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) {