rust-clippy/clippy_lints/src/trivially_copy_pass_by_ref.rs

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// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use std::cmp;
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use matches::matches;
use crate::rustc::hir;
use crate::rustc::hir::*;
use crate::rustc::hir::intravisit::FnKind;
use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use crate::rustc::{declare_tool_lint, lint_array};
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use if_chain::if_chain;
use crate::rustc::ty::TyKind;
use crate::rustc::session::config::Config as SessionConfig;
use crate::rustc_target::spec::abi::Abi;
use crate::rustc_target::abi::LayoutOf;
use crate::syntax::ast::NodeId;
use crate::syntax_pos::Span;
use crate::utils::{in_macro, is_copy, is_self, span_lint_and_sugg, snippet};
/// **What it does:** Checks for functions taking arguments by reference, where
/// the argument type is `Copy` and small enough to be more efficient to always
/// pass by value.
///
/// **Why is this bad?** In many calling conventions instances of structs will
/// be passed through registers if they fit into two or less general purpose
/// registers.
///
/// **Known problems:** This lint is target register size dependent, it is
/// limited to 32-bit to try and reduce portability problems between 32 and
/// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
/// will be different.
///
/// The configuration option `trivial_copy_size_limit` can be set to override
/// this limit for a project.
///
/// This lint attempts to allow passing arguments by reference if a reference
/// to that argument is returned. This is implemented by comparing the lifetime
/// of the argument and return value for equality. However, this can cause
/// false positives in cases involving multiple lifetimes that are bounded by
/// each other.
///
/// **Example:**
/// ```rust
/// fn foo(v: &u32) {
/// assert_eq!(v, 42);
/// }
/// // should be
/// fn foo(v: u32) {
/// assert_eq!(v, 42);
/// }
/// ```
declare_clippy_lint! {
pub TRIVIALLY_COPY_PASS_BY_REF,
perf,
"functions taking small copyable arguments by reference"
}
pub struct TriviallyCopyPassByRef {
limit: u64,
}
impl TriviallyCopyPassByRef {
pub fn new(limit: Option<u64>, target: &SessionConfig) -> Self {
let limit = limit.unwrap_or_else(|| {
let bit_width = target.usize_ty.bit_width().expect("usize should have a width") as u64;
// Cap the calculated bit width at 32-bits to reduce
// portability problems between 32 and 64-bit targets
let bit_width = cmp::min(bit_width, 32);
let byte_width = bit_width / 8;
// Use a limit of 2 times the register bit width
byte_width * 2
});
Self { limit }
}
}
impl LintPass for TriviallyCopyPassByRef {
fn get_lints(&self) -> LintArray {
lint_array![TRIVIALLY_COPY_PASS_BY_REF]
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
fn check_fn(
&mut self,
cx: &LateContext<'a, 'tcx>,
kind: FnKind<'tcx>,
decl: &'tcx FnDecl,
body: &'tcx Body,
span: Span,
node_id: NodeId,
) {
if in_macro(span) {
return;
}
match kind {
FnKind::ItemFn(.., header, _, attrs) => {
if header.abi != Abi::Rust {
return;
}
for a in attrs {
if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
return;
}
}
},
FnKind::Method(..) => (),
_ => return,
}
// Exclude non-inherent impls
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if let Some(Node::Item(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
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if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
ItemKind::Trait(..))
{
return;
}
}
let fn_def_id = cx.tcx.hir.local_def_id(node_id);
let fn_sig = cx.tcx.fn_sig(fn_def_id);
let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
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// Use lifetimes to determine if we're returning a reference to the
// argument. In that case we can't switch to pass-by-value as the
// argument will not live long enough.
let output_lts = match fn_sig.output().sty {
TyKind::Ref(output_lt, _, _) => vec![output_lt],
TyKind::Adt(_, substs) => substs.regions().collect(),
_ => vec![],
};
for ((input, &ty), arg) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments) {
// All spans generated from a proc-macro invocation are the same...
if span == input.span {
return;
}
if_chain! {
if let TyKind::Ref(input_lt, ty, Mutability::MutImmutable) = ty.sty;
if !output_lts.contains(&input_lt);
if is_copy(cx, ty);
if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
if size <= self.limit;
if let hir::TyKind::Rptr(_, MutTy { ty: ref decl_ty, .. }) = input.node;
then {
let value_type = if is_self(arg) {
"self".into()
} else {
snippet(cx, decl_ty.span, "_").into()
};
span_lint_and_sugg(
cx,
TRIVIALLY_COPY_PASS_BY_REF,
input.span,
"this argument is passed by reference, but would be more efficient if passed by value",
"consider passing by value instead",
value_type);
}
}
}
}
}