rust/src/librustc_mir/dataflow/mod.rs

use syntax::ast::{self, MetaItem};

use rustc_data_structures::bit_set::{BitSet, BitSetOperator, HybridBitSet};
use rustc_data_structures::indexed_vec::Idx;
use rustc_data_structures::work_queue::WorkQueue;

use rustc::hir::HirId;
use rustc::ty::{self, TyCtxt};
use rustc::mir::{self, Mir, BasicBlock, BasicBlockData, Location, Statement, Terminator};
use rustc::mir::traversal;
use rustc::session::Session;

use std::borrow::Borrow;
use std::fmt;
use std::io;
use std::path::PathBuf;
use std::usize;

pub use self::impls::{MaybeStorageLive};
pub use self::impls::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
pub use self::impls::DefinitelyInitializedPlaces;
pub use self::impls::EverInitializedPlaces;
pub use self::impls::borrows::Borrows;
pub use self::impls::HaveBeenBorrowedLocals;
pub use self::at_location::{FlowAtLocation, FlowsAtLocation};
pub(crate) use self::drop_flag_effects::*;

use self::move_paths::MoveData;

mod at_location;
pub mod drop_flag_effects;
mod graphviz;
mod impls;
pub mod move_paths;

pub(crate) use self::move_paths::indexes;

pub(crate) struct DataflowBuilder<'a, 'tcx: 'a, BD>
where
    BD: BitDenotation<'tcx>
{
    hir_id: HirId,
    flow_state: DataflowAnalysis<'a, 'tcx, BD>,
    print_preflow_to: Option<String>,
    print_postflow_to: Option<String>,
}

/// `DebugFormatted` encapsulates the "{:?}" rendering of some
/// arbitrary value. This way: you pay cost of allocating an extra
/// string (as well as that of rendering up-front); in exchange, you
/// don't have to hand over ownership of your value or deal with
/// borrowing it.
pub(crate) struct DebugFormatted(String);

impl DebugFormatted {
    pub fn new(input: &dyn fmt::Debug) -> DebugFormatted {
        DebugFormatted(format!("{:?}", input))
    }
}

impl fmt::Debug for DebugFormatted {
    fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(w, "{}", self.0)
    }
}

pub(crate) trait Dataflow<'tcx, BD: BitDenotation<'tcx>> {
    /// Sets up and runs the dataflow problem, using `p` to render results if
    /// implementation so chooses.
    fn dataflow<P>(&mut self, p: P) where P: Fn(&BD, BD::Idx) -> DebugFormatted {
        let _ = p; // default implementation does not instrument process.
        self.build_sets();
        self.propagate();
    }

    /// Sets up the entry, gen, and kill sets for this instance of a dataflow problem.
    fn build_sets(&mut self);

    /// Finds a fixed-point solution to this instance of a dataflow problem.
    fn propagate(&mut self);
}

impl<'a, 'tcx: 'a, BD> Dataflow<'tcx, BD> for DataflowBuilder<'a, 'tcx, BD>
where
    BD: BitDenotation<'tcx>
{
    fn dataflow<P>(&mut self, p: P) where P: Fn(&BD, BD::Idx) -> DebugFormatted {
        self.flow_state.build_sets();
        self.pre_dataflow_instrumentation(|c,i| p(c,i)).unwrap();
        self.flow_state.propagate();
        self.post_dataflow_instrumentation(|c,i| p(c,i)).unwrap();
    }

    fn build_sets(&mut self) { self.flow_state.build_sets(); }
    fn propagate(&mut self) { self.flow_state.propagate(); }
}

pub(crate) fn has_rustc_mir_with(attrs: &[ast::Attribute], name: &str) -> Option<MetaItem> {
    for attr in attrs {
        if attr.check_name("rustc_mir") {
            let items = attr.meta_item_list();
            for item in items.iter().flat_map(|l| l.iter()) {
                match item.meta_item() {
                    Some(mi) if mi.check_name(name) => return Some(mi.clone()),
                    _ => continue
                }
            }
        }
    }
    return None;
}

pub struct MoveDataParamEnv<'gcx, 'tcx> {
    pub(crate) move_data: MoveData<'tcx>,
    pub(crate) param_env: ty::ParamEnv<'gcx>,
}

pub(crate) fn do_dataflow<'a, 'gcx, 'tcx, BD, P>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                                                 mir: &'a Mir<'tcx>,
                                                 hir_id: HirId,
                                                 attributes: &[ast::Attribute],
                                                 dead_unwinds: &BitSet<BasicBlock>,
                                                 bd: BD,
                                                 p: P)
                                                 -> DataflowResults<'tcx, BD>
    where BD: BitDenotation<'tcx> + InitialFlow,
          P: Fn(&BD, BD::Idx) -> DebugFormatted
{
    let flow_state = DataflowAnalysis::new(mir, dead_unwinds, bd);
    flow_state.run(tcx, hir_id, attributes, p)
}

impl<'a, 'gcx: 'tcx, 'tcx: 'a, BD> DataflowAnalysis<'a, 'tcx, BD> where BD: BitDenotation<'tcx>
{
    pub(crate) fn run<P>(self,
                         tcx: TyCtxt<'a, 'gcx, 'tcx>,
                         hir_id: HirId,
                         attributes: &[ast::Attribute],
                         p: P) -> DataflowResults<'tcx, BD>
        where P: Fn(&BD, BD::Idx) -> DebugFormatted
    {
        let name_found = |sess: &Session, attrs: &[ast::Attribute], name| -> Option<String> {
            if let Some(item) = has_rustc_mir_with(attrs, name) {
                if let Some(s) = item.value_str() {
                    return Some(s.to_string())
                } else {
                    sess.span_err(
                        item.span,
                        &format!("{} attribute requires a path", item.ident));
                    return None;
                }
            }
            return None;
        };

        let print_preflow_to =
            name_found(tcx.sess, attributes, "borrowck_graphviz_preflow");
        let print_postflow_to =
            name_found(tcx.sess, attributes, "borrowck_graphviz_postflow");

        let mut mbcx = DataflowBuilder {
            hir_id,
            print_preflow_to, print_postflow_to, flow_state: self,
        };

        mbcx.dataflow(p);
        mbcx.flow_state.results()
    }
}

struct PropagationContext<'b, 'a: 'b, 'tcx: 'a, O> where O: 'b + BitDenotation<'tcx>
{
    builder: &'b mut DataflowAnalysis<'a, 'tcx, O>,
}

impl<'a, 'tcx: 'a, BD> DataflowAnalysis<'a, 'tcx, BD> where BD: BitDenotation<'tcx>
{
    fn propagate(&mut self) {
        let mut temp = BitSet::new_empty(self.flow_state.sets.bits_per_block);
        let mut propcx = PropagationContext {
            builder: self,
        };
        propcx.walk_cfg(&mut temp);
    }

    fn build_sets(&mut self) {
        // First we need to build the entry-, gen- and kill-sets.

        {
            let sets = &mut self.flow_state.sets.for_block(mir::START_BLOCK.index());
            self.flow_state.operator.start_block_effect(&mut sets.on_entry);
        }

        for (bb, data) in self.mir.basic_blocks().iter_enumerated() {
            let &mir::BasicBlockData { ref statements, ref terminator, is_cleanup: _ } = data;

            let mut interim_state;
            let sets = &mut self.flow_state.sets.for_block(bb.index());
            let track_intrablock = BD::accumulates_intrablock_state();
            if track_intrablock {
                debug!("swapping in mutable on_entry, initially {:?}", sets.on_entry);
                interim_state = sets.on_entry.to_owned();
                sets.on_entry = &mut interim_state;
            }
            for j_stmt in 0..statements.len() {
                let location = Location { block: bb, statement_index: j_stmt };
                self.flow_state.operator.before_statement_effect(sets, location);
                self.flow_state.operator.statement_effect(sets, location);
                if track_intrablock {
                    sets.apply_local_effect();
                }
            }

            if terminator.is_some() {
                let location = Location { block: bb, statement_index: statements.len() };
                self.flow_state.operator.before_terminator_effect(sets, location);
                self.flow_state.operator.terminator_effect(sets, location);
                if track_intrablock {
                    sets.apply_local_effect();
                }
            }
        }
    }
}

impl<'b, 'a: 'b, 'tcx: 'a, BD> PropagationContext<'b, 'a, 'tcx, BD> where BD: BitDenotation<'tcx>
{
    fn walk_cfg(&mut self, in_out: &mut BitSet<BD::Idx>) {
        let mut dirty_queue: WorkQueue<mir::BasicBlock> =
            WorkQueue::with_all(self.builder.mir.basic_blocks().len());
        let mir = self.builder.mir;
        while let Some(bb) = dirty_queue.pop() {
            let bb_data = &mir[bb];
            {
                let sets = self.builder.flow_state.sets.for_block(bb.index());
                debug_assert!(in_out.words().len() == sets.on_entry.words().len());
                in_out.overwrite(sets.on_entry);
                in_out.union(sets.gen_set);
                in_out.subtract(sets.kill_set);
            }
            self.builder.propagate_bits_into_graph_successors_of(
                in_out, (bb, bb_data), &mut dirty_queue);
        }
    }
}

fn dataflow_path(context: &str, path: &str) -> PathBuf {
    let mut path = PathBuf::from(path);
    let new_file_name = {
        let orig_file_name = path.file_name().unwrap().to_str().unwrap();
        format!("{}_{}", context, orig_file_name)
    };
    path.set_file_name(new_file_name);
    path
}

impl<'a, 'tcx: 'a, BD> DataflowBuilder<'a, 'tcx, BD> where BD: BitDenotation<'tcx>
{
    fn pre_dataflow_instrumentation<P>(&self, p: P) -> io::Result<()>
        where P: Fn(&BD, BD::Idx) -> DebugFormatted
    {
        if let Some(ref path_str) = self.print_preflow_to {
            let path = dataflow_path(BD::name(), path_str);
            graphviz::print_borrowck_graph_to(self, &path, p)
        } else {
            Ok(())
        }
    }

    fn post_dataflow_instrumentation<P>(&self, p: P) -> io::Result<()>
        where P: Fn(&BD, BD::Idx) -> DebugFormatted
    {
        if let Some(ref path_str) = self.print_postflow_to {
            let path = dataflow_path(BD::name(), path_str);
            graphviz::print_borrowck_graph_to(self, &path, p)
        } else {
            Ok(())
        }
    }
}

/// DataflowResultsConsumer abstracts over walking the MIR with some
/// already constructed dataflow results.
///
/// It abstracts over the FlowState and also completely hides the
/// underlying flow analysis results, because it needs to handle cases
/// where we are combining the results of *multiple* flow analyses
/// (e.g., borrows + inits + uninits).
pub(crate) trait DataflowResultsConsumer<'a, 'tcx: 'a> {
    type FlowState: FlowsAtLocation;

    // Observation Hooks: override (at least one of) these to get analysis feedback.
    fn visit_block_entry(&mut self,
                         _bb: BasicBlock,
                         _flow_state: &Self::FlowState) {}

    fn visit_statement_entry(&mut self,
                             _loc: Location,
                             _stmt: &Statement<'tcx>,
                             _flow_state: &Self::FlowState) {}

    fn visit_terminator_entry(&mut self,
                              _loc: Location,
                              _term: &Terminator<'tcx>,
                              _flow_state: &Self::FlowState) {}

    // Main entry point: this drives the processing of results.

    fn analyze_results(&mut self, flow_uninit: &mut Self::FlowState) {
        let flow = flow_uninit;
        for (bb, _) in traversal::reverse_postorder(self.mir()) {
            flow.reset_to_entry_of(bb);
            self.process_basic_block(bb, flow);
        }
    }

    fn process_basic_block(&mut self, bb: BasicBlock, flow_state: &mut Self::FlowState) {
        let BasicBlockData { ref statements, ref terminator, is_cleanup: _ } =
            self.mir()[bb];
        let mut location = Location { block: bb, statement_index: 0 };
        for stmt in statements.iter() {
            flow_state.reconstruct_statement_effect(location);
            self.visit_statement_entry(location, stmt, flow_state);
            flow_state.apply_local_effect(location);
            location.statement_index += 1;
        }

        if let Some(ref term) = *terminator {
            flow_state.reconstruct_terminator_effect(location);
            self.visit_terminator_entry(location, term, flow_state);

            // We don't need to apply the effect of the terminator,
            // since we are only visiting dataflow state on control
            // flow entry to the various nodes. (But we still need to
            // reconstruct the effect, because the visit method might
            // inspect it.)
        }
    }

    // Delegated Hooks: Provide access to the MIR and process the flow state.

    fn mir(&self) -> &'a Mir<'tcx>;
}

pub fn state_for_location<'tcx, T: BitDenotation<'tcx>>(loc: Location,
                                                        analysis: &T,
                                                        result: &DataflowResults<'tcx, T>,
                                                        mir: &Mir<'tcx>)
    -> BitSet<T::Idx> {
    let mut on_entry = result.sets().on_entry_set_for(loc.block.index()).to_owned();
    let mut kill_set = on_entry.to_hybrid();
    let mut gen_set = kill_set.clone();

    {
        let mut sets = BlockSets {
            on_entry: &mut on_entry,
            kill_set: &mut kill_set,
            gen_set: &mut gen_set,
        };

        for stmt in 0..loc.statement_index {
            let mut stmt_loc = loc;
            stmt_loc.statement_index = stmt;
            analysis.before_statement_effect(&mut sets, stmt_loc);
            analysis.statement_effect(&mut sets, stmt_loc);
        }

        // Apply the pre-statement effect of the statement we're evaluating.
        if loc.statement_index == mir[loc.block].statements.len() {
            analysis.before_terminator_effect(&mut sets, loc);
        } else {
            analysis.before_statement_effect(&mut sets, loc);
        }
    }

    gen_set.to_dense()
}

pub struct DataflowAnalysis<'a, 'tcx: 'a, O> where O: BitDenotation<'tcx>
{
    flow_state: DataflowState<'tcx, O>,
    dead_unwinds: &'a BitSet<mir::BasicBlock>,
    mir: &'a Mir<'tcx>,
}

impl<'a, 'tcx: 'a, O> DataflowAnalysis<'a, 'tcx, O> where O: BitDenotation<'tcx>
{
    pub fn results(self) -> DataflowResults<'tcx, O> {
        DataflowResults(self.flow_state)
    }

    pub fn mir(&self) -> &'a Mir<'tcx> { self.mir }
}

pub struct DataflowResults<'tcx, O>(pub(crate) DataflowState<'tcx, O>) where O: BitDenotation<'tcx>;

impl<'tcx, O: BitDenotation<'tcx>> DataflowResults<'tcx, O> {
    pub fn sets(&self) -> &AllSets<O::Idx> {
        &self.0.sets
    }

    pub fn operator(&self) -> &O {
        &self.0.operator
    }
}

/// State of a dataflow analysis; couples a collection of bit sets
/// with operator used to initialize and merge bits during analysis.
pub struct DataflowState<'tcx, O: BitDenotation<'tcx>>
{
    /// All the sets for the analysis. (Factored into its
    /// own structure so that we can borrow it mutably
    /// on its own separate from other fields.)
    pub sets: AllSets<O::Idx>,

    /// operator used to initialize, combine, and interpret bits.
    pub(crate) operator: O,
}

impl<'tcx, O: BitDenotation<'tcx>> DataflowState<'tcx, O> {
    pub(crate) fn interpret_set<'c, P>(&self,
                                       o: &'c O,
                                       set: &BitSet<O::Idx>,
                                       render_idx: &P)
                                       -> Vec<DebugFormatted>
        where P: Fn(&O, O::Idx) -> DebugFormatted
    {
        set.iter().map(|i| render_idx(o, i)).collect()
    }

    pub(crate) fn interpret_hybrid_set<'c, P>(&self,
                                              o: &'c O,
                                              set: &HybridBitSet<O::Idx>,
                                              render_idx: &P)
                                              -> Vec<DebugFormatted>
        where P: Fn(&O, O::Idx) -> DebugFormatted
    {
        set.iter().map(|i| render_idx(o, i)).collect()
    }
}

#[derive(Debug)]
pub struct AllSets<E: Idx> {
    /// Analysis bitwidth for each block.
    bits_per_block: usize,

    /// For each block, bits valid on entry to the block.
    on_entry_sets: Vec<BitSet<E>>,

    /// For each block, bits generated by executing the statements +
    /// terminator in the block -- with one caveat. In particular, for
    /// *call terminators*, the effect of storing the destination is
    /// not included, since that only takes effect on the **success**
    /// edge (and not the unwind edge).
    gen_sets: Vec<HybridBitSet<E>>,

    /// For each block, bits killed by executing the statements +
    /// terminator in the block -- with one caveat. In particular, for
    /// *call terminators*, the effect of storing the destination is
    /// not included, since that only takes effect on the **success**
    /// edge (and not the unwind edge).
    kill_sets: Vec<HybridBitSet<E>>,
}

/// Triple of sets associated with a given block.
///
/// Generally, one sets up `on_entry`, `gen_set`, and `kill_set` for
/// each block individually, and then runs the dataflow analysis which
/// iteratively modifies the various `on_entry` sets (but leaves the
/// other two sets unchanged, since they represent the effect of the
/// block, which should be invariant over the course of the analysis).
///
/// It is best to ensure that the intersection of `gen_set` and
/// `kill_set` is empty; otherwise the results of the dataflow will
/// have a hidden dependency on what order the bits are generated and
/// killed during the iteration. (This is such a good idea that the
/// `fn gen` and `fn kill` methods that set their state enforce this
/// for you.)
#[derive(Debug)]
pub struct BlockSets<'a, E: Idx> {
    /// Dataflow state immediately before control flow enters the given block.
    pub(crate) on_entry: &'a mut BitSet<E>,

    /// Bits that are set to 1 by the time we exit the given block. Hybrid
    /// because it usually contains only 0 or 1 elements.
    pub(crate) gen_set: &'a mut HybridBitSet<E>,

    /// Bits that are set to 0 by the time we exit the given block. Hybrid
    /// because it usually contains only 0 or 1 elements.
    pub(crate) kill_set: &'a mut HybridBitSet<E>,
}

impl<'a, E:Idx> BlockSets<'a, E> {
    fn gen(&mut self, e: E) {
        self.gen_set.insert(e);
        self.kill_set.remove(e);
    }
    fn gen_all<I>(&mut self, i: I)
        where I: IntoIterator,
              I::Item: Borrow<E>
    {
        for j in i {
            self.gen(*j.borrow());
        }
    }

    fn kill(&mut self, e: E) {
        self.gen_set.remove(e);
        self.kill_set.insert(e);
    }

    fn kill_all<I>(&mut self, i: I)
        where I: IntoIterator,
              I::Item: Borrow<E>
    {
        for j in i {
            self.kill(*j.borrow());
        }
    }

    fn apply_local_effect(&mut self) {
        self.on_entry.union(self.gen_set);
        self.on_entry.subtract(self.kill_set);
    }
}

impl<E:Idx> AllSets<E> {
    pub fn bits_per_block(&self) -> usize { self.bits_per_block }
    pub fn for_block(&mut self, block_idx: usize) -> BlockSets<'_, E> {
        BlockSets {
            on_entry: &mut self.on_entry_sets[block_idx],
            gen_set: &mut self.gen_sets[block_idx],
            kill_set: &mut self.kill_sets[block_idx],
        }
    }

    pub fn on_entry_set_for(&self, block_idx: usize) -> &BitSet<E> {
        &self.on_entry_sets[block_idx]
    }
    pub fn gen_set_for(&self, block_idx: usize) -> &HybridBitSet<E> {
        &self.gen_sets[block_idx]
    }
    pub fn kill_set_for(&self, block_idx: usize) -> &HybridBitSet<E> {
        &self.kill_sets[block_idx]
    }
}

/// Parameterization for the precise form of data flow that is used.
/// `InitialFlow` handles initializing the bitvectors before any
/// code is inspected by the analysis. Analyses that need more nuanced
/// initialization (e.g., they need to consult the results of some other
/// dataflow analysis to set up the initial bitvectors) should not
/// implement this.
pub trait InitialFlow {
    /// Specifies the initial value for each bit in the `on_entry` set
    fn bottom_value() -> bool;
}

pub trait BitDenotation<'tcx>: BitSetOperator {
    /// Specifies what index type is used to access the bitvector.
    type Idx: Idx;

    /// Some analyses want to accumulate knowledge within a block when
    /// analyzing its statements for building the gen/kill sets. Override
    /// this method to return true in such cases.
    ///
    /// When this returns true, the statement-effect (re)construction
    /// will clone the `on_entry` state and pass along a reference via
    /// `sets.on_entry` to that local clone into `statement_effect` and
    /// `terminator_effect`).
    ///
    /// When it's false, no local clone is constructed; instead a
    /// reference directly into `on_entry` is passed along via
    /// `sets.on_entry` instead, which represents the flow state at
    /// the block's start, not necessarily the state immediately prior
    /// to the statement/terminator under analysis.
    ///
    /// In either case, the passed reference is mutable, but this is a
    /// wart from using the `BlockSets` type in the API; the intention
    /// is that the `statement_effect` and `terminator_effect` methods
    /// mutate only the gen/kill sets.
    //
    // FIXME: we should consider enforcing the intention described in
    // the previous paragraph by passing the three sets in separate
    // parameters to encode their distinct mutabilities.
    fn accumulates_intrablock_state() -> bool { false }

    /// A name describing the dataflow analysis that this
    /// `BitDenotation` is supporting. The name should be something
    /// suitable for plugging in as part of a filename (i.e., avoid
    /// space-characters or other things that tend to look bad on a
    /// file system, like slashes or periods). It is also better for
    /// the name to be reasonably short, again because it will be
    /// plugged into a filename.
    fn name() -> &'static str;

    /// Size of each bitvector allocated for each block in the analysis.
    fn bits_per_block(&self) -> usize;

    /// Mutates the entry set according to the effects that
    /// have been established *prior* to entering the start
    /// block. This can't access the gen/kill sets, because
    /// these won't be accounted for correctly.
    ///
    /// (For example, establishing the call arguments.)
    fn start_block_effect(&self, entry_set: &mut BitSet<Self::Idx>);

    /// Similar to `statement_effect`, except it applies
    /// *just before* the statement rather than *just after* it.
    ///
    /// This matters for "dataflow at location" APIs, because the
    /// before-statement effect is visible while visiting the
    /// statement, while the after-statement effect only becomes
    /// visible at the next statement.
    ///
    /// Both the before-statement and after-statement effects are
    /// applied, in that order, before moving for the next
    /// statement.
    fn before_statement_effect(&self,
                               _sets: &mut BlockSets<'_, Self::Idx>,
                               _location: Location) {}

    /// Mutates the block-sets (the flow sets for the given
    /// basic block) according to the effects of evaluating statement.
    ///
    /// This is used, in particular, for building up the
    /// "transfer-function" representing the overall-effect of the
    /// block, represented via GEN and KILL sets.
    ///
    /// The statement is identified as `bb_data[idx_stmt]`, where
    /// `bb_data` is the sequence of statements identified by `bb` in
    /// the MIR.
    fn statement_effect(&self,
                        sets: &mut BlockSets<'_, Self::Idx>,
                        location: Location);

    /// Similar to `terminator_effect`, except it applies
    /// *just before* the terminator rather than *just after* it.
    ///
    /// This matters for "dataflow at location" APIs, because the
    /// before-terminator effect is visible while visiting the
    /// terminator, while the after-terminator effect only becomes
    /// visible at the terminator's successors.
    ///
    /// Both the before-terminator and after-terminator effects are
    /// applied, in that order, before moving for the next
    /// terminator.
    fn before_terminator_effect(&self,
                                _sets: &mut BlockSets<'_, Self::Idx>,
                                _location: Location) {}

    /// Mutates the block-sets (the flow sets for the given
    /// basic block) according to the effects of evaluating
    /// the terminator.
    ///
    /// This is used, in particular, for building up the
    /// "transfer-function" representing the overall-effect of the
    /// block, represented via GEN and KILL sets.
    ///
    /// The effects applied here cannot depend on which branch the
    /// terminator took.
    fn terminator_effect(&self,
                         sets: &mut BlockSets<'_, Self::Idx>,
                         location: Location);

    /// Mutates the block-sets according to the (flow-dependent)
    /// effect of a successful return from a Call terminator.
    ///
    /// If basic-block BB_x ends with a call-instruction that, upon
    /// successful return, flows to BB_y, then this method will be
    /// called on the exit flow-state of BB_x in order to set up the
    /// entry flow-state of BB_y.
    ///
    /// This is used, in particular, as a special case during the
    /// "propagate" loop where all of the basic blocks are repeatedly
    /// visited. Since the effects of a Call terminator are
    /// flow-dependent, the current MIR cannot encode them via just
    /// GEN and KILL sets attached to the block, and so instead we add
    /// this extra machinery to represent the flow-dependent effect.
    //
    // FIXME: right now this is a bit of a wart in the API. It might
    // be better to represent this as an additional gen- and
    // kill-sets associated with each edge coming out of the basic
    // block.
    fn propagate_call_return(
        &self,
        in_out: &mut BitSet<Self::Idx>,
        call_bb: mir::BasicBlock,
        dest_bb: mir::BasicBlock,
        dest_place: &mir::Place<'tcx>,
    );
}

impl<'a, 'tcx, D> DataflowAnalysis<'a, 'tcx, D> where D: BitDenotation<'tcx>
{
    pub fn new(mir: &'a Mir<'tcx>,
               dead_unwinds: &'a BitSet<mir::BasicBlock>,
               denotation: D) -> Self where D: InitialFlow {
        let bits_per_block = denotation.bits_per_block();
        let num_blocks = mir.basic_blocks().len();

        let on_entry_sets = if D::bottom_value() {
            vec![BitSet::new_filled(bits_per_block); num_blocks]
        } else {
            vec![BitSet::new_empty(bits_per_block); num_blocks]
        };
        let gen_sets = vec![HybridBitSet::new_empty(bits_per_block); num_blocks];
        let kill_sets = gen_sets.clone();

        DataflowAnalysis {
            mir,
            dead_unwinds,
            flow_state: DataflowState {
                sets: AllSets {
                    bits_per_block,
                    on_entry_sets,
                    gen_sets,
                    kill_sets,
                },
                operator: denotation,
            }
        }
    }
}

impl<'a, 'tcx: 'a, D> DataflowAnalysis<'a, 'tcx, D> where D: BitDenotation<'tcx> {
    /// Propagates the bits of `in_out` into all the successors of `bb`,
    /// using bitwise operator denoted by `self.operator`.
    ///
    /// For most blocks, this is entirely uniform. However, for blocks
    /// that end with a call terminator, the effect of the call on the
    /// dataflow state may depend on whether the call returned
    /// successfully or unwound.
    ///
    /// To reflect this, the `propagate_call_return` method of the
    /// `BitDenotation` mutates `in_out` when propagating `in_out` via
    /// a call terminator; such mutation is performed *last*, to
    /// ensure its side-effects do not leak elsewhere (e.g., into
    /// unwind target).
    fn propagate_bits_into_graph_successors_of(
        &mut self,
        in_out: &mut BitSet<D::Idx>,
        (bb, bb_data): (mir::BasicBlock, &mir::BasicBlockData<'tcx>),
        dirty_list: &mut WorkQueue<mir::BasicBlock>)
    {
        match bb_data.terminator().kind {
            mir::TerminatorKind::Return |
            mir::TerminatorKind::Resume |
            mir::TerminatorKind::Abort |
            mir::TerminatorKind::GeneratorDrop |
            mir::TerminatorKind::Unreachable => {}
            mir::TerminatorKind::Goto { target } |
            mir::TerminatorKind::Assert { target, cleanup: None, .. } |
            mir::TerminatorKind::Yield { resume: target, drop: None, .. } |
            mir::TerminatorKind::Drop { target, location: _, unwind: None } |
            mir::TerminatorKind::DropAndReplace {
                target, value: _, location: _, unwind: None
            } => {
                self.propagate_bits_into_entry_set_for(in_out, target, dirty_list);
            }
            mir::TerminatorKind::Yield { resume: target, drop: Some(drop), .. } => {
                self.propagate_bits_into_entry_set_for(in_out, target, dirty_list);
                self.propagate_bits_into_entry_set_for(in_out, drop, dirty_list);
            }
            mir::TerminatorKind::Assert { target, cleanup: Some(unwind), .. } |
            mir::TerminatorKind::Drop { target, location: _, unwind: Some(unwind) } |
            mir::TerminatorKind::DropAndReplace {
                target, value: _, location: _, unwind: Some(unwind)
            } => {
                self.propagate_bits_into_entry_set_for(in_out, target, dirty_list);
                if !self.dead_unwinds.contains(bb) {
                    self.propagate_bits_into_entry_set_for(in_out, unwind, dirty_list);
                }
            }
            mir::TerminatorKind::SwitchInt { ref targets, .. } => {
                for target in targets {
                    self.propagate_bits_into_entry_set_for(in_out, *target, dirty_list);
                }
            }
            mir::TerminatorKind::Call { cleanup, ref destination, .. } => {
                if let Some(unwind) = cleanup {
                    if !self.dead_unwinds.contains(bb) {
                        self.propagate_bits_into_entry_set_for(in_out, unwind, dirty_list);
                    }
                }
                if let Some((ref dest_place, dest_bb)) = *destination {
                    // N.B.: This must be done *last*, after all other
                    // propagation, as documented in comment above.
                    self.flow_state.operator.propagate_call_return(
                        in_out, bb, dest_bb, dest_place);
                    self.propagate_bits_into_entry_set_for(in_out, dest_bb, dirty_list);
                }
            }
            mir::TerminatorKind::FalseEdges { real_target, ref imaginary_targets } => {
                self.propagate_bits_into_entry_set_for(in_out, real_target, dirty_list);
                for target in imaginary_targets {
                    self.propagate_bits_into_entry_set_for(in_out, *target, dirty_list);
                }
            }
            mir::TerminatorKind::FalseUnwind { real_target, unwind } => {
                self.propagate_bits_into_entry_set_for(in_out, real_target, dirty_list);
                if let Some(unwind) = unwind {
                    if !self.dead_unwinds.contains(bb) {
                        self.propagate_bits_into_entry_set_for(in_out, unwind, dirty_list);
                    }
                }
            }
        }
    }

    fn propagate_bits_into_entry_set_for(&mut self,
                                         in_out: &BitSet<D::Idx>,
                                         bb: mir::BasicBlock,
                                         dirty_queue: &mut WorkQueue<mir::BasicBlock>) {
        let entry_set = &mut self.flow_state.sets.for_block(bb.index()).on_entry;
        let set_changed = self.flow_state.operator.join(entry_set, &in_out);
        if set_changed {
            dirty_queue.insert(bb);
        }
    }
}