rust-clippy/clippy_lints/src/escape.rs

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use rustc_hir::intravisit;
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use rustc_hir::{self, Body, FnDecl, HirId, HirIdSet, ItemKind, Node};
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use rustc_infer::infer::TyCtxtInferExt;
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use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, Ty};
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use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::source_map::Span;
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use rustc_target::abi::LayoutOf;
use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
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use crate::utils::span_lint;
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#[derive(Copy, Clone)]
pub struct BoxedLocal {
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pub too_large_for_stack: u64,
}
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declare_clippy_lint! {
/// **What it does:** Checks for usage of `Box<T>` where an unboxed `T` would
/// work fine.
///
/// **Why is this bad?** This is an unnecessary allocation, and bad for
/// performance. It is only necessary to allocate if you wish to move the box
/// into something.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// # fn foo(bar: usize) {}
///
/// // Bad
/// let x = Box::new(1);
/// foo(*x);
/// println!("{}", *x);
///
/// // Good
/// let x = 1;
/// foo(x);
/// println!("{}", x);
/// ```
pub BOXED_LOCAL,
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perf,
"using `Box<T>` where unnecessary"
}
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fn is_non_trait_box(ty: Ty<'_>) -> bool {
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ty.is_box() && !ty.boxed_ty().is_trait()
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}
struct EscapeDelegate<'a, 'tcx> {
cx: &'a LateContext<'a, 'tcx>,
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set: HirIdSet,
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too_large_for_stack: u64,
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}
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impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoxedLocal {
fn check_fn(
&mut self,
cx: &LateContext<'a, 'tcx>,
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_: intravisit::FnKind<'tcx>,
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_: &'tcx FnDecl<'_>,
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body: &'tcx Body<'_>,
_: Span,
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hir_id: HirId,
) {
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// If the method is an impl for a trait, don't warn.
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let parent_id = cx.tcx.hir().get_parent_item(hir_id);
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let parent_node = cx.tcx.hir().find(parent_id);
if let Some(Node::Item(item)) = parent_node {
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if let ItemKind::Impl { of_trait: Some(_), .. } = item.kind {
return;
}
}
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let mut v = EscapeDelegate {
cx,
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set: HirIdSet::default(),
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too_large_for_stack: self.too_large_for_stack,
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};
let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
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cx.tcx.infer_ctxt().enter(|infcx| {
ExprUseVisitor::new(&mut v, &infcx, fn_def_id, cx.param_env, cx.tables()).consume_body(body);
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});
for node in v.set {
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span_lint(
cx,
BOXED_LOCAL,
cx.tcx.hir().span(node),
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"local variable doesn't need to be boxed here",
);
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}
}
}
// TODO: Replace with Map::is_argument(..) when it's fixed
fn is_argument(map: rustc_middle::hir::map::Map<'_>, id: HirId) -> bool {
match map.find(id) {
Some(Node::Binding(_)) => (),
_ => return false,
}
match map.find(map.get_parent_node(id)) {
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Some(Node::Param(_)) => true,
_ => false,
}
}
impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
fn consume(&mut self, cmt: &PlaceWithHirId<'tcx>, mode: ConsumeMode) {
if cmt.place.projections.is_empty() {
if let PlaceBase::Local(lid) = cmt.place.base {
if let ConsumeMode::Move = mode {
// moved out or in. clearly can't be localized
self.set.remove(&lid);
}
let map = &self.cx.tcx.hir();
if let Some(Node::Binding(_)) = map.find(cmt.hir_id) {
if self.set.contains(&lid) {
// let y = x where x is known
// remove x, insert y
self.set.insert(cmt.hir_id);
self.set.remove(&lid);
}
}
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}
}
}
fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, _: ty::BorrowKind) {
if cmt.place.projections.is_empty() {
if let PlaceBase::Local(lid) = cmt.place.base {
self.set.remove(&lid);
}
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}
}
fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>) {
if cmt.place.projections.is_empty() {
let map = &self.cx.tcx.hir();
if is_argument(*map, cmt.hir_id) {
// Skip closure arguments
let parent_id = map.get_parent_node(cmt.hir_id);
if let Some(Node::Expr(..)) = map.find(map.get_parent_node(parent_id)) {
return;
}
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if is_non_trait_box(cmt.place.ty) && !self.is_large_box(cmt.place.ty) {
self.set.insert(cmt.hir_id);
}
return;
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}
}
}
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}
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impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
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// Large types need to be boxed to avoid stack overflows.
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if ty.is_box() {
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self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack
} else {
false
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}
}
}