4 use clippy_utils::diagnostics::span_lint_and_sugg;
5 use clippy_utils::is_self_ty;
6 use clippy_utils::source::snippet;
7 use clippy_utils::ty::is_copy;
8 use if_chain::if_chain;
10 use rustc_errors::Applicability;
12 use rustc_hir::intravisit::FnKind;
13 use rustc_hir::{BindingAnnotation, Body, FnDecl, HirId, Impl, ItemKind, MutTy, Mutability, Node, PatKind};
14 use rustc_lint::{LateContext, LateLintPass};
16 use rustc_session::{declare_tool_lint, impl_lint_pass};
17 use rustc_span::{sym, Span};
18 use rustc_target::abi::LayoutOf;
19 use rustc_target::spec::abi::Abi;
20 use rustc_target::spec::Target;
22 declare_clippy_lint! {
24 /// Checks for functions taking arguments by reference, where
25 /// the argument type is `Copy` and small enough to be more efficient to always
28 /// ### Why is this bad?
29 /// In many calling conventions instances of structs will
30 /// be passed through registers if they fit into two or less general purpose
33 /// ### Known problems
34 /// This lint is target register size dependent, it is
35 /// limited to 32-bit to try and reduce portability problems between 32 and
36 /// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
37 /// will be different.
39 /// The configuration option `trivial_copy_size_limit` can be set to override
40 /// this limit for a project.
42 /// This lint attempts to allow passing arguments by reference if a reference
43 /// to that argument is returned. This is implemented by comparing the lifetime
44 /// of the argument and return value for equality. However, this can cause
45 /// false positives in cases involving multiple lifetimes that are bounded by
48 /// Also, it does not take account of other similar cases where getting memory addresses
49 /// matters; namely, returning the pointer to the argument in question,
50 /// and passing the argument, as both references and pointers,
51 /// to a function that needs the memory address. For further details, refer to
52 /// [this issue](https://github.com/rust-lang/rust-clippy/issues/5953)
53 /// that explains a real case in which this false positive
54 /// led to an **undefined behaviour** introduced with unsafe code.
60 /// fn foo(v: &u32) {}
67 pub TRIVIALLY_COPY_PASS_BY_REF,
69 "functions taking small copyable arguments by reference"
72 declare_clippy_lint! {
74 /// Checks for functions taking arguments by value, where
75 /// the argument type is `Copy` and large enough to be worth considering
76 /// passing by reference. Does not trigger if the function is being exported,
77 /// because that might induce API breakage, if the parameter is declared as mutable,
78 /// or if the argument is a `self`.
80 /// ### Why is this bad?
81 /// Arguments passed by value might result in an unnecessary
82 /// shallow copy, taking up more space in the stack and requiring a call to
83 /// `memcpy`, which can be expensive.
87 /// #[derive(Clone, Copy)]
88 /// struct TooLarge([u8; 2048]);
91 /// fn foo(v: TooLarge) {}
94 /// #[derive(Clone, Copy)]
95 /// struct TooLarge([u8; 2048]);
98 /// fn foo(v: &TooLarge) {}
100 pub LARGE_TYPES_PASSED_BY_VALUE,
102 "functions taking large arguments by value"
105 #[derive(Copy, Clone)]
106 pub struct PassByRefOrValue {
109 avoid_breaking_exported_api: bool,
112 impl<'tcx> PassByRefOrValue {
114 ref_min_size: Option<u64>,
116 avoid_breaking_exported_api: bool,
119 let ref_min_size = ref_min_size.unwrap_or_else(|| {
120 let bit_width = u64::from(target.pointer_width);
121 // Cap the calculated bit width at 32-bits to reduce
122 // portability problems between 32 and 64-bit targets
123 let bit_width = cmp::min(bit_width, 32);
124 #[allow(clippy::integer_division)]
125 let byte_width = bit_width / 8;
126 // Use a limit of 2 times the register byte width
133 avoid_breaking_exported_api,
137 fn check_poly_fn(&mut self, cx: &LateContext<'tcx>, hir_id: HirId, decl: &FnDecl<'_>, span: Option<Span>) {
138 if self.avoid_breaking_exported_api && cx.access_levels.is_exported(hir_id) {
141 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
143 let fn_sig = cx.tcx.fn_sig(fn_def_id);
144 let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
146 let fn_body = cx.enclosing_body.map(|id| cx.tcx.hir().body(id));
148 for (index, (input, &ty)) in iter::zip(decl.inputs, fn_sig.inputs()).enumerate() {
149 // All spans generated from a proc-macro invocation are the same...
151 Some(s) if s == input.span => return,
156 ty::Ref(input_lt, ty, Mutability::Not) => {
157 // Use lifetimes to determine if we're returning a reference to the
158 // argument. In that case we can't switch to pass-by-value as the
159 // argument will not live long enough.
160 let output_lts = match *fn_sig.output().kind() {
161 ty::Ref(output_lt, _, _) => vec![output_lt],
162 ty::Adt(_, substs) => substs.regions().collect(),
167 if !output_lts.contains(input_lt);
169 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
170 if size <= self.ref_min_size;
171 if let hir::TyKind::Rptr(_, MutTy { ty: decl_ty, .. }) = input.kind;
173 let value_type = if is_self_ty(decl_ty) {
176 snippet(cx, decl_ty.span, "_").into()
180 TRIVIALLY_COPY_PASS_BY_REF,
182 &format!("this argument ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, self.ref_min_size),
183 "consider passing by value instead",
185 Applicability::Unspecified,
191 ty::Adt(_, _) | ty::Array(_, _) | ty::Tuple(_) => {
192 // if function has a body and parameter is annotated with mut, ignore
193 if let Some(param) = fn_body.and_then(|body| body.params.get(index)) {
194 match param.pat.kind {
195 PatKind::Binding(BindingAnnotation::Unannotated, _, _, _) => {},
202 if !is_self_ty(input);
203 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
204 if size > self.value_max_size;
208 LARGE_TYPES_PASSED_BY_VALUE,
210 &format!("this argument ({} byte) is passed by value, but might be more efficient if passed by reference (limit: {} byte)", size, self.value_max_size),
211 "consider passing by reference instead",
212 format!("&{}", snippet(cx, input.span, "_")),
213 Applicability::MaybeIncorrect,
225 impl_lint_pass!(PassByRefOrValue => [TRIVIALLY_COPY_PASS_BY_REF, LARGE_TYPES_PASSED_BY_VALUE]);
227 impl<'tcx> LateLintPass<'tcx> for PassByRefOrValue {
228 fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'_>) {
229 if item.span.from_expansion() {
233 if let hir::TraitItemKind::Fn(method_sig, _) = &item.kind {
234 self.check_poly_fn(cx, item.hir_id(), &*method_sig.decl, None);
240 cx: &LateContext<'tcx>,
242 decl: &'tcx FnDecl<'_>,
243 _body: &'tcx Body<'_>,
247 if span.from_expansion() {
252 FnKind::ItemFn(.., header, _) => {
253 if header.abi != Abi::Rust {
256 let attrs = cx.tcx.hir().attrs(hir_id);
258 if let Some(meta_items) = a.meta_item_list() {
259 if a.has_name(sym::proc_macro_derive)
260 || (a.has_name(sym::inline) && attr::list_contains_name(&meta_items, sym::always))
267 FnKind::Method(..) => (),
268 FnKind::Closure => return,
271 // Exclude non-inherent impls
272 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
275 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
281 self.check_poly_fn(cx, hir_id, decl, Some(span));