop(ty),
 

src/librustc_mir/dataflow/impls/indirect_mutation.rs

@@ -104,25 +104,16 @@ impl<'tcx> TransferFunction<'_, '_, 'tcx> {
         kind: mir::BorrowKind,
         borrowed_place: &mir::Place<'tcx>,
     ) -> bool {
-        let borrowed_ty = borrowed_place.ty(self.body, self.tcx).ty;
-
-        // Zero-sized types cannot be mutated, since there is nothing inside to mutate.
-        //
-        // FIXME: For now, we only exempt arrays of length zero. We need to carefully
-        // consider the effects before extending this to all ZSTs.
-        if let ty::Array(_, len) = borrowed_ty.kind {
-            if len.try_eval_usize(self.tcx, self.param_env) == Some(0) {
-                return false;
-            }
-        }
-
         match kind {
             mir::BorrowKind::Mut { .. } => true,
 
             | mir::BorrowKind::Shared
             | mir::BorrowKind::Shallow
             | mir::BorrowKind::Unique
-            => !borrowed_ty.is_freeze(self.tcx, self.param_env, DUMMY_SP),
+            => !borrowed_place
+                .ty(self.body, self.tcx)
+                .ty
+                .is_freeze(self.tcx, self.param_env, DUMMY_SP),
         }
     }
 }

src/test/ui/consts/issue-65348.rs

@@ -0,0 +1,23 @@
+// check-pass
+
+struct Generic<T>(T);
+
+impl<T> Generic<T> {
+    const ARRAY: [T; 0] = [];
+    const NEWTYPE_ARRAY: Generic<[T; 0]> = Generic([]);
+    const ARRAY_FIELD: Generic<(i32, [T; 0])> = Generic((0, []));
+}
+
+pub const fn array<T>() ->  &'static T {
+    &Generic::<T>::ARRAY[0]
+}
+
+pub const fn newtype_array<T>() ->  &'static T {
+    &Generic::<T>::NEWTYPE_ARRAY.0[0]
+}
+
+pub const fn array_field<T>() ->  &'static T {
+    &(Generic::<T>::ARRAY_FIELD.0).1[0]
+}
+
+fn main() {}