3 use crate::utils::{is_copy, is_self_ty, snippet, span_lint_and_sugg};
4 use if_chain::if_chain;
5 use rustc_errors::Applicability;
7 use rustc_hir::intravisit::FnKind;
8 use rustc_hir::{Body, FnDecl, HirId, ItemKind, MutTy, Mutability, Node};
9 use rustc_lint::{LateContext, LateLintPass};
11 use rustc_session::config::Config as SessionConfig;
12 use rustc_session::{declare_tool_lint, impl_lint_pass};
14 use rustc_target::abi::LayoutOf;
15 use rustc_target::spec::abi::Abi;
17 declare_clippy_lint! {
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
44 /// fn foo(v: &u32) {}
51 pub TRIVIALLY_COPY_PASS_BY_REF,
53 "functions taking small copyable arguments by reference"
56 #[derive(Copy, Clone)]
57 pub struct TriviallyCopyPassByRef {
61 impl<'a, 'tcx> TriviallyCopyPassByRef {
62 pub fn new(limit: Option<u64>, target: &SessionConfig) -> Self {
63 let limit = limit.unwrap_or_else(|| {
64 let bit_width = u64::from(target.ptr_width);
65 // Cap the calculated bit width at 32-bits to reduce
66 // portability problems between 32 and 64-bit targets
67 let bit_width = cmp::min(bit_width, 32);
68 #[allow(clippy::integer_division)]
69 let byte_width = bit_width / 8;
70 // Use a limit of 2 times the register byte width
76 fn check_poly_fn(&mut self, cx: &LateContext<'_, 'tcx>, hir_id: HirId, decl: &FnDecl<'_>, span: Option<Span>) {
77 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
79 let fn_sig = cx.tcx.fn_sig(fn_def_id);
80 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
82 // Use lifetimes to determine if we're returning a reference to the
83 // argument. In that case we can't switch to pass-by-value as the
84 // argument will not live long enough.
85 let output_lts = match fn_sig.output().kind {
86 ty::Ref(output_lt, _, _) => vec![output_lt],
87 ty::Adt(_, substs) => substs.regions().collect(),
91 for (input, &ty) in decl.inputs.iter().zip(fn_sig.inputs()) {
92 // All spans generated from a proc-macro invocation are the same...
94 Some(s) if s == input.span => return,
99 if let ty::Ref(input_lt, ty, Mutability::Not) = ty.kind;
100 if !output_lts.contains(&input_lt);
102 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
103 if size <= self.limit;
104 if let hir::TyKind::Rptr(_, MutTy { ty: ref decl_ty, .. }) = input.kind;
106 let value_type = if is_self_ty(decl_ty) {
109 snippet(cx, decl_ty.span, "_").into()
113 TRIVIALLY_COPY_PASS_BY_REF,
115 &format!("this argument ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, self.limit),
116 "consider passing by value instead",
118 Applicability::Unspecified,
126 impl_lint_pass!(TriviallyCopyPassByRef => [TRIVIALLY_COPY_PASS_BY_REF]);
128 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
129 fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::TraitItem<'_>) {
130 if item.span.from_expansion() {
134 if let hir::TraitItemKind::Fn(method_sig, _) = &item.kind {
135 self.check_poly_fn(cx, item.hir_id, &*method_sig.decl, None);
141 cx: &LateContext<'a, 'tcx>,
143 decl: &'tcx FnDecl<'_>,
144 _body: &'tcx Body<'_>,
148 if span.from_expansion() {
153 FnKind::ItemFn(.., header, _, attrs) => {
154 if header.abi != Abi::Rust {
158 if a.meta_item_list().is_some() && a.check_name(sym!(proc_macro_derive)) {
163 FnKind::Method(..) => (),
167 // Exclude non-inherent impls
168 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
169 if matches!(item.kind, ItemKind::Impl{ of_trait: Some(_), .. } |
176 self.check_poly_fn(cx, hir_id, decl, Some(span));