rust/src/librustc_mir/dataflow/impls/borrows.rs

// Copyright 2012-2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use rustc::hir;
use rustc::hir::def_id::DefId;
use rustc::middle::region;
use rustc::mir::{self, Location, Mir};
use rustc::mir::visit::Visitor;
use rustc::ty::{self, Region, TyCtxt};
use rustc::ty::RegionKind;
use rustc::ty::RegionKind::ReScope;
use rustc::util::nodemap::{FxHashMap, FxHashSet};

use rustc_data_structures::bitslice::{BitwiseOperator};
use rustc_data_structures::indexed_set::{IdxSet};
use rustc_data_structures::indexed_vec::{IndexVec};

use dataflow::{BitDenotation, BlockSets, DataflowOperator};
pub use dataflow::indexes::BorrowIndex;
use borrow_check::nll::region_infer::RegionInferenceContext;
use borrow_check::nll::ToRegionVid;

use syntax_pos::Span;

use std::fmt;
use std::rc::Rc;

// `Borrows` maps each dataflow bit to an `Rvalue::Ref`, which can be
// uniquely identified in the MIR by the `Location` of the assigment
// statement in which it appears on the right hand side.
pub struct Borrows<'a, 'gcx: 'tcx, 'tcx: 'a> {
    tcx: TyCtxt<'a, 'gcx, 'tcx>,
    mir: &'a Mir<'tcx>,
    scope_tree: Rc<region::ScopeTree>,
    root_scope: Option<region::Scope>,
    borrows: IndexVec<BorrowIndex, BorrowData<'tcx>>,
    location_map: FxHashMap<Location, BorrowIndex>,
    region_map: FxHashMap<Region<'tcx>, FxHashSet<BorrowIndex>>,
    local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
    region_span_map: FxHashMap<RegionKind, Span>,
    nonlexical_regioncx: Option<RegionInferenceContext<'tcx>>,
}

// temporarily allow some dead fields: `kind` and `region` will be
// needed by borrowck; `place` will probably be a MovePathIndex when
// that is extended to include borrowed data paths.
#[allow(dead_code)]
#[derive(Debug)]
pub struct BorrowData<'tcx> {
    pub(crate) location: Location,
    pub(crate) kind: mir::BorrowKind,
    pub(crate) region: Region<'tcx>,
    pub(crate) place: mir::Place<'tcx>,
}

impl<'tcx> fmt::Display for BorrowData<'tcx> {
    fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
        let kind = match self.kind {
            mir::BorrowKind::Shared => "",
            mir::BorrowKind::Unique => "uniq ",
            mir::BorrowKind::Mut => "mut ",
        };
        let region = format!("{}", self.region);
        let region = if region.len() > 0 { format!("{} ", region) } else { region };
        write!(w, "&{}{}{:?}", region, kind, self.place)
    }
}

impl<'a, 'gcx, 'tcx> Borrows<'a, 'gcx, 'tcx> {
    pub fn new(tcx: TyCtxt<'a, 'gcx, 'tcx>,
               mir: &'a Mir<'tcx>,
               nonlexical_regioncx: Option<RegionInferenceContext<'tcx>>,
               def_id: DefId,
               body_id: Option<hir::BodyId>)
               -> Self {
        let scope_tree = tcx.region_scope_tree(def_id);
        let root_scope = body_id.map(|body_id| {
            region::Scope::CallSite(tcx.hir.body(body_id).value.hir_id.local_id)
        });
        let mut visitor = GatherBorrows {
            tcx,
            mir,
            idx_vec: IndexVec::new(),
            location_map: FxHashMap(),
            region_map: FxHashMap(),
            local_map: FxHashMap(),
            region_span_map: FxHashMap()
        };
        visitor.visit_mir(mir);
        return Borrows { tcx: tcx,
                         mir: mir,
                         borrows: visitor.idx_vec,
                         scope_tree,
                         root_scope,
                         location_map: visitor.location_map,
                         region_map: visitor.region_map,
                         local_map: visitor.local_map,
                         region_span_map: visitor.region_span_map,
                         nonlexical_regioncx };

        struct GatherBorrows<'a, 'gcx: 'tcx, 'tcx: 'a> {
            tcx: TyCtxt<'a, 'gcx, 'tcx>,
            mir: &'a Mir<'tcx>,
            idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
            location_map: FxHashMap<Location, BorrowIndex>,
            region_map: FxHashMap<Region<'tcx>, FxHashSet<BorrowIndex>>,
            local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
            region_span_map: FxHashMap<RegionKind, Span>,
        }

        impl<'a, 'gcx, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'gcx, 'tcx> {
            fn visit_rvalue(&mut self,
                            rvalue: &mir::Rvalue<'tcx>,
                            location: mir::Location) {
                fn root_local(mut p: &mir::Place<'_>) -> Option<mir::Local> {
                    loop { match p {
                        mir::Place::Projection(pi) => p = &pi.base,
                        mir::Place::Static(_) => return None,
                        mir::Place::Local(l) => return Some(*l)
                    }}
                }

                if let mir::Rvalue::Ref(region, kind, ref place) = *rvalue {
                    if is_unsafe_place(self.tcx, self.mir, place) { return; }

                    let borrow = BorrowData {
                        location: location, kind: kind, region: region, place: place.clone(),
                    };
                    let idx = self.idx_vec.push(borrow);
                    self.location_map.insert(location, idx);

                    let borrows = self.region_map.entry(region).or_insert(FxHashSet());
                    borrows.insert(idx);

                    if let Some(local) = root_local(place) {
                        let borrows = self.local_map.entry(local).or_insert(FxHashSet());
                        borrows.insert(idx);
                    }
                }
            }

            fn visit_statement(&mut self,
                               block: mir::BasicBlock,
                               statement: &mir::Statement<'tcx>,
                               location: Location) {
                if let mir::StatementKind::EndRegion(region_scope) = statement.kind {
                    self.region_span_map.insert(ReScope(region_scope), statement.source_info.span);
                }
                self.super_statement(block, statement, location);
            }
        }
    }

    pub fn borrows(&self) -> &IndexVec<BorrowIndex, BorrowData<'tcx>> { &self.borrows }

    pub fn location(&self, idx: BorrowIndex) -> &Location {
        &self.borrows[idx].location
    }

    /// Returns the span for the "end point" given region. This will
    /// return `None` if NLL is enabled, since that concept has no
    /// meaning there.  Otherwise, return region span if it exists and
    /// span for end of the function if it doesn't exist.
    pub fn opt_region_end_span(&self, region: &Region) -> Option<Span> {
        match self.nonlexical_regioncx {
            Some(_) => None,
            None => {
                match self.region_span_map.get(region) {
                    Some(span) => Some(span.end_point()),
                    None => Some(self.mir.span.end_point())
                }
            }
        }
    }

    /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
    fn kill_loans_out_of_scope_at_location(&self,
                                           sets: &mut BlockSets<BorrowIndex>,
                                           location: Location) {
        if let Some(ref regioncx) = self.nonlexical_regioncx {
            for (borrow_index, borrow_data) in self.borrows.iter_enumerated() {
                let borrow_region = borrow_data.region.to_region_vid();
                if !regioncx.region_contains_point(borrow_region, location) {
                    // The region checker really considers the borrow
                    // to start at the point **after** the location of
                    // the borrow, but the borrow checker puts the gen
                    // directly **on** the location of the
                    // borrow. This results in a gen/kill both being
                    // generated for same point if we are not
                    // careful. Probably we should change the point of
                    // the gen, but for now we hackily account for the
                    // mismatch here by not generating a kill for the
                    // location on the borrow itself.
                    if location != borrow_data.location {
                        sets.kill(&borrow_index);
                    }
                }
            }
        }
    }
}

impl<'a, 'gcx, 'tcx> BitDenotation for Borrows<'a, 'gcx, 'tcx> {
    type Idx = BorrowIndex;
    fn name() -> &'static str { "borrows" }
    fn bits_per_block(&self) -> usize {
        self.borrows.len()
    }
    fn start_block_effect(&self, _sets: &mut BlockSets<BorrowIndex>)  {
        // no borrows of code region_scopes have been taken prior to
        // function execution, so this method has no effect on
        // `_sets`.
    }
    fn statement_effect(&self,
                        sets: &mut BlockSets<BorrowIndex>,
                        location: Location) {
        let block = &self.mir.basic_blocks().get(location.block).unwrap_or_else(|| {
            panic!("could not find block at location {:?}", location);
        });
        let stmt = block.statements.get(location.statement_index).unwrap_or_else(|| {
            panic!("could not find statement at location {:?}");
        });
        match stmt.kind {
            mir::StatementKind::EndRegion(region_scope) => {
                if let Some(borrow_indexes) = self.region_map.get(&ReScope(region_scope)) {
                    assert!(self.nonlexical_regioncx.is_none());
                    sets.kill_all(borrow_indexes);
                } else {
                    // (if there is no entry, then there are no borrows to be tracked)
                }
            }

            mir::StatementKind::Assign(_, ref rhs) => {
                if let mir::Rvalue::Ref(region, _, ref place) = *rhs {
                    if is_unsafe_place(self.tcx, self.mir, place) { return; }
                    if let RegionKind::ReEmpty = region {
                        // If the borrowed value is dead, the region for it
                        // can be empty. Don't track the borrow in that case.
                        return
                    }

                    let index = self.location_map.get(&location).unwrap_or_else(|| {
                        panic!("could not find BorrowIndex for location {:?}", location);
                    });
                    assert!(self.region_map.get(region).unwrap_or_else(|| {
                        panic!("could not find BorrowIndexs for region {:?}", region);
                    }).contains(&index));
                    sets.gen(&index);
                }
            }

            mir::StatementKind::StorageDead(local) => {
                // Make sure there are no remaining borrows for locals that
                // are gone out of scope.
                //
                // FIXME: expand this to variables that are assigned over.
                if let Some(borrow_indexes) = self.local_map.get(&local) {
                    sets.kill_all(borrow_indexes);
                }
            }

            mir::StatementKind::InlineAsm { .. } |
            mir::StatementKind::SetDiscriminant { .. } |
            mir::StatementKind::StorageLive(..) |
            mir::StatementKind::Validate(..) |
            mir::StatementKind::Nop => {}

        }

        self.kill_loans_out_of_scope_at_location(sets, location);
    }

    fn terminator_effect(&self,
                         sets: &mut BlockSets<BorrowIndex>,
                         location: Location) {
        let block = &self.mir.basic_blocks().get(location.block).unwrap_or_else(|| {
            panic!("could not find block at location {:?}", location);
        });
        match block.terminator().kind {
            mir::TerminatorKind::Resume |
            mir::TerminatorKind::Return |
            mir::TerminatorKind::GeneratorDrop => {
                // When we return from the function, then all `ReScope`-style regions
                // are guaranteed to have ended.
                // Normally, there would be `EndRegion` statements that come before,
                // and hence most of these loans will already be dead -- but, in some cases
                // like unwind paths, we do not always emit `EndRegion` statements, so we
                // add some kills here as a "backup" and to avoid spurious error messages.
                for (borrow_index, borrow_data) in self.borrows.iter_enumerated() {
                    if let ReScope(scope) = borrow_data.region {
                        // Check that the scope is not actually a scope from a function that is
                        // a parent of our closure. Note that the CallSite scope itself is
                        // *outside* of the closure, for some weird reason.
                        if let Some(root_scope) = self.root_scope {
                            if *scope != root_scope &&
                                self.scope_tree.is_subscope_of(*scope, root_scope)
                            {
                                sets.kill(&borrow_index);
                            }
                        }
                    }
                }
            }
            mir::TerminatorKind::SwitchInt {..} |
            mir::TerminatorKind::Drop {..} |
            mir::TerminatorKind::DropAndReplace {..} |
            mir::TerminatorKind::Call {..} |
            mir::TerminatorKind::Assert {..} |
            mir::TerminatorKind::Yield {..} |
            mir::TerminatorKind::Goto {..} |
            mir::TerminatorKind::FalseEdges {..} |
            mir::TerminatorKind::Unreachable => {}
        }
        self.kill_loans_out_of_scope_at_location(sets, location);
    }

    fn propagate_call_return(&self,
                             _in_out: &mut IdxSet<BorrowIndex>,
                             _call_bb: mir::BasicBlock,
                             _dest_bb: mir::BasicBlock,
                             _dest_place: &mir::Place) {
        // there are no effects on the region scopes from method calls.
    }
}

impl<'a, 'gcx, 'tcx> BitwiseOperator for Borrows<'a, 'gcx, 'tcx> {
    #[inline]
    fn join(&self, pred1: usize, pred2: usize) -> usize {
        pred1 | pred2 // union effects of preds when computing borrows
    }
}

impl<'a, 'gcx, 'tcx> DataflowOperator for Borrows<'a, 'gcx, 'tcx> {
    #[inline]
    fn bottom_value() -> bool {
        false // bottom = no Rvalue::Refs are active by default
    }
}

fn is_unsafe_place<'a, 'gcx: 'tcx, 'tcx: 'a>(
    tcx: TyCtxt<'a, 'gcx, 'tcx>,
    mir: &'a Mir<'tcx>,
    place: &mir::Place<'tcx>
) -> bool {
    use self::mir::Place::*;
    use self::mir::ProjectionElem;

    match *place {
        Local(_) => false,
        Static(ref static_) => tcx.is_static_mut(static_.def_id),
        Projection(ref proj) => {
            match proj.elem {
                ProjectionElem::Field(..) |
                ProjectionElem::Downcast(..) |
                ProjectionElem::Subslice { .. } |
                ProjectionElem::ConstantIndex { .. } |
                ProjectionElem::Index(_) => {
                    is_unsafe_place(tcx, mir, &proj.base)
                }
                ProjectionElem::Deref => {
                    let ty = proj.base.ty(mir, tcx).to_ty(tcx);
                    match ty.sty {
                        ty::TyRawPtr(..) => true,
                        _ => is_unsafe_place(tcx, mir, &proj.base),
                    }
                }
            }
        }
    }
}