| use clippy_utils::diagnostics::span_lint_and_help; |
| use rustc_hir::{HirId, Item, ItemKind}; |
| use rustc_lint::{LateContext, LateLintPass}; |
| use rustc_middle::ty::layout::LayoutOf; |
| use rustc_middle::ty::{self, FieldDef, GenericArg, List}; |
| use rustc_session::declare_lint_pass; |
| use rustc_span::sym; |
| |
| declare_clippy_lint! { |
| /// ### What it does |
| /// Displays a warning when a union is declared with the default representation (without a `#[repr(C)]` attribute). |
| /// |
| /// ### Why is this bad? |
| /// Unions in Rust have unspecified layout by default, despite many people thinking that they |
| /// lay out each field at the start of the union (like C does). That is, there are no guarantees |
| /// about the offset of the fields for unions with multiple non-ZST fields without an explicitly |
| /// specified layout. These cases may lead to undefined behavior in unsafe blocks. |
| /// |
| /// ### Example |
| /// ```no_run |
| /// union Foo { |
| /// a: i32, |
| /// b: u32, |
| /// } |
| /// |
| /// fn main() { |
| /// let _x: u32 = unsafe { |
| /// Foo { a: 0_i32 }.b // Undefined behavior: `b` is allowed to be padding |
| /// }; |
| /// } |
| /// ``` |
| /// Use instead: |
| /// ```no_run |
| /// #[repr(C)] |
| /// union Foo { |
| /// a: i32, |
| /// b: u32, |
| /// } |
| /// |
| /// fn main() { |
| /// let _x: u32 = unsafe { |
| /// Foo { a: 0_i32 }.b // Now defined behavior, this is just an i32 -> u32 transmute |
| /// }; |
| /// } |
| /// ``` |
| #[clippy::version = "1.60.0"] |
| pub DEFAULT_UNION_REPRESENTATION, |
| restriction, |
| "unions without a `#[repr(C)]` attribute" |
| } |
| declare_lint_pass!(DefaultUnionRepresentation => [DEFAULT_UNION_REPRESENTATION]); |
| |
| impl<'tcx> LateLintPass<'tcx> for DefaultUnionRepresentation { |
| fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) { |
| if !item.span.from_expansion() |
| && is_union_with_two_non_zst_fields(cx, item) |
| && !has_c_repr_attr(cx, item.hir_id()) |
| { |
| span_lint_and_help( |
| cx, |
| DEFAULT_UNION_REPRESENTATION, |
| item.span, |
| "this union has the default representation", |
| None, |
| format!( |
| "consider annotating `{}` with `#[repr(C)]` to explicitly specify memory layout", |
| cx.tcx.def_path_str(item.owner_id) |
| ), |
| ); |
| } |
| } |
| } |
| |
| /// Returns true if the given item is a union with at least two non-ZST fields. |
| /// (ZST fields having an arbitrary offset is completely inconsequential, and |
| /// if there is only one field left after ignoring ZST fields then the offset |
| /// of that field does not matter either.) |
| fn is_union_with_two_non_zst_fields<'tcx>(cx: &LateContext<'tcx>, item: &Item<'tcx>) -> bool { |
| if let ItemKind::Union(..) = &item.kind |
| && let ty::Adt(adt_def, args) = cx.tcx.type_of(item.owner_id).instantiate_identity().kind() |
| { |
| adt_def.all_fields().filter(|f| !is_zst(cx, f, args)).count() >= 2 |
| } else { |
| false |
| } |
| } |
| |
| fn is_zst<'tcx>(cx: &LateContext<'tcx>, field: &FieldDef, args: &'tcx List<GenericArg<'tcx>>) -> bool { |
| let ty = field.ty(cx.tcx, args); |
| if let Ok(layout) = cx.layout_of(ty) { |
| layout.is_zst() |
| } else { |
| false |
| } |
| } |
| |
| fn has_c_repr_attr(cx: &LateContext<'_>, hir_id: HirId) -> bool { |
| cx.tcx.hir().attrs(hir_id).iter().any(|attr| { |
| if attr.has_name(sym::repr) { |
| if let Some(items) = attr.meta_item_list() { |
| for item in items { |
| if item.is_word() && matches!(item.name_or_empty(), sym::C) { |
| return true; |
| } |
| } |
| } |
| } |
| false |
| }) |
| } |