3 use crate::utils::{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;
12 use rustc::{declare_tool_lint, impl_lint_pass};
13 use rustc_errors::Applicability;
14 use rustc_target::abi::LayoutOf;
15 use rustc_target::spec::abi::Abi;
18 declare_clippy_lint! {
19 /// **What it does:** Checks for functions taking arguments by reference, where
20 /// the argument type is `Copy` and small enough to be more efficient to always
23 /// **Why is this bad?** In many calling conventions instances of structs will
24 /// be passed through registers if they fit into two or less general purpose
27 /// **Known problems:** This lint is target register size dependent, it is
28 /// limited to 32-bit to try and reduce portability problems between 32 and
29 /// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
30 /// will be different.
32 /// The configuration option `trivial_copy_size_limit` can be set to override
33 /// this limit for a project.
35 /// This lint attempts to allow passing arguments by reference if a reference
36 /// to that argument is returned. This is implemented by comparing the lifetime
37 /// of the argument and return value for equality. However, this can cause
38 /// false positives in cases involving multiple lifetimes that are bounded by
45 /// fn foo(v: &u32) {}
52 pub TRIVIALLY_COPY_PASS_BY_REF,
54 "functions taking small copyable arguments by reference"
57 #[derive(Copy, Clone)]
58 pub struct TriviallyCopyPassByRef {
62 impl<'a, 'tcx> TriviallyCopyPassByRef {
63 pub fn new(limit: Option<u64>, target: &SessionConfig) -> Self {
64 let limit = limit.unwrap_or_else(|| {
65 let bit_width = u64::from(target.ptr_width);
66 // Cap the calculated bit width at 32-bits to reduce
67 // portability problems between 32 and 64-bit targets
68 let bit_width = cmp::min(bit_width, 32);
69 #[allow(clippy::integer_division)]
70 let byte_width = bit_width / 8;
71 // Use a limit of 2 times the register byte width
77 fn check_poly_fn(&mut self, cx: &LateContext<'_, 'tcx>, hir_id: HirId, decl: &FnDecl, span: Option<Span>) {
78 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
80 let fn_sig = cx.tcx.fn_sig(fn_def_id);
81 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
83 // Use lifetimes to determine if we're returning a reference to the
84 // argument. In that case we can't switch to pass-by-value as the
85 // argument will not live long enough.
86 let output_lts = match fn_sig.output().kind {
87 ty::Ref(output_lt, _, _) => vec![output_lt],
88 ty::Adt(_, substs) => substs.regions().collect(),
92 for (input, &ty) in decl.inputs.iter().zip(fn_sig.inputs()) {
93 // All spans generated from a proc-macro invocation are the same...
95 Some(s) if s == input.span => return,
100 if let ty::Ref(input_lt, ty, Mutability::Immutable) = ty.kind;
101 if !output_lts.contains(&input_lt);
103 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
104 if size <= self.limit;
105 if let hir::TyKind::Rptr(_, MutTy { ty: ref decl_ty, .. }) = input.kind;
107 let value_type = if is_self_ty(decl_ty) {
110 snippet(cx, decl_ty.span, "_").into()
114 TRIVIALLY_COPY_PASS_BY_REF,
116 &format!("this argument ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, self.limit),
117 "consider passing by value instead",
119 Applicability::Unspecified,
127 impl_lint_pass!(TriviallyCopyPassByRef => [TRIVIALLY_COPY_PASS_BY_REF]);
129 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
130 fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::TraitItem) {
131 if item.span.from_expansion() {
135 if let hir::TraitItemKind::Method(method_sig, _) = &item.kind {
136 self.check_poly_fn(cx, item.hir_id, &*method_sig.decl, None);
142 cx: &LateContext<'a, 'tcx>,
149 if span.from_expansion() {
154 FnKind::ItemFn(.., header, _, attrs) => {
155 if header.abi != Abi::Rust {
159 if a.meta_item_list().is_some() && a.check_name(sym!(proc_macro_derive)) {
164 FnKind::Method(..) => (),
168 // Exclude non-inherent impls
169 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
170 if matches!(item.kind, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
177 self.check_poly_fn(cx, hir_id, decl, Some(span));