3 use crate::utils::{in_macro, is_copy, is_self_ty, snippet, span_lint_and_sugg};
4 use if_chain::if_chain;
7 use rustc::hir::intravisit::FnKind;
9 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
10 use rustc::session::config::Config as SessionConfig;
11 use rustc::ty::{self, FnSig};
12 use rustc::{declare_tool_lint, lint_array};
13 use rustc_errors::Applicability;
14 use rustc_target::abi::LayoutOf;
15 use rustc_target::spec::abi::Abi;
18 /// **What it does:** Checks for functions taking arguments by reference, where
19 /// the argument type is `Copy` and small enough to be more efficient to always
22 /// **Why is this bad?** In many calling conventions instances of structs will
23 /// be passed through registers if they fit into two or less general purpose
26 /// **Known problems:** This lint is target register size dependent, it is
27 /// limited to 32-bit to try and reduce portability problems between 32 and
28 /// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
29 /// will be different.
31 /// The configuration option `trivial_copy_size_limit` can be set to override
32 /// this limit for a project.
34 /// This lint attempts to allow passing arguments by reference if a reference
35 /// to that argument is returned. This is implemented by comparing the lifetime
36 /// of the argument and return value for equality. However, this can cause
37 /// false positives in cases involving multiple lifetimes that are bounded by
43 /// assert_eq!(v, 42);
47 /// assert_eq!(v, 42);
50 declare_clippy_lint! {
51 pub TRIVIALLY_COPY_PASS_BY_REF,
53 "functions taking small copyable arguments by reference"
56 pub struct TriviallyCopyPassByRef {
60 impl<'a, 'tcx> TriviallyCopyPassByRef {
61 pub fn new(limit: Option<u64>, target: &SessionConfig) -> Self {
62 let limit = limit.unwrap_or_else(|| {
63 let bit_width = target.usize_ty.bit_width().expect("usize should have a width") as u64;
64 // Cap the calculated bit width at 32-bits to reduce
65 // portability problems between 32 and 64-bit targets
66 let bit_width = cmp::min(bit_width, 32);
67 let byte_width = bit_width / 8;
68 // Use a limit of 2 times the register bit width
74 fn check_trait_method(&mut self, cx: &LateContext<'_, 'tcx>, item: &TraitItemRef) {
75 let method_def_id = cx.tcx.hir().local_def_id(item.id.node_id);
76 let method_sig = cx.tcx.fn_sig(method_def_id);
77 let method_sig = cx.tcx.erase_late_bound_regions(&method_sig);
79 let decl = match cx.tcx.hir().fn_decl(item.id.node_id) {
84 self.check_poly_fn(cx, &decl, &method_sig, None);
87 fn check_poly_fn(&mut self, cx: &LateContext<'_, 'tcx>, decl: &FnDecl, sig: &FnSig<'tcx>, span: Option<Span>) {
88 // Use lifetimes to determine if we're returning a reference to the
89 // argument. In that case we can't switch to pass-by-value as the
90 // argument will not live long enough.
91 let output_lts = match sig.output().sty {
92 ty::Ref(output_lt, _, _) => vec![output_lt],
93 ty::Adt(_, substs) => substs.regions().collect(),
97 for (input, &ty) in decl.inputs.iter().zip(sig.inputs()) {
98 // All spans generated from a proc-macro invocation are the same...
100 Some(s) if s == input.span => return,
105 if let ty::Ref(input_lt, ty, Mutability::MutImmutable) = ty.sty;
106 if !output_lts.contains(&input_lt);
108 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
109 if size <= self.limit;
110 if let hir::TyKind::Rptr(_, MutTy { ty: ref decl_ty, .. }) = input.node;
112 let value_type = if is_self_ty(decl_ty) {
115 snippet(cx, decl_ty.span, "_").into()
119 TRIVIALLY_COPY_PASS_BY_REF,
121 "this argument is passed by reference, but would be more efficient if passed by value",
122 "consider passing by value instead",
124 Applicability::Unspecified,
131 fn check_trait_items(&mut self, cx: &LateContext<'_, '_>, trait_items: &[TraitItemRef]) {
132 for item in trait_items {
133 if let AssociatedItemKind::Method { .. } = item.kind {
134 self.check_trait_method(cx, item);
140 impl LintPass for TriviallyCopyPassByRef {
141 fn get_lints(&self) -> LintArray {
142 lint_array![TRIVIALLY_COPY_PASS_BY_REF]
145 fn name(&self) -> &'static str {
146 "TrivallyCopyPassByRef"
150 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
151 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
152 if in_macro(item.span) {
155 if let ItemKind::Trait(_, _, _, _, ref trait_items) = item.node {
156 self.check_trait_items(cx, trait_items);
162 cx: &LateContext<'a, 'tcx>,
174 FnKind::ItemFn(.., header, _, attrs) => {
175 if header.abi != Abi::Rust {
179 if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
184 FnKind::Method(..) => (),
188 // Exclude non-inherent impls
189 if let Some(Node::Item(item)) = cx
192 .find_by_hir_id(cx.tcx.hir().get_parent_node_by_hir_id(hir_id))
194 if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
201 let fn_def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
203 let fn_sig = cx.tcx.fn_sig(fn_def_id);
204 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
206 self.check_poly_fn(cx, decl, &fn_sig, Some(span));