rust-clippy/clippy_lints/src/escape.rs

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use rustc::hir::intravisit as visit;
use rustc::hir::{self, *};
use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::middle::expr_use_visitor::*;
use rustc::middle::mem_categorization::{cmt_, Categorization};
use rustc::ty::layout::LayoutOf;
use rustc::ty::{self, Ty};
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use rustc::util::nodemap::HirIdSet;
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use rustc::{declare_tool_lint, impl_lint_pass};
use syntax::source_map::Span;
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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) {}
/// let x = Box::new(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>,
_: visit::FnKind<'tcx>,
_: &'tcx FnDecl,
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(_, _, _, _, 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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let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
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ExprUseVisitor::new(
&mut v,
cx.tcx,
fn_def_id,
cx.param_env,
region_scope_tree,
cx.tables,
None,
)
.consume_body(body);
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: &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> {
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fn consume(&mut self, _: HirId, _: Span, cmt: &cmt_<'tcx>, mode: ConsumeMode) {
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if let Categorization::Local(lid) = cmt.cat {
if let Move(DirectRefMove) | Move(CaptureMove) = mode {
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// moved out or in. clearly can't be localized
self.set.remove(&lid);
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}
}
}
fn matched_pat(&mut self, _: &Pat, _: &cmt_<'tcx>, _: MatchMode) {}
fn consume_pat(&mut self, consume_pat: &Pat, cmt: &cmt_<'tcx>, _: ConsumeMode) {
let map = &self.cx.tcx.hir();
if is_argument(map, consume_pat.hir_id) {
// Skip closure arguments
let parent_id = map.get_parent_node(consume_pat.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.ty) && !self.is_large_box(cmt.ty) {
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self.set.insert(consume_pat.hir_id);
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}
return;
}
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if let Categorization::Rvalue(..) = cmt.cat {
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if let Some(Node::Stmt(st)) = map.find(map.get_parent_node(cmt.hir_id)) {
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if let StmtKind::Local(ref loc) = st.kind {
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if let Some(ref ex) = loc.init {
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if let ExprKind::Box(..) = ex.kind {
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if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
// let x = box (...)
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self.set.insert(consume_pat.hir_id);
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}
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// TODO Box::new
// TODO vec![]
// TODO "foo".to_owned() and friends
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}
}
}
}
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}
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if let Categorization::Local(lid) = cmt.cat {
if self.set.contains(&lid) {
// let y = x where x is known
// remove x, insert y
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self.set.insert(consume_pat.hir_id);
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self.set.remove(&lid);
}
}
}
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fn borrow(
&mut self,
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_: HirId,
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_: Span,
cmt: &cmt_<'tcx>,
_: ty::Region<'_>,
_: ty::BorrowKind,
loan_cause: LoanCause,
) {
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if let Categorization::Local(lid) = cmt.cat {
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match loan_cause {
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// `x.foo()`
// Used without autoderef-ing (i.e., `x.clone()`).
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LoanCause::AutoRef |
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// `&x`
// `foo(&x)` where no extra autoref-ing is happening.
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LoanCause::AddrOf |
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// `match x` can move.
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LoanCause::MatchDiscriminant => {
self.set.remove(&lid);
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}
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// Do nothing for matches, etc. These can't escape.
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_ => {}
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}
}
}
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fn decl_without_init(&mut self, _: HirId, _: Span) {}
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fn mutate(&mut self, _: HirId, _: Span, _: &cmt_<'tcx>, _: MutateMode) {}
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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() {
self.cx.layout_of(ty.boxed_ty()).ok().map_or(0, |l| l.size.bytes()) > self.too_large_for_stack
} else {
false
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}
}
}