compiler: allow transmute of ZST arrays with generics

compiler/rustc_middle/src/ty/layout.rs

@@ -345,11 +345,16 @@ impl<'tcx> SizeSkeleton<'tcx> {
             ty::Array(inner, len)
                 if len.ty() == tcx.types.usize && tcx.features().transmute_generic_consts =>
             {
+                let len_eval = len.try_eval_target_usize(tcx, param_env);
+                if len_eval == Some(0) {
+                    return Ok(SizeSkeleton::Known(Size::from_bytes(0)));
+                }
+
                 match SizeSkeleton::compute(inner, tcx, param_env)? {
                     // This may succeed because the multiplication of two types may overflow
                     // but a single size of a nested array will not.
                     SizeSkeleton::Known(s) => {
-                        if let Some(c) = len.try_eval_target_usize(tcx, param_env) {
+                        if let Some(c) = len_eval {
                             let size = s
                                 .bytes()
                                 .checked_mul(c)

tests/ui/transmute/transmute-zst-generics.rs

@@ -0,0 +1,34 @@
+//@ run-pass
+
+// Transmuting to/from ZSTs that contain generics.
+
+#![feature(transmute_generic_consts)]
+
+// Verify non-generic ZST -> generic ZST transmute
+unsafe fn cast_zst0<T>(from: ()) -> [T; 0] {
+    ::std::mem::transmute::<(), [T; 0]>(from)
+}
+
+// Verify generic ZST -> non-generic ZST transmute
+unsafe fn cast_zst1<T>(from: [T; 0]) -> () {
+    ::std::mem::transmute::<[T; 0], ()>(from)
+}
+
+// Verify transmute with generic compound types
+unsafe fn cast_zst2<T>(from: ()) -> [(T, T); 0] {
+    ::std::mem::transmute::<(), [(T, T); 0]>(from)
+}
+
+// Verify transmute with ZST propagation through arrays
+unsafe fn cast_zst3<T>(from: ()) -> [[T; 0]; 8] {
+    ::std::mem::transmute::<(), [[T; 0]; 8]>(from)
+}
+
+pub fn main() {
+    unsafe {
+        let _: [u32; 0] = cast_zst0(());
+        let _ = cast_zst1::<u32>([]);
+        let _: [(u32, u32); 0] = cast_zst2(());
+        let _: [[u32; 0]; 8] = cast_zst3(());
+    };
+}