286 lines
11 KiB
Rust
286 lines
11 KiB
Rust
use std::cmp;
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use std::iter;
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use clippy_utils::diagnostics::span_lint_and_sugg;
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use clippy_utils::source::snippet;
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use clippy_utils::ty::is_copy;
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use clippy_utils::{is_self, is_self_ty};
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use if_chain::if_chain;
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use rustc_ast::attr;
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use rustc_errors::Applicability;
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use rustc_hir as hir;
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use rustc_hir::intravisit::FnKind;
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use rustc_hir::{BindingAnnotation, Body, FnDecl, HirId, Impl, ItemKind, MutTy, Mutability, Node, PatKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::ty;
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use rustc_middle::ty::layout::LayoutOf;
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use rustc_session::{declare_tool_lint, impl_lint_pass};
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use rustc_span::def_id::LocalDefId;
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use rustc_span::{sym, Span};
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use rustc_target::spec::abi::Abi;
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use rustc_target::spec::Target;
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declare_clippy_lint! {
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/// ### What it does
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/// Checks for functions taking arguments by reference, where
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/// the argument type is `Copy` and small enough to be more efficient to always
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/// pass by value.
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///
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/// ### Why is this bad?
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/// In many calling conventions instances of structs will
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/// be passed through registers if they fit into two or less general purpose
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/// registers.
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///
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/// ### Known problems
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/// This lint is target register size dependent, it is
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/// limited to 32-bit to try and reduce portability problems between 32 and
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/// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
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/// will be different.
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///
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/// The configuration option `trivial_copy_size_limit` can be set to override
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/// this limit for a project.
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///
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/// This lint attempts to allow passing arguments by reference if a reference
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/// to that argument is returned. This is implemented by comparing the lifetime
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/// of the argument and return value for equality. However, this can cause
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/// false positives in cases involving multiple lifetimes that are bounded by
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/// each other.
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///
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/// Also, it does not take account of other similar cases where getting memory addresses
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/// matters; namely, returning the pointer to the argument in question,
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/// and passing the argument, as both references and pointers,
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/// to a function that needs the memory address. For further details, refer to
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/// [this issue](https://github.com/rust-lang/rust-clippy/issues/5953)
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/// that explains a real case in which this false positive
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/// led to an **undefined behaviour** introduced with unsafe code.
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///
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/// ### Example
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///
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/// ```rust
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/// // Bad
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/// fn foo(v: &u32) {}
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/// ```
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///
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/// ```rust
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/// // Better
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/// fn foo(v: u32) {}
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/// ```
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#[clippy::version = "pre 1.29.0"]
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pub TRIVIALLY_COPY_PASS_BY_REF,
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pedantic,
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"functions taking small copyable arguments by reference"
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}
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declare_clippy_lint! {
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/// ### What it does
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/// Checks for functions taking arguments by value, where
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/// the argument type is `Copy` and large enough to be worth considering
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/// passing by reference. Does not trigger if the function is being exported,
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/// because that might induce API breakage, if the parameter is declared as mutable,
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/// or if the argument is a `self`.
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///
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/// ### Why is this bad?
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/// Arguments passed by value might result in an unnecessary
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/// shallow copy, taking up more space in the stack and requiring a call to
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/// `memcpy`, which can be expensive.
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///
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/// ### Example
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/// ```rust
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/// #[derive(Clone, Copy)]
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/// struct TooLarge([u8; 2048]);
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///
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/// // Bad
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/// fn foo(v: TooLarge) {}
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/// ```
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/// ```rust
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/// #[derive(Clone, Copy)]
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/// struct TooLarge([u8; 2048]);
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///
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/// // Good
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/// fn foo(v: &TooLarge) {}
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/// ```
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#[clippy::version = "1.49.0"]
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pub LARGE_TYPES_PASSED_BY_VALUE,
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pedantic,
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"functions taking large arguments by value"
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}
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#[derive(Copy, Clone)]
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pub struct PassByRefOrValue {
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ref_min_size: u64,
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value_max_size: u64,
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avoid_breaking_exported_api: bool,
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}
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impl<'tcx> PassByRefOrValue {
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pub fn new(
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ref_min_size: Option<u64>,
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value_max_size: u64,
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avoid_breaking_exported_api: bool,
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target: &Target,
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) -> Self {
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let ref_min_size = ref_min_size.unwrap_or_else(|| {
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let bit_width = u64::from(target.pointer_width);
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// Cap the calculated bit width at 32-bits to reduce
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// portability problems between 32 and 64-bit targets
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let bit_width = cmp::min(bit_width, 32);
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#[allow(clippy::integer_division)]
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let byte_width = bit_width / 8;
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// Use a limit of 2 times the register byte width
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byte_width * 2
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});
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Self {
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ref_min_size,
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value_max_size,
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avoid_breaking_exported_api,
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}
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}
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fn check_poly_fn(&mut self, cx: &LateContext<'tcx>, def_id: LocalDefId, decl: &FnDecl<'_>, span: Option<Span>) {
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if self.avoid_breaking_exported_api && cx.access_levels.is_exported(def_id) {
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return;
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}
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let fn_sig = cx.tcx.fn_sig(def_id);
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let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
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let fn_body = cx.enclosing_body.map(|id| cx.tcx.hir().body(id));
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for (index, (input, &ty)) in iter::zip(decl.inputs, fn_sig.inputs()).enumerate() {
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// All spans generated from a proc-macro invocation are the same...
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match span {
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Some(s) if s == input.span => return,
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_ => (),
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}
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match ty.kind() {
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ty::Ref(input_lt, ty, Mutability::Not) => {
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// Use lifetimes to determine if we're returning a reference to the
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// argument. In that case we can't switch to pass-by-value as the
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// argument will not live long enough.
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let output_lts = match *fn_sig.output().kind() {
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ty::Ref(output_lt, _, _) => vec![output_lt],
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ty::Adt(_, substs) => substs.regions().collect(),
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_ => vec![],
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};
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if_chain! {
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if !output_lts.contains(input_lt);
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if is_copy(cx, ty);
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if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
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if size <= self.ref_min_size;
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if let hir::TyKind::Rptr(_, MutTy { ty: decl_ty, .. }) = input.kind;
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then {
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let value_type = if fn_body.and_then(|body| body.params.get(index)).map_or(false, is_self) {
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"self".into()
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} else {
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snippet(cx, decl_ty.span, "_").into()
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};
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span_lint_and_sugg(
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cx,
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TRIVIALLY_COPY_PASS_BY_REF,
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input.span,
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&format!("this argument ({} byte) is passed by reference, but would be more efficient if passed by value (limit: {} byte)", size, self.ref_min_size),
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"consider passing by value instead",
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value_type,
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Applicability::Unspecified,
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);
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}
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}
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},
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ty::Adt(_, _) | ty::Array(_, _) | ty::Tuple(_) => {
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// if function has a body and parameter is annotated with mut, ignore
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if let Some(param) = fn_body.and_then(|body| body.params.get(index)) {
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match param.pat.kind {
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PatKind::Binding(BindingAnnotation::Unannotated, _, _, _) => {},
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_ => continue,
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}
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}
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if_chain! {
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if is_copy(cx, ty);
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if !is_self_ty(input);
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if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
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if size > self.value_max_size;
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then {
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span_lint_and_sugg(
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cx,
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LARGE_TYPES_PASSED_BY_VALUE,
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input.span,
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&format!("this argument ({} byte) is passed by value, but might be more efficient if passed by reference (limit: {} byte)", size, self.value_max_size),
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"consider passing by reference instead",
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format!("&{}", snippet(cx, input.span, "_")),
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Applicability::MaybeIncorrect,
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);
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}
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}
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},
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_ => {},
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}
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}
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}
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}
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impl_lint_pass!(PassByRefOrValue => [TRIVIALLY_COPY_PASS_BY_REF, LARGE_TYPES_PASSED_BY_VALUE]);
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impl<'tcx> LateLintPass<'tcx> for PassByRefOrValue {
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fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'_>) {
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if item.span.from_expansion() {
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return;
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}
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if let hir::TraitItemKind::Fn(method_sig, _) = &item.kind {
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self.check_poly_fn(cx, item.def_id, &*method_sig.decl, None);
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}
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}
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fn check_fn(
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&mut self,
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cx: &LateContext<'tcx>,
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kind: FnKind<'tcx>,
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decl: &'tcx FnDecl<'_>,
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_body: &'tcx Body<'_>,
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span: Span,
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hir_id: HirId,
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) {
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if span.from_expansion() {
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return;
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}
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match kind {
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FnKind::ItemFn(.., header, _) => {
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if header.abi != Abi::Rust {
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return;
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}
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let attrs = cx.tcx.hir().attrs(hir_id);
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for a in attrs {
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if let Some(meta_items) = a.meta_item_list() {
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if a.has_name(sym::proc_macro_derive)
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|| (a.has_name(sym::inline) && attr::list_contains_name(&meta_items, sym::always))
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{
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return;
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}
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}
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}
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},
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FnKind::Method(..) => (),
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FnKind::Closure => return,
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}
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// 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!(
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item.kind,
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ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
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) {
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return;
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}
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}
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self.check_poly_fn(cx, cx.tcx.hir().local_def_id(hir_id), decl, Some(span));
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}
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}
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