compiler: allow transmute of ZST arrays with generics

Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus
considering `[T; 0]` to have a compile-time fixed-size of 0.

The existing evaluator already deals with generic arrays under the
feature-guard `transmute_const_generics`. However, it merely allows
comparing fixed-size types with fixed-size types, and generic types with
generic types. For generic types, it merely compares whether their
arguments match (ordering them first). Even if their exact sizes are not
known at compile time, it can ensure that they will eventually be the
same.

This patch extends this by shortcutting the size-evaluation of zero
sized arrays and thus allowing size comparisons of `()` with `[T; 0]`,
where one contains generics and the other does not.

This code is guarded by `transmute_const_generics` (#109929), even
though it is unclear whether it should be. However, this assumes that a
separate stabilization PR is required to move this out of the feature
guard.

Initially reported in #98104.

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 +1,14 @@
+// run-pass
+// Transmuting to/from ZSTs that contain generics.
+
+#![feature(transmute_generic_consts)]
+
+unsafe fn cast_zst<T>(from: ()) -> [T; 0] {
+    ::std::mem::transmute::<(), [T; 0]>(from)
+}
+
+pub fn main() {
+    unsafe {
+        let _: [u32; 0] = cast_zst(());
+    };
+}