rust/clippy_lints/src/escape.rs

use rustc_hir::intravisit;
use rustc_hir::{self, Body, FnDecl, HirId, HirIdSet, ItemKind, Node};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, Ty};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::source_map::Span;
use rustc_target::abi::LayoutOf;
use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, Place, PlaceBase};

use crate::utils::span_lint;

#[derive(Copy, Clone)]
pub struct BoxedLocal {
    pub too_large_for_stack: u64,
}

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,
    perf,
    "using `Box<T>` where unnecessary"
}

fn is_non_trait_box(ty: Ty<'_>) -> bool {
    ty.is_box() && !ty.boxed_ty().is_trait()
}

struct EscapeDelegate<'a, 'tcx> {
    cx: &'a LateContext<'a, 'tcx>,
    set: HirIdSet,
    too_large_for_stack: u64,
}

impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BoxedLocal {
    fn check_fn(
        &mut self,
        cx: &LateContext<'a, 'tcx>,
        _: intravisit::FnKind<'tcx>,
        _: &'tcx FnDecl<'_>,
        body: &'tcx Body<'_>,
        _: Span,
        hir_id: HirId,
    ) {
        // If the method is an impl for a trait, don't warn.
        let parent_id = cx.tcx.hir().get_parent_item(hir_id);
        let parent_node = cx.tcx.hir().find(parent_id);

        if let Some(Node::Item(item)) = parent_node {
            if let ItemKind::Impl { of_trait: Some(_), .. } = item.kind {
                return;
            }
        }

        let mut v = EscapeDelegate {
            cx,
            set: HirIdSet::default(),
            too_large_for_stack: self.too_large_for_stack,
        };

        let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
        cx.tcx.infer_ctxt().enter(|infcx| {
            ExprUseVisitor::new(&mut v, &infcx, fn_def_id.to_def_id(), cx.param_env, cx.tables).consume_body(body);
        });

        for node in v.set {
            span_lint(
                cx,
                BOXED_LOCAL,
                cx.tcx.hir().span(node),
                "local variable doesn't need to be boxed here",
            );
        }
    }
}

// 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)) {
        Some(Node::Param(_)) => true,
        _ => false,
    }
}

impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
    fn consume(&mut self, cmt: &Place<'tcx>, mode: ConsumeMode) {
        if cmt.projections.is_empty() {
            if let PlaceBase::Local(lid) = cmt.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);
                    }
                }
            }
        }
    }

    fn borrow(&mut self, cmt: &Place<'tcx>, _: ty::BorrowKind) {
        if cmt.projections.is_empty() {
            if let PlaceBase::Local(lid) = cmt.base {
                self.set.remove(&lid);
            }
        }
    }

    fn mutate(&mut self, cmt: &Place<'tcx>) {
        if cmt.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;
                }

                if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
                    self.set.insert(cmt.hir_id);
                }
                return;
            }
        }
    }
}

impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
    fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
        // Large types need to be boxed to avoid stack overflows.
        if ty.is_box() {
            self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack
        } else {
            false
        }
    }
}