rust-clippy/clippy_lints/src/trivially_copy_pass_by_ref.rs

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use std::cmp;
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use crate::utils::{is_copy, is_self_ty, snippet, span_lint_and_sugg};
use if_chain::if_chain;
use matches::matches;
use rustc::hir;
use rustc::hir::intravisit::FnKind;
use rustc::hir::*;
use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::session::config::Config as SessionConfig;
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use rustc::ty;
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use rustc::{declare_tool_lint, impl_lint_pass};
use rustc_errors::Applicability;
use rustc_target::abi::LayoutOf;
use rustc_target::spec::abi::Abi;
use syntax_pos::Span;
declare_clippy_lint! {
/// **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
/// // Bad
/// fn foo(v: &u32) {}
/// ```
///
/// ```rust
/// // Better
/// fn foo(v: u32) {}
/// ```
pub TRIVIALLY_COPY_PASS_BY_REF,
perf,
"functions taking small copyable arguments by reference"
}
pub struct TriviallyCopyPassByRef {
limit: u64,
}
impl<'a, 'tcx> 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);
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#[allow(clippy::integer_division)]
let byte_width = bit_width / 8;
// Use a limit of 2 times the register byte width
byte_width * 2
});
Self { limit }
}
fn check_poly_fn(&mut self, cx: &LateContext<'_, 'tcx>, hir_id: HirId, decl: &FnDecl, span: Option<Span>) {
let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
let fn_sig = cx.tcx.fn_sig(fn_def_id);
let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
// 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 {
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ty::Ref(output_lt, _, _) => vec![output_lt],
ty::Adt(_, substs) => substs.regions().collect(),
_ => vec![],
};
for (input, &ty) in decl.inputs.iter().zip(fn_sig.inputs()) {
// All spans generated from a proc-macro invocation are the same...
match span {
Some(s) if s == input.span => return,
_ => (),
}
if_chain! {
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if let ty::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_ty(decl_ty) {
"self".into()
} else {
snippet(cx, decl_ty.span, "_").into()
};
span_lint_and_sugg(
cx,
TRIVIALLY_COPY_PASS_BY_REF,
input.span,
&format!("this argument ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, self.limit),
"consider passing by value instead",
value_type,
Applicability::Unspecified,
);
}
}
}
}
}
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impl_lint_pass!(TriviallyCopyPassByRef => [TRIVIALLY_COPY_PASS_BY_REF]);
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::TraitItem) {
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if item.span.from_expansion() {
return;
}
if let hir::TraitItemKind::Method(method_sig, _) = &item.node {
self.check_poly_fn(cx, item.hir_id, &*method_sig.decl, None);
}
}
fn check_fn(
&mut self,
cx: &LateContext<'a, 'tcx>,
kind: FnKind<'tcx>,
decl: &'tcx FnDecl,
_body: &'tcx Body,
span: Span,
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hir_id: HirId,
) {
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if span.from_expansion() {
return;
}
match kind {
FnKind::ItemFn(.., header, _, attrs) => {
if header.abi != Abi::Rust {
return;
}
for a in attrs {
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if a.meta_item_list().is_some() && a.check_name(sym!(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(hir_id)) {
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if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
ItemKind::Trait(..))
{
return;
}
}
self.check_poly_fn(cx, hir_id, decl, Some(span));
}
}