]> git.proxmox.com Git - rustc.git/blame - src/tools/clippy/clippy_lints/src/default_union_representation.rs
New upstream version 1.70.0+dfsg2
[rustc.git] / src / tools / clippy / clippy_lints / src / default_union_representation.rs
CommitLineData
5099ac24
FG
1use clippy_utils::diagnostics::span_lint_and_help;
2use rustc_hir::{self as hir, HirId, Item, ItemKind};
2b03887a 3use rustc_hir_analysis::hir_ty_to_ty;
5099ac24
FG
4use rustc_lint::{LateContext, LateLintPass};
5use rustc_middle::ty::layout::LayoutOf;
6use rustc_session::{declare_lint_pass, declare_tool_lint};
7use rustc_span::sym;
5099ac24
FG
8
9declare_clippy_lint! {
10 /// ### What it does
11 /// Displays a warning when a union is declared with the default representation (without a `#[repr(C)]` attribute).
12 ///
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.
18 ///
19 /// ### Example
20 /// ```rust
21 /// union Foo {
22 /// a: i32,
23 /// b: u32,
24 /// }
25 ///
26 /// fn main() {
27 /// let _x: u32 = unsafe {
923072b8 28 /// Foo { a: 0_i32 }.b // Undefined behavior: `b` is allowed to be padding
5099ac24
FG
29 /// };
30 /// }
31 /// ```
32 /// Use instead:
33 /// ```rust
34 /// #[repr(C)]
35 /// union Foo {
36 /// a: i32,
37 /// b: u32,
38 /// }
39 ///
40 /// fn main() {
41 /// let _x: u32 = unsafe {
923072b8 42 /// Foo { a: 0_i32 }.b // Now defined behavior, this is just an i32 -> u32 transmute
5099ac24
FG
43 /// };
44 /// }
45 /// ```
46 #[clippy::version = "1.60.0"]
47 pub DEFAULT_UNION_REPRESENTATION,
48 restriction,
49 "unions without a `#[repr(C)]` attribute"
50}
51declare_lint_pass!(DefaultUnionRepresentation => [DEFAULT_UNION_REPRESENTATION]);
52
53impl<'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()) {
56 span_lint_and_help(
57 cx,
58 DEFAULT_UNION_REPRESENTATION,
59 item.span,
60 "this union has the default representation",
61 None,
62 &format!(
63 "consider annotating `{}` with `#[repr(C)]` to explicitly specify memory layout",
2b03887a 64 cx.tcx.def_path_str(item.owner_id.to_def_id())
5099ac24
FG
65 ),
66 );
67 }
68 }
69}
70
71/// Returns true if the given item is a union with at least two non-ZST fields.
72fn 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
75 } else {
76 false
77 }
78}
79
80fn is_zst(cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>) -> bool {
81 if hir_ty.span.from_expansion() {
82 return false;
83 }
84 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
85 if let Ok(layout) = cx.layout_of(ty) {
86 layout.is_zst()
87 } else {
88 false
89 }
90}
91
92fn 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() {
96 for item in items {
97 if item.is_word() && matches!(item.name_or_empty(), sym::C) {
98 return true;
99 }
100 }
101 }
102 }
103 false
104 })
105}