1 use clippy_utils
::diagnostics
::span_lint_and_help
;
2 use rustc_hir
::{self as hir, HirId, Item, ItemKind}
;
3 use rustc_lint
::{LateContext, LateLintPass}
;
4 use rustc_middle
::ty
::layout
::LayoutOf
;
5 use rustc_session
::{declare_lint_pass, declare_tool_lint}
;
7 use rustc_typeck
::hir_ty_to_ty
;
11 /// Displays a warning when a union is declared with the default representation (without a `#[repr(C)]` attribute).
13 /// ### Why is this bad?
14 /// Unions in Rust have unspecified layout by default, despite many people thinking that they
15 /// lay out each field at the start of the union (like C does). That is, there are no guarantees
16 /// about the offset of the fields for unions with multiple non-ZST fields without an explicitly
17 /// specified layout. These cases may lead to undefined behavior in unsafe blocks.
27 /// let _x: u32 = unsafe {
28 /// Foo { a: 0_i32 }.b // Undefined behavior: `b` is allowed to be padding
41 /// let _x: u32 = unsafe {
42 /// Foo { a: 0_i32 }.b // Now defined behavior, this is just an i32 -> u32 transmute
46 #[clippy::version = "1.60.0"]
47 pub DEFAULT_UNION_REPRESENTATION
,
49 "unions without a `#[repr(C)]` attribute"
51 declare_lint_pass
!(DefaultUnionRepresentation
=> [DEFAULT_UNION_REPRESENTATION
]);
53 impl<'tcx
> LateLintPass
<'tcx
> for DefaultUnionRepresentation
{
54 fn check_item(&mut self, cx
: &LateContext
<'tcx
>, item
: &'tcx Item
<'tcx
>) {
55 if is_union_with_two_non_zst_fields(cx
, item
) && !has_c_repr_attr(cx
, item
.hir_id()) {
58 DEFAULT_UNION_REPRESENTATION
,
60 "this union has the default representation",
63 "consider annotating `{}` with `#[repr(C)]` to explicitly specify memory layout",
64 cx
.tcx
.def_path_str(item
.def_id
.to_def_id())
71 /// Returns true if the given item is a union with at least two non-ZST fields.
72 fn is_union_with_two_non_zst_fields(cx
: &LateContext
<'_
>, item
: &Item
<'_
>) -> bool
{
73 if let ItemKind
::Union(data
, _
) = &item
.kind
{
74 data
.fields().iter().filter(|f
| !is_zst(cx
, f
.ty
)).count() >= 2
80 fn is_zst(cx
: &LateContext
<'_
>, hir_ty
: &hir
::Ty
<'_
>) -> bool
{
81 if hir_ty
.span
.from_expansion() {
84 let ty
= hir_ty_to_ty(cx
.tcx
, hir_ty
);
85 if let Ok(layout
) = cx
.layout_of(ty
) {
92 fn has_c_repr_attr(cx
: &LateContext
<'_
>, hir_id
: HirId
) -> bool
{
93 cx
.tcx
.hir().attrs(hir_id
).iter().any(|attr
| {
94 if attr
.has_name(sym
::repr
) {
95 if let Some(items
) = attr
.meta_item_list() {
97 if item
.is_word() && matches
!(item
.name_or_empty(), sym
::C
) {