mirror of
https://github.com/rust-lang/rust-clippy
synced 2024-12-24 12:03:28 +00:00
106 lines
3.4 KiB
Rust
106 lines
3.4 KiB
Rust
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use clippy_utils::diagnostics::span_lint_and_help;
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use rustc_hir::{self as hir, HirId, Item, ItemKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::ty::layout::LayoutOf;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::sym;
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use rustc_typeck::hir_ty_to_ty;
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declare_clippy_lint! {
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/// ### What it does
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/// Displays a warning when a union is declared with the default representation (without a `#[repr(C)]` attribute).
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///
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/// ### Why is this bad?
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/// Unions in Rust have unspecified layout by default, despite many people thinking that they
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/// lay out each field at the start of the union (like C does). That is, there are no guarantees
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/// about the offset of the fields for unions with multiple non-ZST fields without an explicitly
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/// specified layout. These cases may lead to undefined behavior in unsafe blocks.
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///
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/// ### Example
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/// ```rust
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/// union Foo {
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/// a: i32,
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/// b: u32,
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/// }
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///
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/// fn main() {
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/// let _x: u32 = unsafe {
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/// Foo { a: 0_i32 }.b // Undefined behaviour: `b` is allowed to be padding
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/// };
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/// }
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/// ```
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/// Use instead:
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/// ```rust
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/// #[repr(C)]
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/// union Foo {
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/// a: i32,
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/// b: u32,
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/// }
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///
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/// fn main() {
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/// let _x: u32 = unsafe {
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/// Foo { a: 0_i32 }.b // Now defined behaviour, this is just an i32 -> u32 transmute
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/// };
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/// }
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/// ```
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#[clippy::version = "1.60.0"]
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pub DEFAULT_UNION_REPRESENTATION,
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restriction,
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"unions without a `#[repr(C)]` attribute"
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}
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declare_lint_pass!(DefaultUnionRepresentation => [DEFAULT_UNION_REPRESENTATION]);
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impl<'tcx> LateLintPass<'tcx> for DefaultUnionRepresentation {
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fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
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if is_union_with_two_non_zst_fields(cx, item) && !has_c_repr_attr(cx, item.hir_id()) {
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span_lint_and_help(
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cx,
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DEFAULT_UNION_REPRESENTATION,
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item.span,
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"this union has the default representation",
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None,
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&format!(
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"consider annotating `{}` with `#[repr(C)]` to explicitly specify memory layout",
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cx.tcx.def_path_str(item.def_id.to_def_id())
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),
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);
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}
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}
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}
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/// Returns true if the given item is a union with at least two non-ZST fields.
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fn is_union_with_two_non_zst_fields(cx: &LateContext<'_>, item: &Item<'_>) -> bool {
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if let ItemKind::Union(data, _) = &item.kind {
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data.fields().iter().filter(|f| !is_zst(cx, f.ty)).count() >= 2
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} else {
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false
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}
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}
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fn is_zst(cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>) -> bool {
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if hir_ty.span.from_expansion() {
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return false;
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}
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let ty = hir_ty_to_ty(cx.tcx, hir_ty);
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if let Ok(layout) = cx.layout_of(ty) {
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layout.is_zst()
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} else {
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false
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}
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}
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fn has_c_repr_attr(cx: &LateContext<'_>, hir_id: HirId) -> bool {
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cx.tcx.hir().attrs(hir_id).iter().any(|attr| {
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if attr.has_name(sym::repr) {
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if let Some(items) = attr.meta_item_list() {
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for item in items {
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if item.is_word() && matches!(item.name_or_empty(), sym::C) {
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return true;
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}
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}
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}
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}
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false
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})
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}
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