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Sema: disallow empty extern/packed unions

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These types don't really make much sense: you can't pack together bits
of a type which cannot exist, nor can you pass it over an ABI boundary.
This commit is contained in:
Matthew Lugg 2026-02-08 13:56:01 +00:00
parent 1dd1172ed8
commit c9ba213da7
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1 changed files with 24 additions and 4 deletions
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28
src/Sema/type_resolution.zig
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@ -347,6 +347,12 @@ pub fn resolveStructLayout(sema: *Sema, struct_ty: Type) CompileError!void {
}
};
switch (struct_obj.layout) {
.auto => {},
.@"extern" => assert(class != .no_possible_value), // field types are all extern, so are not NPV
.@"packed" => unreachable,
}
if (struct_obj.layout == .auto) {
const runtime_order = struct_obj.field_runtime_order.get(ip);
// This logic does not reorder fields; it only moves the omitted ones to the end so that logic
@ -451,7 +457,12 @@ fn resolvePackedStructLayout(
try sema.addDeclaredHereNote(msg, field_ty);
break :msg msg;
});
assert(!field_ty.comptimeOnly(zcu)); // packable types are not comptime-only
switch (field_ty.classify(zcu)) {
.one_possible_value, .runtime => {},
.no_possible_value => unreachable, // packable types are not NPV
.partially_comptime => unreachable, // packable types are not comptime-only
.fully_comptime => unreachable, // packable types are not comptime-only
}
field_bits += field_ty.bitSize(zcu);
}
@ -749,6 +760,13 @@ pub fn resolveUnionLayout(sema: *Sema, union_ty: Type) CompileError!void {
}
}
// Uninstantiable `extern union`s don't make sense; disallow them.
if (possible_tags == 0 and union_obj.layout != .auto) {
// Field types are all extern, so not NPV; thus zero possible tags means no tags at all.
assert(union_obj.field_types.len == 0);
return sema.fail(&block, union_ty.srcLoc(zcu), "extern union has no fields", .{});
}
// We only need a runtime tag if there are multiple possible active fields *and* the union is
// not going to be comptime-only. Even if there are still runtime bits in the payload, the tag
// does not require runtime bits in a comptime-only union, because it is impossible to get a
@ -874,6 +892,11 @@ fn resolvePackedUnionLayout(
const gpa = comp.gpa;
const ip = &zcu.intern_pool;
// Uninstantiable `packed union`s don't make sense; disallow them.
if (union_obj.field_types.len == 0) {
return sema.fail(block, union_ty.srcLoc(zcu), "packed union has no fields", .{});
}
// Resolve the layout of all fields, and check their types are allowed.
for (union_obj.field_types.get(ip), 0..) |field_ty_ip, field_index| {
const field_ty: Type = .fromInterned(field_ty_ip);
@ -937,9 +960,6 @@ fn resolvePackedUnionLayout(
});
}
break :ty backing_ty;
} else if (union_obj.field_types.len == 0) ty: {
// Special case: there is no first field to infer the type from. Treat the union as empty (zero-bit).
break :ty .u0;
} else ty: {
const field_types = union_obj.field_types.get(ip);
const first_field_type: Type = .fromInterned(field_types[0]);