Sema: allow @ptrCast slice of zero-bit type to slice of non-zero-bit type

This is actually completely well-defined. The resulting slice always has
0 elements. The only disallowed case is casting *to* a slice of a
zero-bit type, because in that case, you cna't figure out how many
destination elements to use (and there's *no* valid destination length
if the source slice corresponds to more than 0 bits).

Author: mlugg

src/Sema.zig

@@ -22837,11 +22837,14 @@ fn ptrCastFull(
         if (src_slice_like_elem.comptimeOnly(zcu) or dest_elem.comptimeOnly(zcu)) {
             return sema.fail(block, src, "cannot infer length of slice of '{}' from slice of '{}'", .{ dest_elem.fmt(pt), src_slice_like_elem.fmt(pt) });
         }
-        const src_elem_size = src_slice_like_elem.abiSize(zcu);
+        // It's okay for `src_slice_like_elem` to be 0-bit; the resulting slice will just always have 0 elements.
+        // However, `dest_elem` can't be 0-bit. If it were, then either the source slice has 0 bits and we don't
+        // know how what `result.len` should be, or the source has >0 bits and there is no valid `result.len`.
         const dest_elem_size = dest_elem.abiSize(zcu);
-        if (src_elem_size == 0 or dest_elem_size == 0) {
+        if (dest_elem_size == 0) {
             return sema.fail(block, src, "cannot infer length of slice of '{}' from slice of '{}'", .{ dest_elem.fmt(pt), src_slice_like_elem.fmt(pt) });
         }
+        const src_elem_size = src_slice_like_elem.abiSize(zcu);
         break :need_len_change src_elem_size != dest_elem_size;
     } else false;
 

test/behavior/ptrcast.zig

@@ -507,3 +507,23 @@ test "@ptrCast array pointer to slice with complex length decrease" {
     try S.doTheTest(@splat(0));
     try comptime S.doTheTest(@splat(0));
 }
+
+test "@ptrCast slice of zero-bit type to different slice" {
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    const S = struct {
+        fn doTheTest(comptime T: type, zero_bits: []const T) !void {
+            const out: []const u8 = @ptrCast(zero_bits);
+            try expect(out.len == 0);
+        }
+    };
+    try S.doTheTest(void, &.{ {}, {}, {} });
+    try S.doTheTest(u0, &.{ 0, 0, 0, 0 });
+    try S.doTheTest(packed struct(u0) {}, &.{ .{}, .{} });
+    try comptime S.doTheTest(void, &.{ {}, {}, {} });
+    try comptime S.doTheTest(u0, &.{ 0, 0, 0, 0 });
+    try comptime S.doTheTest(packed struct(u0) {}, &.{ .{}, .{} });
+}