rust/clippy_lints/src/escape.rs

use rustc::hir::*;
use rustc::hir::intravisit as visit;
use rustc::hir::map::Node::{NodeExpr, NodeStmt};
use rustc::lint::*;
use rustc::middle::expr_use_visitor::*;
use rustc::middle::mem_categorization::{cmt, Categorization};
use rustc::ty;
use rustc::ty::layout::TargetDataLayout;
use rustc::traits::Reveal;
use rustc::util::nodemap::NodeSet;
use syntax::ast::NodeId;
use syntax::codemap::Span;
use utils::span_lint;

pub struct Pass {
    pub too_large_for_stack: u64,
}

/// **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 main() {
///     let x = Box::new(1);
///     foo(*x);
///     println!("{}", *x);
/// }
/// ```
declare_lint! {
    pub BOXED_LOCAL,
    Warn,
    "using `Box<T>` where unnecessary"
}

fn is_non_trait_box(ty: ty::Ty) -> bool {
    match ty.sty {
        ty::TyBox(inner) => !inner.is_trait(),
        _ => false,
    }
}

struct EscapeDelegate<'a, 'tcx: 'a> {
    set: NodeSet,
    tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
    tables: &'a ty::Tables<'tcx>,
    target: TargetDataLayout,
    too_large_for_stack: u64,
}

impl LintPass for Pass {
    fn get_lints(&self) -> LintArray {
        lint_array!(BOXED_LOCAL)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
    fn check_fn(
        &mut self,
        cx: &LateContext<'a, 'tcx>,
        _: visit::FnKind<'tcx>,
        _: &'tcx FnDecl,
        body: &'tcx Body,
        _: Span,
        _id: NodeId
    ) {
        // we store the infcx because it is expensive to recreate
        // the context each time.
        let mut v = EscapeDelegate {
            set: NodeSet(),
            tcx: cx.tcx,
            tables: cx.tables,
            target: TargetDataLayout::parse(cx.sess()),
            too_large_for_stack: self.too_large_for_stack,
        };

        let infcx = cx.tcx.borrowck_fake_infer_ctxt(body.id());
        {
            let mut vis = ExprUseVisitor::new(&mut v, &infcx);
            vis.consume_body(body);
        }

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

impl<'a, 'tcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
    fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
        if let Categorization::Local(lid) = cmt.cat {
            if self.set.contains(&lid) {
                if let Move(DirectRefMove) = mode {
                    // moved out or in. clearly can't be localized
                    self.set.remove(&lid);
                }
            }
        }
    }
    fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
    fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
        let map = &self.tcx.map;
        if map.is_argument(consume_pat.id) {
            // Skip closure arguments
            if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
                return;
            }
            if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
                self.set.insert(consume_pat.id);
            }
            return;
        }
        if let Categorization::Rvalue(..) = cmt.cat {
            if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
                if let StmtDecl(ref decl, _) = st.node {
                    if let DeclLocal(ref loc) = decl.node {
                        if let Some(ref ex) = loc.init {
                            if let ExprBox(..) = ex.node {
                                if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
                                    // let x = box (...)
                                    self.set.insert(consume_pat.id);
                                }
                                // TODO Box::new
                                // TODO vec![]
                                // TODO "foo".to_owned() and friends
                            }
                        }
                    }
                }
            }
        }
        if let Categorization::Local(lid) = cmt.cat {
            if self.set.contains(&lid) {
                // let y = x where x is known
                // remove x, insert y
                self.set.insert(consume_pat.id);
                self.set.remove(&lid);
            }
        }

    }
    fn borrow(
        &mut self,
        borrow_id: NodeId,
        _: Span,
        cmt: cmt<'tcx>,
        _: &ty::Region,
        _: ty::BorrowKind,
        loan_cause: LoanCause
    ) {
        use rustc::ty::adjustment::Adjust;

        if let Categorization::Local(lid) = cmt.cat {
            if self.set.contains(&lid) {
                if let Some(&Adjust::DerefRef { autoderefs, .. }) =
                    self.tables
                        .adjustments
                        .get(&borrow_id)
                        .map(|a| &a.kind) {
                    if LoanCause::AutoRef == loan_cause {
                        // x.foo()
                        if autoderefs == 0 {
                            self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
                        }
                    } else {
                        span_bug!(cmt.span, "Unknown adjusted AutoRef");
                    }
                } else if LoanCause::AddrOf == loan_cause {
                    // &x
                    if let Some(&Adjust::DerefRef { autoderefs, .. }) =
                        self.tables
                            .adjustments
                            .get(&self.tcx
                                .map
                                .get_parent_node(borrow_id))
                            .map(|a| &a.kind) {
                        if autoderefs <= 1 {
                            // foo(&x) where no extra autoreffing is happening
                            self.set.remove(&lid);
                        }
                    }

                } else if LoanCause::MatchDiscriminant == loan_cause {
                    self.set.remove(&lid); // `match x` can move
                }
                // do nothing for matches, etc. These can't escape
            }
        }
    }
    fn decl_without_init(&mut self, _: NodeId, _: Span) {}
    fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
}

impl<'a, 'tcx: 'a> EscapeDelegate<'a, 'tcx> {
    fn is_large_box(&self, ty: ty::Ty<'tcx>) -> bool {
        // Large types need to be boxed to avoid stack
        // overflows.
        match ty.sty {
            ty::TyBox(inner) => {
                self.tcx.infer_ctxt((), Reveal::All).enter(|infcx| {
                    if let Ok(layout) = inner.layout(&infcx) {
                        let size = layout.size(&self.target);
                        size.bytes() > self.too_large_for_stack
                    } else {
                        false
                    }
                })
            },
            _ => false,
        }
    }
}