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

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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).
This commit is contained in:
mlugg 2025-03-31 09:11:18 +01:00 • committed by Andrew Kelley
parent 0bdc0bb534
commit d53cc5e5b2
2 changed files with 25 additions and 2 deletions
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7
src/Sema.zig
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@ -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;

20
test/behavior/ptrcast.zig
View file

@ -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) {}, &.{ .{}, .{} });
}