rust-clippy/clippy_lints/src/redundant_clone.rs

use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
use clippy_utils::source::snippet_opt;
use clippy_utils::ty::{has_drop, is_copy, is_type_diagnostic_item, walk_ptrs_ty_depth};
use clippy_utils::{fn_has_unsatisfiable_preds, match_def_path, paths};
use if_chain::if_chain;
use rustc_data_structures::{fx::FxHashMap, transitive_relation::TransitiveRelation};
use rustc_errors::Applicability;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{def_id, Body, FnDecl, HirId};
use rustc_index::bit_set::{BitSet, HybridBitSet};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::mir::{
    self, traversal,
    visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor as _},
    Mutability,
};
use rustc_middle::ty::{self, fold::TypeVisitor, Ty};
use rustc_mir_dataflow::{Analysis, AnalysisDomain, CallReturnPlaces, GenKill, GenKillAnalysis, ResultsCursor};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::{BytePos, Span};
use rustc_span::sym;
use std::convert::TryFrom;
use std::ops::ControlFlow;

macro_rules! unwrap_or_continue {
    ($x:expr) => {
        match $x {
            Some(x) => x,
            None => continue,
        }
    };
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for a redundant `clone()` (and its relatives) which clones an owned
    /// value that is going to be dropped without further use.
    ///
    /// ### Why is this bad?
    /// It is not always possible for the compiler to eliminate useless
    /// allocations and deallocations generated by redundant `clone()`s.
    ///
    /// ### Known problems
    /// False-negatives: analysis performed by this lint is conservative and limited.
    ///
    /// ### Example
    /// ```rust
    /// # use std::path::Path;
    /// # #[derive(Clone)]
    /// # struct Foo;
    /// # impl Foo {
    /// #     fn new() -> Self { Foo {} }
    /// # }
    /// # fn call(x: Foo) {}
    /// {
    ///     let x = Foo::new();
    ///     call(x.clone());
    ///     call(x.clone()); // this can just pass `x`
    /// }
    ///
    /// ["lorem", "ipsum"].join(" ").to_string();
    ///
    /// Path::new("/a/b").join("c").to_path_buf();
    /// ```
    #[clippy::version = "1.32.0"]
    pub REDUNDANT_CLONE,
    perf,
    "`clone()` of an owned value that is going to be dropped immediately"
}

declare_lint_pass!(RedundantClone => [REDUNDANT_CLONE]);

impl<'tcx> LateLintPass<'tcx> for RedundantClone {
    #[allow(clippy::too_many_lines)]
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        _: FnKind<'tcx>,
        _: &'tcx FnDecl<'_>,
        body: &'tcx Body<'_>,
        _: Span,
        _: HirId,
    ) {
        let def_id = cx.tcx.hir().body_owner_def_id(body.id());

        // Building MIR for `fn`s with unsatisfiable preds results in ICE.
        if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
            return;
        }

        let mir = cx.tcx.optimized_mir(def_id.to_def_id());

        let possible_origin = {
            let mut vis = PossibleOriginVisitor::new(mir);
            vis.visit_body(mir);
            vis.into_map(cx)
        };
        let maybe_storage_live_result = MaybeStorageLive
            .into_engine(cx.tcx, mir)
            .pass_name("redundant_clone")
            .iterate_to_fixpoint()
            .into_results_cursor(mir);
        let mut possible_borrower = {
            let mut vis = PossibleBorrowerVisitor::new(cx, mir, possible_origin);
            vis.visit_body(mir);
            vis.into_map(cx, maybe_storage_live_result)
        };

        for (bb, bbdata) in mir.basic_blocks().iter_enumerated() {
            let terminator = bbdata.terminator();

            if terminator.source_info.span.from_expansion() {
                continue;
            }

            // Give up on loops
            if terminator.successors().any(|s| *s == bb) {
                continue;
            }

            let (fn_def_id, arg, arg_ty, clone_ret) =
                unwrap_or_continue!(is_call_with_ref_arg(cx, mir, &terminator.kind));

            let from_borrow = match_def_path(cx, fn_def_id, &paths::CLONE_TRAIT_METHOD)
                || match_def_path(cx, fn_def_id, &paths::TO_OWNED_METHOD)
                || (match_def_path(cx, fn_def_id, &paths::TO_STRING_METHOD)
                    && is_type_diagnostic_item(cx, arg_ty, sym::String));

            let from_deref = !from_borrow
                && (match_def_path(cx, fn_def_id, &paths::PATH_TO_PATH_BUF)
                    || match_def_path(cx, fn_def_id, &paths::OS_STR_TO_OS_STRING));

            if !from_borrow && !from_deref {
                continue;
            }

            if let ty::Adt(def, _) = arg_ty.kind() {
                if def.is_manually_drop() {
                    continue;
                }
            }

            // `{ arg = &cloned; clone(move arg); }` or `{ arg = &cloned; to_path_buf(arg); }`
            let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(cx, mir, arg, from_borrow, bb));

            let loc = mir::Location {
                block: bb,
                statement_index: bbdata.statements.len(),
            };

            // `Local` to be cloned, and a local of `clone` call's destination
            let (local, ret_local) = if from_borrow {
                // `res = clone(arg)` can be turned into `res = move arg;`
                // if `arg` is the only borrow of `cloned` at this point.

                if cannot_move_out || !possible_borrower.only_borrowers(&[arg], cloned, loc) {
                    continue;
                }

                (cloned, clone_ret)
            } else {
                // `arg` is a reference as it is `.deref()`ed in the previous block.
                // Look into the predecessor block and find out the source of deref.

                let ps = &mir.predecessors()[bb];
                if ps.len() != 1 {
                    continue;
                }
                let pred_terminator = mir[ps[0]].terminator();

                // receiver of the `deref()` call
                let (pred_arg, deref_clone_ret) = if_chain! {
                    if let Some((pred_fn_def_id, pred_arg, pred_arg_ty, res)) =
                        is_call_with_ref_arg(cx, mir, &pred_terminator.kind);
                    if res == cloned;
                    if cx.tcx.is_diagnostic_item(sym::deref_method, pred_fn_def_id);
                    if is_type_diagnostic_item(cx, pred_arg_ty, sym::PathBuf)
                        || is_type_diagnostic_item(cx, pred_arg_ty, sym::OsString);
                    then {
                        (pred_arg, res)
                    } else {
                        continue;
                    }
                };

                let (local, cannot_move_out) =
                    unwrap_or_continue!(find_stmt_assigns_to(cx, mir, pred_arg, true, ps[0]));
                let loc = mir::Location {
                    block: bb,
                    statement_index: mir.basic_blocks()[bb].statements.len(),
                };

                // This can be turned into `res = move local` if `arg` and `cloned` are not borrowed
                // at the last statement:
                //
                // ```
                // pred_arg = &local;
                // cloned = deref(pred_arg);
                // arg = &cloned;
                // StorageDead(pred_arg);
                // res = to_path_buf(cloned);
                // ```
                if cannot_move_out || !possible_borrower.only_borrowers(&[arg, cloned], local, loc) {
                    continue;
                }

                (local, deref_clone_ret)
            };

            let clone_usage = if local == ret_local {
                CloneUsage {
                    cloned_used: false,
                    cloned_consume_or_mutate_loc: None,
                    clone_consumed_or_mutated: true,
                }
            } else {
                let clone_usage = visit_clone_usage(local, ret_local, mir, bb);
                if clone_usage.cloned_used && clone_usage.clone_consumed_or_mutated {
                    // cloned value is used, and the clone is modified or moved
                    continue;
                } else if let Some(loc) = clone_usage.cloned_consume_or_mutate_loc {
                    // cloned value is mutated, and the clone is alive.
                    if possible_borrower.local_is_alive_at(ret_local, loc) {
                        continue;
                    }
                }
                clone_usage
            };

            let span = terminator.source_info.span;
            let scope = terminator.source_info.scope;
            let node = mir.source_scopes[scope]
                .local_data
                .as_ref()
                .assert_crate_local()
                .lint_root;

            if_chain! {
                if let Some(snip) = snippet_opt(cx, span);
                if let Some(dot) = snip.rfind('.');
                then {
                    let sugg_span = span.with_lo(
                        span.lo() + BytePos(u32::try_from(dot).unwrap())
                    );
                    let mut app = Applicability::MaybeIncorrect;

                    let call_snip = &snip[dot + 1..];
                    // Machine applicable when `call_snip` looks like `foobar()`
                    if let Some(call_snip) = call_snip.strip_suffix("()").map(str::trim) {
                        if call_snip.as_bytes().iter().all(|b| b.is_ascii_alphabetic() || *b == b'_') {
                            app = Applicability::MachineApplicable;
                        }
                    }

                    span_lint_hir_and_then(cx, REDUNDANT_CLONE, node, sugg_span, "redundant clone", |diag| {
                        diag.span_suggestion(
                            sugg_span,
                            "remove this",
                            String::new(),
                            app,
                        );
                        if clone_usage.cloned_used {
                            diag.span_note(
                                span,
                                "cloned value is neither consumed nor mutated",
                            );
                        } else {
                            diag.span_note(
                                span.with_hi(span.lo() + BytePos(u32::try_from(dot).unwrap())),
                                "this value is dropped without further use",
                            );
                        }
                    });
                } else {
                    span_lint_hir(cx, REDUNDANT_CLONE, node, span, "redundant clone");
                }
            }
        }
    }
}

/// If `kind` is `y = func(x: &T)` where `T: !Copy`, returns `(DefId of func, x, T, y)`.
fn is_call_with_ref_arg<'tcx>(
    cx: &LateContext<'tcx>,
    mir: &'tcx mir::Body<'tcx>,
    kind: &'tcx mir::TerminatorKind<'tcx>,
) -> Option<(def_id::DefId, mir::Local, Ty<'tcx>, mir::Local)> {
    if_chain! {
        if let mir::TerminatorKind::Call { func, args, destination, .. } = kind;
        if args.len() == 1;
        if let mir::Operand::Move(mir::Place { local, .. }) = &args[0];
        if let ty::FnDef(def_id, _) = *func.ty(&*mir, cx.tcx).kind();
        if let (inner_ty, 1) = walk_ptrs_ty_depth(args[0].ty(&*mir, cx.tcx));
        if !is_copy(cx, inner_ty);
        then {
            Some((def_id, *local, inner_ty, destination.as_ref().map(|(dest, _)| dest)?.as_local()?))
        } else {
            None
        }
    }
}

type CannotMoveOut = bool;

/// Finds the first `to = (&)from`, and returns
/// ``Some((from, whether `from` cannot be moved out))``.
fn find_stmt_assigns_to<'tcx>(
    cx: &LateContext<'tcx>,
    mir: &mir::Body<'tcx>,
    to_local: mir::Local,
    by_ref: bool,
    bb: mir::BasicBlock,
) -> Option<(mir::Local, CannotMoveOut)> {
    let rvalue = mir.basic_blocks()[bb].statements.iter().rev().find_map(|stmt| {
        if let mir::StatementKind::Assign(box (mir::Place { local, .. }, v)) = &stmt.kind {
            return if *local == to_local { Some(v) } else { None };
        }

        None
    })?;

    match (by_ref, &*rvalue) {
        (true, mir::Rvalue::Ref(_, _, place)) | (false, mir::Rvalue::Use(mir::Operand::Copy(place))) => {
            Some(base_local_and_movability(cx, mir, *place))
        },
        (false, mir::Rvalue::Ref(_, _, place)) => {
            if let [mir::ProjectionElem::Deref] = place.as_ref().projection {
                Some(base_local_and_movability(cx, mir, *place))
            } else {
                None
            }
        },
        _ => None,
    }
}

/// Extracts and returns the undermost base `Local` of given `place`. Returns `place` itself
/// if it is already a `Local`.
///
/// Also reports whether given `place` cannot be moved out.
fn base_local_and_movability<'tcx>(
    cx: &LateContext<'tcx>,
    mir: &mir::Body<'tcx>,
    place: mir::Place<'tcx>,
) -> (mir::Local, CannotMoveOut) {
    use rustc_middle::mir::PlaceRef;

    // Dereference. You cannot move things out from a borrowed value.
    let mut deref = false;
    // Accessing a field of an ADT that has `Drop`. Moving the field out will cause E0509.
    let mut field = false;
    // If projection is a slice index then clone can be removed only if the
    // underlying type implements Copy
    let mut slice = false;

    let PlaceRef { local, mut projection } = place.as_ref();
    while let [base @ .., elem] = projection {
        projection = base;
        deref |= matches!(elem, mir::ProjectionElem::Deref);
        field |= matches!(elem, mir::ProjectionElem::Field(..))
            && has_drop(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
        slice |= matches!(elem, mir::ProjectionElem::Index(..))
            && !is_copy(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
    }

    (local, deref || field || slice)
}

#[derive(Default)]
struct CloneUsage {
    /// Whether the cloned value is used after the clone.
    cloned_used: bool,
    /// The first location where the cloned value is consumed or mutated, if any.
    cloned_consume_or_mutate_loc: Option<mir::Location>,
    /// Whether the clone value is mutated.
    clone_consumed_or_mutated: bool,
}
fn visit_clone_usage(cloned: mir::Local, clone: mir::Local, mir: &mir::Body<'_>, bb: mir::BasicBlock) -> CloneUsage {
    struct V {
        cloned: mir::Local,
        clone: mir::Local,
        result: CloneUsage,
    }
    impl<'tcx> mir::visit::Visitor<'tcx> for V {
        fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
            let statements = &data.statements;
            for (statement_index, statement) in statements.iter().enumerate() {
                self.visit_statement(statement, mir::Location { block, statement_index });
            }

            self.visit_terminator(
                data.terminator(),
                mir::Location {
                    block,
                    statement_index: statements.len(),
                },
            );
        }

        fn visit_place(&mut self, place: &mir::Place<'tcx>, ctx: PlaceContext, loc: mir::Location) {
            let local = place.local;

            if local == self.cloned
                && !matches!(
                    ctx,
                    PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_)
                )
            {
                self.result.cloned_used = true;
                self.result.cloned_consume_or_mutate_loc = self.result.cloned_consume_or_mutate_loc.or_else(|| {
                    matches!(
                        ctx,
                        PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
                            | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
                    )
                    .then(|| loc)
                });
            } else if local == self.clone {
                match ctx {
                    PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
                    | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
                        self.result.clone_consumed_or_mutated = true;
                    },
                    _ => {},
                }
            }
        }
    }

    let init = CloneUsage {
        cloned_used: false,
        cloned_consume_or_mutate_loc: None,
        // Consider non-temporary clones consumed.
        // TODO: Actually check for mutation of non-temporaries.
        clone_consumed_or_mutated: mir.local_kind(clone) != mir::LocalKind::Temp,
    };
    traversal::ReversePostorder::new(mir, bb)
        .skip(1)
        .fold(init, |usage, (tbb, tdata)| {
            // Short-circuit
            if (usage.cloned_used && usage.clone_consumed_or_mutated) ||
                // Give up on loops
                tdata.terminator().successors().any(|s| *s == bb)
            {
                return CloneUsage {
                    cloned_used: true,
                    clone_consumed_or_mutated: true,
                    ..usage
                };
            }

            let mut v = V {
                cloned,
                clone,
                result: usage,
            };
            v.visit_basic_block_data(tbb, tdata);
            v.result
        })
}

/// Determines liveness of each local purely based on `StorageLive`/`Dead`.
#[derive(Copy, Clone)]
struct MaybeStorageLive;

impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
    type Domain = BitSet<mir::Local>;
    const NAME: &'static str = "maybe_storage_live";

    fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
        // bottom = dead
        BitSet::new_empty(body.local_decls.len())
    }

    fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain) {
        for arg in body.args_iter() {
            state.insert(arg);
        }
    }
}

impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
    type Idx = mir::Local;

    fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
        match stmt.kind {
            mir::StatementKind::StorageLive(l) => trans.gen(l),
            mir::StatementKind::StorageDead(l) => trans.kill(l),
            _ => (),
        }
    }

    fn terminator_effect(
        &self,
        _trans: &mut impl GenKill<Self::Idx>,
        _terminator: &mir::Terminator<'tcx>,
        _loc: mir::Location,
    ) {
    }

    fn call_return_effect(
        &self,
        _trans: &mut impl GenKill<Self::Idx>,
        _block: mir::BasicBlock,
        _return_places: CallReturnPlaces<'_, 'tcx>,
    ) {
        // Nothing to do when a call returns successfully
    }
}

/// Collects the possible borrowers of each local.
/// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
/// possible borrowers of `a`.
struct PossibleBorrowerVisitor<'a, 'tcx> {
    possible_borrower: TransitiveRelation<mir::Local>,
    body: &'a mir::Body<'tcx>,
    cx: &'a LateContext<'tcx>,
    possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
}

impl<'a, 'tcx> PossibleBorrowerVisitor<'a, 'tcx> {
    fn new(
        cx: &'a LateContext<'tcx>,
        body: &'a mir::Body<'tcx>,
        possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
    ) -> Self {
        Self {
            possible_borrower: TransitiveRelation::default(),
            cx,
            body,
            possible_origin,
        }
    }

    fn into_map(
        self,
        cx: &LateContext<'tcx>,
        maybe_live: ResultsCursor<'tcx, 'tcx, MaybeStorageLive>,
    ) -> PossibleBorrowerMap<'a, 'tcx> {
        let mut map = FxHashMap::default();
        for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
            if is_copy(cx, self.body.local_decls[row].ty) {
                continue;
            }

            let borrowers = self.possible_borrower.reachable_from(&row);
            if !borrowers.is_empty() {
                let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
                for &c in borrowers {
                    if c != mir::Local::from_usize(0) {
                        bs.insert(c);
                    }
                }

                if !bs.is_empty() {
                    map.insert(row, bs);
                }
            }
        }

        let bs = BitSet::new_empty(self.body.local_decls.len());
        PossibleBorrowerMap {
            map,
            maybe_live,
            bitset: (bs.clone(), bs),
        }
    }
}

impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
    fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
        let lhs = place.local;
        match rvalue {
            mir::Rvalue::Ref(_, _, borrowed) => {
                self.possible_borrower.add(borrowed.local, lhs);
            },
            other => {
                if ContainsRegion
                    .visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty)
                    .is_continue()
                {
                    return;
                }
                rvalue_locals(other, |rhs| {
                    if lhs != rhs {
                        self.possible_borrower.add(rhs, lhs);
                    }
                });
            },
        }
    }

    fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
        if let mir::TerminatorKind::Call {
            args,
            destination: Some((mir::Place { local: dest, .. }, _)),
            ..
        } = &terminator.kind
        {
            // TODO add doc
            // If the call returns something with lifetimes,
            // let's conservatively assume the returned value contains lifetime of all the arguments.
            // For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.

            let mut immutable_borrowers = vec![];
            let mut mutable_borrowers = vec![];

            for op in args {
                match op {
                    mir::Operand::Copy(p) | mir::Operand::Move(p) => {
                        if let ty::Ref(_, _, Mutability::Mut) = self.body.local_decls[p.local].ty.kind() {
                            mutable_borrowers.push(p.local);
                        } else {
                            immutable_borrowers.push(p.local);
                        }
                    },
                    mir::Operand::Constant(..) => (),
                }
            }

            let mut mutable_variables: Vec<mir::Local> = mutable_borrowers
                .iter()
                .filter_map(|r| self.possible_origin.get(r))
                .flat_map(HybridBitSet::iter)
                .collect();

            if ContainsRegion.visit_ty(self.body.local_decls[*dest].ty).is_break() {
                mutable_variables.push(*dest);
            }

            for y in mutable_variables {
                for x in &immutable_borrowers {
                    self.possible_borrower.add(*x, y);
                }
                for x in &mutable_borrowers {
                    self.possible_borrower.add(*x, y);
                }
            }
        }
    }
}

/// Collect possible borrowed for every `&mut` local.
/// For exampel, `_1 = &mut _2` generate _1: {_2,...}
/// Known Problems: not sure all borrowed are tracked
struct PossibleOriginVisitor<'a, 'tcx> {
    possible_origin: TransitiveRelation<mir::Local>,
    body: &'a mir::Body<'tcx>,
}

impl<'a, 'tcx> PossibleOriginVisitor<'a, 'tcx> {
    fn new(body: &'a mir::Body<'tcx>) -> Self {
        Self {
            possible_origin: TransitiveRelation::default(),
            body,
        }
    }

    fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, HybridBitSet<mir::Local>> {
        let mut map = FxHashMap::default();
        for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
            if is_copy(cx, self.body.local_decls[row].ty) {
                continue;
            }

            let borrowers = self.possible_origin.reachable_from(&row);
            if !borrowers.is_empty() {
                let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
                for &c in borrowers {
                    if c != mir::Local::from_usize(0) {
                        bs.insert(c);
                    }
                }

                if !bs.is_empty() {
                    map.insert(row, bs);
                }
            }
        }
        map
    }
}

impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleOriginVisitor<'a, 'tcx> {
    fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
        let lhs = place.local;
        match rvalue {
            // Only consider `&mut`, which can modify origin place
            mir::Rvalue::Ref(_, rustc_middle::mir::BorrowKind::Mut { .. }, borrowed) |
            // _2: &mut _;
            // _3 = move _2
            mir::Rvalue::Use(mir::Operand::Move(borrowed))  |
            // _3 = move _2 as &mut _;
            mir::Rvalue::Cast(_, mir::Operand::Move(borrowed), _)
                => {
                self.possible_origin.add(lhs, borrowed.local);
            },
            _ => {},
        }
    }
}

struct ContainsRegion;

impl TypeVisitor<'_> for ContainsRegion {
    type BreakTy = ();

    fn visit_region(&mut self, _: ty::Region<'_>) -> ControlFlow<Self::BreakTy> {
        ControlFlow::BREAK
    }
}

fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
    use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};

    let mut visit_op = |op: &mir::Operand<'_>| match op {
        mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
        mir::Operand::Constant(..) => (),
    };

    match rvalue {
        Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
        Aggregate(_, ops) => ops.iter().for_each(visit_op),
        BinaryOp(_, box (lhs, rhs)) | CheckedBinaryOp(_, box (lhs, rhs)) => {
            visit_op(lhs);
            visit_op(rhs);
        },
        _ => (),
    }
}

/// Result of `PossibleBorrowerVisitor`.
struct PossibleBorrowerMap<'a, 'tcx> {
    /// Mapping `Local -> its possible borrowers`
    map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
    maybe_live: ResultsCursor<'a, 'tcx, MaybeStorageLive>,
    // Caches to avoid allocation of `BitSet` on every query
    bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
}

impl PossibleBorrowerMap<'_, '_> {
    /// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
    fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
        self.maybe_live.seek_after_primary_effect(at);

        self.bitset.0.clear();
        let maybe_live = &mut self.maybe_live;
        if let Some(bitset) = self.map.get(&borrowed) {
            for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
                self.bitset.0.insert(b);
            }
        } else {
            return false;
        }

        self.bitset.1.clear();
        for b in borrowers {
            self.bitset.1.insert(*b);
        }

        self.bitset.0 == self.bitset.1
    }

    fn local_is_alive_at(&mut self, local: mir::Local, at: mir::Location) -> bool {
        self.maybe_live.seek_after_primary_effect(at);
        self.maybe_live.contains(local)
    }
}