Rollup merge of #128438 - Bryanskiy:empty-array-dropck, r=lcnr

Add special-case for [T, 0] in dropck_outlives

implements/fixes #110288.

r? `@lcnr`

compiler/rustc_trait_selection/src/traits/query/dropck_outlives.rs

@@ -42,8 +42,15 @@ pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
         | ty::Foreign(..)
         | ty::Error(_) => true,
 
-        // `T is PAT`, `[T; N]`, and `[T]` have same properties as T.
-        ty::Pat(ty, _) | ty::Array(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, *ty),
+        // `T is PAT` and `[T]` have same properties as T.
+        ty::Pat(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, *ty),
+        ty::Array(ty, size) => {
+            // Empty array never has a dtor. See issue #110288.
+            match size.try_to_target_usize(tcx) {
+                Some(0) => true,
+                _ => trivial_dropck_outlives(tcx, *ty),
+            }
+        }
 
         // (T1..Tn) and closures have same properties as T1..Tn --
         // check if *all* of them are trivial.

tests/ui/dropck/dropck-empty-array.rs

@@ -0,0 +1,23 @@
+//@ run-pass
+
+#[allow(dead_code)]
+struct Struct<'s>(&'s str);
+
+impl<'s> Drop for Struct<'s> {
+    fn drop(&mut self) {}
+}
+
+fn to_array_zero<T>(_: T) -> [T; 0] {
+    []
+}
+
+pub fn array_zero_in_tuple() {
+    let mut x = ([], String::new());
+    {
+        let s = String::from("temporary");
+        let p = Struct(&s);
+        x.0 = to_array_zero(p);
+    }
+}
+
+fn main() {}