1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution.
4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
7 // option. This file may not be copied, modified, or distributed
8 // except according to those terms.
14 use crate::rustc::hir;
15 use crate::rustc::hir::*;
16 use crate::rustc::hir::intravisit::FnKind;
17 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
18 use crate::rustc::{declare_tool_lint, lint_array};
19 use if_chain::if_chain;
20 use crate::rustc::ty::TyKind;
21 use crate::rustc::ty::FnSig;
22 use crate::rustc::session::config::Config as SessionConfig;
23 use crate::rustc_target::spec::abi::Abi;
24 use crate::rustc_target::abi::LayoutOf;
25 use crate::syntax::ast::NodeId;
26 use crate::syntax_pos::Span;
27 use crate::utils::{in_macro, is_copy, is_self_ty, span_lint_and_sugg, snippet};
29 /// **What it does:** Checks for functions taking arguments by reference, where
30 /// the argument type is `Copy` and small enough to be more efficient to always
33 /// **Why is this bad?** In many calling conventions instances of structs will
34 /// be passed through registers if they fit into two or less general purpose
37 /// **Known problems:** This lint is target register size dependent, it is
38 /// limited to 32-bit to try and reduce portability problems between 32 and
39 /// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
40 /// will be different.
42 /// The configuration option `trivial_copy_size_limit` can be set to override
43 /// this limit for a project.
45 /// This lint attempts to allow passing arguments by reference if a reference
46 /// to that argument is returned. This is implemented by comparing the lifetime
47 /// of the argument and return value for equality. However, this can cause
48 /// false positives in cases involving multiple lifetimes that are bounded by
54 /// assert_eq!(v, 42);
58 /// assert_eq!(v, 42);
61 declare_clippy_lint! {
62 pub TRIVIALLY_COPY_PASS_BY_REF,
64 "functions taking small copyable arguments by reference"
67 pub struct TriviallyCopyPassByRef {
71 impl<'a, 'tcx> TriviallyCopyPassByRef {
72 pub fn new(limit: Option<u64>, target: &SessionConfig) -> Self {
73 let limit = limit.unwrap_or_else(|| {
74 let bit_width = target.usize_ty.bit_width().expect("usize should have a width") as u64;
75 // Cap the calculated bit width at 32-bits to reduce
76 // portability problems between 32 and 64-bit targets
77 let bit_width = cmp::min(bit_width, 32);
78 let byte_width = bit_width / 8;
79 // Use a limit of 2 times the register bit width
85 fn check_trait_method(
87 cx: &LateContext<'_, 'tcx>,
90 let method_def_id = cx.tcx.hir.local_def_id(item.id.node_id);
91 let method_sig = cx.tcx.fn_sig(method_def_id);
92 let method_sig = cx.tcx.erase_late_bound_regions(&method_sig);
94 let decl = match cx.tcx.hir.fn_decl(item.id.node_id) {
99 self.check_poly_fn(cx, &decl, &method_sig, None);
104 cx: &LateContext<'_, 'tcx>,
109 // Use lifetimes to determine if we're returning a reference to the
110 // argument. In that case we can't switch to pass-by-value as the
111 // argument will not live long enough.
112 let output_lts = match sig.output().sty {
113 TyKind::Ref(output_lt, _, _) => vec![output_lt],
114 TyKind::Adt(_, substs) => substs.regions().collect(),
118 for (input, &ty) in decl.inputs.iter().zip(sig.inputs()) {
119 // All spans generated from a proc-macro invocation are the same...
121 Some(s) if s == input.span => return,
126 if let TyKind::Ref(input_lt, ty, Mutability::MutImmutable) = ty.sty;
127 if !output_lts.contains(&input_lt);
129 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
130 if size <= self.limit;
131 if let hir::TyKind::Rptr(_, MutTy { ty: ref decl_ty, .. }) = input.node;
133 let value_type = if is_self_ty(decl_ty) {
136 snippet(cx, decl_ty.span, "_").into()
140 TRIVIALLY_COPY_PASS_BY_REF,
142 "this argument is passed by reference, but would be more efficient if passed by value",
143 "consider passing by value instead",
150 fn check_trait_items(
152 cx: &LateContext<'_, '_>,
153 trait_items: &[TraitItemRef]
155 for item in trait_items {
156 if let AssociatedItemKind::Method{..} = item.kind {
157 self.check_trait_method(cx, item);
163 impl LintPass for TriviallyCopyPassByRef {
164 fn get_lints(&self) -> LintArray {
165 lint_array![TRIVIALLY_COPY_PASS_BY_REF]
169 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
170 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
171 if in_macro(item.span) {
174 if let ItemKind::Trait(_, _, _, _, ref trait_items) = item.node {
175 self.check_trait_items(cx, trait_items);
181 cx: &LateContext<'a, 'tcx>,
193 FnKind::ItemFn(.., header, _, attrs) => {
194 if header.abi != Abi::Rust {
198 if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
203 FnKind::Method(..) => (),
207 // Exclude non-inherent impls
208 if let Some(Node::Item(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
209 if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
216 let fn_def_id = cx.tcx.hir.local_def_id(node_id);
218 let fn_sig = cx.tcx.fn_sig(fn_def_id);
219 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
221 self.check_poly_fn(cx, decl, &fn_sig, Some(span));