Move rustc_mir::transform to rustc_mir_transform.

1 files changed, 0 insertions, 769 deletions

diff --git a/compiler/rustc_mir/src/transform/coverage/graph.rs b/compiler/rustc_mir/src/transform/coverage/graph.rs
deleted file mode 100644
index d78ad6ce97f..00000000000
--- a/compiler/rustc_mir/src/transform/coverage/graph.rs
+++ /dev/null
@@ -1,769 +0,0 @@
-use super::Error;
-
-use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::graph::dominators::{self, Dominators};
-use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes, WithStartNode};
-use rustc_index::bit_set::BitSet;
-use rustc_index::vec::IndexVec;
-use rustc_middle::mir::coverage::*;
-use rustc_middle::mir::{self, BasicBlock, BasicBlockData, Terminator, TerminatorKind};
-
-use std::ops::{Index, IndexMut};
-
-const ID_SEPARATOR: &str = ",";
-
-/// A coverage-specific simplification of the MIR control flow graph (CFG). The `CoverageGraph`s
-/// nodes are `BasicCoverageBlock`s, which encompass one or more MIR `BasicBlock`s, plus a
-/// `CoverageKind` counter (to be added by `CoverageCounters::make_bcb_counters`), and an optional
-/// set of additional counters--if needed--to count incoming edges, if there are more than one.
-/// (These "edge counters" are eventually converted into new MIR `BasicBlock`s.)
-#[derive(Debug)]
-pub(super) struct CoverageGraph {
-    bcbs: IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
-    bb_to_bcb: IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
-    pub successors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
-    pub predecessors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
-    dominators: Option<Dominators<BasicCoverageBlock>>,
-}
-
-impl CoverageGraph {
-    pub fn from_mir(mir_body: &mir::Body<'tcx>) -> Self {
-        let (bcbs, bb_to_bcb) = Self::compute_basic_coverage_blocks(mir_body);
-
-        // Pre-transform MIR `BasicBlock` successors and predecessors into the BasicCoverageBlock
-        // equivalents. Note that since the BasicCoverageBlock graph has been fully simplified, the
-        // each predecessor of a BCB leader_bb should be in a unique BCB. It is possible for a
-        // `SwitchInt` to have multiple targets to the same destination `BasicBlock`, so
-        // de-duplication is required. This is done without reordering the successors.
-
-        let bcbs_len = bcbs.len();
-        let mut seen = IndexVec::from_elem_n(false, bcbs_len);
-        let successors = IndexVec::from_fn_n(
-            |bcb| {
-                for b in seen.iter_mut() {
-                    *b = false;
-                }
-                let bcb_data = &bcbs[bcb];
-                let mut bcb_successors = Vec::new();
-                for successor in
-                    bcb_filtered_successors(&mir_body, &bcb_data.terminator(mir_body).kind)
-                        .filter_map(|&successor_bb| bb_to_bcb[successor_bb])
-                {
-                    if !seen[successor] {
-                        seen[successor] = true;
-                        bcb_successors.push(successor);
-                    }
-                }
-                bcb_successors
-            },
-            bcbs.len(),
-        );
-
-        let mut predecessors = IndexVec::from_elem_n(Vec::new(), bcbs.len());
-        for (bcb, bcb_successors) in successors.iter_enumerated() {
-            for &successor in bcb_successors {
-                predecessors[successor].push(bcb);
-            }
-        }
-
-        let mut basic_coverage_blocks =
-            Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None };
-        let dominators = dominators::dominators(&basic_coverage_blocks);
-        basic_coverage_blocks.dominators = Some(dominators);
-        basic_coverage_blocks
-    }
-
-    fn compute_basic_coverage_blocks(
-        mir_body: &mir::Body<'tcx>,
-    ) -> (
-        IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
-        IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
-    ) {
-        let num_basic_blocks = mir_body.num_nodes();
-        let mut bcbs = IndexVec::with_capacity(num_basic_blocks);
-        let mut bb_to_bcb = IndexVec::from_elem_n(None, num_basic_blocks);
-
-        // Walk the MIR CFG using a Preorder traversal, which starts from `START_BLOCK` and follows
-        // each block terminator's `successors()`. Coverage spans must map to actual source code,
-        // so compiler generated blocks and paths can be ignored. To that end, the CFG traversal
-        // intentionally omits unwind paths.
-        // FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and
-        // `catch_unwind()` handlers.
-        let mir_cfg_without_unwind = ShortCircuitPreorder::new(&mir_body, bcb_filtered_successors);
-
-        let mut basic_blocks = Vec::new();
-        for (bb, data) in mir_cfg_without_unwind {
-            if let Some(last) = basic_blocks.last() {
-                let predecessors = &mir_body.predecessors()[bb];
-                if predecessors.len() > 1 || !predecessors.contains(last) {
-                    // The `bb` has more than one _incoming_ edge, and should start its own
-                    // `BasicCoverageBlockData`. (Note, the `basic_blocks` vector does not yet
-                    // include `bb`; it contains a sequence of one or more sequential basic_blocks
-                    // with no intermediate branches in or out. Save these as a new
-                    // `BasicCoverageBlockData` before starting the new one.)
-                    Self::add_basic_coverage_block(
-                        &mut bcbs,
-                        &mut bb_to_bcb,
-                        basic_blocks.split_off(0),
-                    );
-                    debug!(
-                        "  because {}",
-                        if predecessors.len() > 1 {
-                            "predecessors.len() > 1".to_owned()
-                        } else {
-                            format!("bb {} is not in precessors: {:?}", bb.index(), predecessors)
-                        }
-                    );
-                }
-            }
-            basic_blocks.push(bb);
-
-            let term = data.terminator();
-
-            match term.kind {
-                TerminatorKind::Return { .. }
-                | TerminatorKind::Abort
-                | TerminatorKind::Yield { .. }
-                | TerminatorKind::SwitchInt { .. } => {
-                    // The `bb` has more than one _outgoing_ edge, or exits the function. Save the
-                    // current sequence of `basic_blocks` gathered to this point, as a new
-                    // `BasicCoverageBlockData`.
-                    Self::add_basic_coverage_block(
-                        &mut bcbs,
-                        &mut bb_to_bcb,
-                        basic_blocks.split_off(0),
-                    );
-                    debug!("  because term.kind = {:?}", term.kind);
-                    // Note that this condition is based on `TerminatorKind`, even though it
-                    // theoretically boils down to `successors().len() != 1`; that is, either zero
-                    // (e.g., `Return`, `Abort`) or multiple successors (e.g., `SwitchInt`), but
-                    // since the BCB CFG ignores things like unwind branches (which exist in the
-                    // `Terminator`s `successors()` list) checking the number of successors won't
-                    // work.
-                }
-
-                // The following `TerminatorKind`s are either not expected outside an unwind branch,
-                // or they should not (under normal circumstances) branch. Coverage graphs are
-                // simplified by assuring coverage results are accurate for program executions that
-                // don't panic.
-                //
-                // Programs that panic and unwind may record slightly inaccurate coverage results
-                // for a coverage region containing the `Terminator` that began the panic. This
-                // is as intended. (See Issue #78544 for a possible future option to support
-                // coverage in test programs that panic.)
-                TerminatorKind::Goto { .. }
-                | TerminatorKind::Resume
-                | TerminatorKind::Unreachable
-                | TerminatorKind::Drop { .. }
-                | TerminatorKind::DropAndReplace { .. }
-                | TerminatorKind::Call { .. }
-                | TerminatorKind::GeneratorDrop
-                | TerminatorKind::Assert { .. }
-                | TerminatorKind::FalseEdge { .. }
-                | TerminatorKind::FalseUnwind { .. }
-                | TerminatorKind::InlineAsm { .. } => {}
-            }
-        }
-
-        if !basic_blocks.is_empty() {
-            // process any remaining basic_blocks into a final `BasicCoverageBlockData`
-            Self::add_basic_coverage_block(&mut bcbs, &mut bb_to_bcb, basic_blocks.split_off(0));
-            debug!("  because the end of the MIR CFG was reached while traversing");
-        }
-
-        (bcbs, bb_to_bcb)
-    }
-
-    fn add_basic_coverage_block(
-        bcbs: &mut IndexVec<BasicCoverageBlock, BasicCoverageBlockData>,
-        bb_to_bcb: &mut IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
-        basic_blocks: Vec<BasicBlock>,
-    ) {
-        let bcb = BasicCoverageBlock::from_usize(bcbs.len());
-        for &bb in basic_blocks.iter() {
-            bb_to_bcb[bb] = Some(bcb);
-        }
-        let bcb_data = BasicCoverageBlockData::from(basic_blocks);
-        debug!("adding bcb{}: {:?}", bcb.index(), bcb_data);
-        bcbs.push(bcb_data);
-    }
-
-    #[inline(always)]
-    pub fn iter_enumerated(
-        &self,
-    ) -> impl Iterator<Item = (BasicCoverageBlock, &BasicCoverageBlockData)> {
-        self.bcbs.iter_enumerated()
-    }
-
-    #[inline(always)]
-    pub fn iter_enumerated_mut(
-        &mut self,
-    ) -> impl Iterator<Item = (BasicCoverageBlock, &mut BasicCoverageBlockData)> {
-        self.bcbs.iter_enumerated_mut()
-    }
-
-    #[inline(always)]
-    pub fn bcb_from_bb(&self, bb: BasicBlock) -> Option<BasicCoverageBlock> {
-        if bb.index() < self.bb_to_bcb.len() { self.bb_to_bcb[bb] } else { None }
-    }
-
-    #[inline(always)]
-    pub fn is_dominated_by(&self, node: BasicCoverageBlock, dom: BasicCoverageBlock) -> bool {
-        self.dominators.as_ref().unwrap().is_dominated_by(node, dom)
-    }
-
-    #[inline(always)]
-    pub fn dominators(&self) -> &Dominators<BasicCoverageBlock> {
-        self.dominators.as_ref().unwrap()
-    }
-}
-
-impl Index<BasicCoverageBlock> for CoverageGraph {
-    type Output = BasicCoverageBlockData;
-
-    #[inline]
-    fn index(&self, index: BasicCoverageBlock) -> &BasicCoverageBlockData {
-        &self.bcbs[index]
-    }
-}
-
-impl IndexMut<BasicCoverageBlock> for CoverageGraph {
-    #[inline]
-    fn index_mut(&mut self, index: BasicCoverageBlock) -> &mut BasicCoverageBlockData {
-        &mut self.bcbs[index]
-    }
-}
-
-impl graph::DirectedGraph for CoverageGraph {
-    type Node = BasicCoverageBlock;
-}
-
-impl graph::WithNumNodes for CoverageGraph {
-    #[inline]
-    fn num_nodes(&self) -> usize {
-        self.bcbs.len()
-    }
-}
-
-impl graph::WithStartNode for CoverageGraph {
-    #[inline]
-    fn start_node(&self) -> Self::Node {
-        self.bcb_from_bb(mir::START_BLOCK)
-            .expect("mir::START_BLOCK should be in a BasicCoverageBlock")
-    }
-}
-
-type BcbSuccessors<'graph> = std::slice::Iter<'graph, BasicCoverageBlock>;
-
-impl<'graph> graph::GraphSuccessors<'graph> for CoverageGraph {
-    type Item = BasicCoverageBlock;
-    type Iter = std::iter::Cloned<BcbSuccessors<'graph>>;
-}
-
-impl graph::WithSuccessors for CoverageGraph {
-    #[inline]
-    fn successors(&self, node: Self::Node) -> <Self as GraphSuccessors<'_>>::Iter {
-        self.successors[node].iter().cloned()
-    }
-}
-
-impl graph::GraphPredecessors<'graph> for CoverageGraph {
-    type Item = BasicCoverageBlock;
-    type Iter = std::iter::Copied<std::slice::Iter<'graph, BasicCoverageBlock>>;
-}
-
-impl graph::WithPredecessors for CoverageGraph {
-    #[inline]
-    fn predecessors(&self, node: Self::Node) -> <Self as graph::GraphPredecessors<'_>>::Iter {
-        self.predecessors[node].iter().copied()
-    }
-}
-
-rustc_index::newtype_index! {
-    /// A node in the [control-flow graph][CFG] of CoverageGraph.
-    pub(super) struct BasicCoverageBlock {
-        DEBUG_FORMAT = "bcb{}",
-        const START_BCB = 0,
-    }
-}
-
-/// `BasicCoverageBlockData` holds the data indexed by a `BasicCoverageBlock`.
-///
-/// A `BasicCoverageBlock` (BCB) represents the maximal-length sequence of MIR `BasicBlock`s without
-/// conditional branches, and form a new, simplified, coverage-specific Control Flow Graph, without
-/// altering the original MIR CFG.
-///
-/// Note that running the MIR `SimplifyCfg` transform is not sufficient (and therefore not
-/// necessary). The BCB-based CFG is a more aggressive simplification. For example:
-///
-///   * The BCB CFG ignores (trims) branches not relevant to coverage, such as unwind-related code,
-///     that is injected by the Rust compiler but has no physical source code to count. This also
-///     means a BasicBlock with a `Call` terminator can be merged into its primary successor target
-///     block, in the same BCB. (But, note: Issue #78544: "MIR InstrumentCoverage: Improve coverage
-///     of `#[should_panic]` tests and `catch_unwind()` handlers")
-///   * Some BasicBlock terminators support Rust-specific concerns--like borrow-checking--that are
-///     not relevant to coverage analysis. `FalseUnwind`, for example, can be treated the same as
-///     a `Goto`, and merged with its successor into the same BCB.
-///
-/// Each BCB with at least one computed `CoverageSpan` will have no more than one `Counter`.
-/// In some cases, a BCB's execution count can be computed by `Expression`. Additional
-/// disjoint `CoverageSpan`s in a BCB can also be counted by `Expression` (by adding `ZERO`
-/// to the BCB's primary counter or expression).
-///
-/// The BCB CFG is critical to simplifying the coverage analysis by ensuring graph path-based
-/// queries (`is_dominated_by()`, `predecessors`, `successors`, etc.) have branch (control flow)
-/// significance.
-#[derive(Debug, Clone)]
-pub(super) struct BasicCoverageBlockData {
-    pub basic_blocks: Vec<BasicBlock>,
-    pub counter_kind: Option<CoverageKind>,
-    edge_from_bcbs: Option<FxHashMap<BasicCoverageBlock, CoverageKind>>,
-}
-
-impl BasicCoverageBlockData {
-    pub fn from(basic_blocks: Vec<BasicBlock>) -> Self {
-        assert!(basic_blocks.len() > 0);
-        Self { basic_blocks, counter_kind: None, edge_from_bcbs: None }
-    }
-
-    #[inline(always)]
-    pub fn leader_bb(&self) -> BasicBlock {
-        self.basic_blocks[0]
-    }
-
-    #[inline(always)]
-    pub fn last_bb(&self) -> BasicBlock {
-        *self.basic_blocks.last().unwrap()
-    }
-
-    #[inline(always)]
-    pub fn terminator<'a, 'tcx>(&self, mir_body: &'a mir::Body<'tcx>) -> &'a Terminator<'tcx> {
-        &mir_body[self.last_bb()].terminator()
-    }
-
-    pub fn set_counter(
-        &mut self,
-        counter_kind: CoverageKind,
-    ) -> Result<ExpressionOperandId, Error> {
-        debug_assert!(
-            // If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also
-            // have an expression (to be injected into an existing `BasicBlock` represented by this
-            // `BasicCoverageBlock`).
-            self.edge_from_bcbs.is_none() || counter_kind.is_expression(),
-            "attempt to add a `Counter` to a BCB target with existing incoming edge counters"
-        );
-        let operand = counter_kind.as_operand_id();
-        if let Some(replaced) = self.counter_kind.replace(counter_kind) {
-            Error::from_string(format!(
-                "attempt to set a BasicCoverageBlock coverage counter more than once; \
-                {:?} already had counter {:?}",
-                self, replaced,
-            ))
-        } else {
-            Ok(operand)
-        }
-    }
-
-    #[inline(always)]
-    pub fn counter(&self) -> Option<&CoverageKind> {
-        self.counter_kind.as_ref()
-    }
-
-    #[inline(always)]
-    pub fn take_counter(&mut self) -> Option<CoverageKind> {
-        self.counter_kind.take()
-    }
-
-    pub fn set_edge_counter_from(
-        &mut self,
-        from_bcb: BasicCoverageBlock,
-        counter_kind: CoverageKind,
-    ) -> Result<ExpressionOperandId, Error> {
-        if level_enabled!(tracing::Level::DEBUG) {
-            // If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also
-            // have an expression (to be injected into an existing `BasicBlock` represented by this
-            // `BasicCoverageBlock`).
-            if !self.counter_kind.as_ref().map_or(true, |c| c.is_expression()) {
-                return Error::from_string(format!(
-                    "attempt to add an incoming edge counter from {:?} when the target BCB already \
-                    has a `Counter`",
-                    from_bcb
-                ));
-            }
-        }
-        let operand = counter_kind.as_operand_id();
-        if let Some(replaced) =
-            self.edge_from_bcbs.get_or_insert_default().insert(from_bcb, counter_kind)
-        {
-            Error::from_string(format!(
-                "attempt to set an edge counter more than once; from_bcb: \
-                {:?} already had counter {:?}",
-                from_bcb, replaced,
-            ))
-        } else {
-            Ok(operand)
-        }
-    }
-
-    #[inline]
-    pub fn edge_counter_from(&self, from_bcb: BasicCoverageBlock) -> Option<&CoverageKind> {
-        if let Some(edge_from_bcbs) = &self.edge_from_bcbs {
-            edge_from_bcbs.get(&from_bcb)
-        } else {
-            None
-        }
-    }
-
-    #[inline]
-    pub fn take_edge_counters(
-        &mut self,
-    ) -> Option<impl Iterator<Item = (BasicCoverageBlock, CoverageKind)>> {
-        self.edge_from_bcbs.take().map_or(None, |m| Some(m.into_iter()))
-    }
-
-    pub fn id(&self) -> String {
-        format!(
-            "@{}",
-            self.basic_blocks
-                .iter()
-                .map(|bb| bb.index().to_string())
-                .collect::<Vec<_>>()
-                .join(ID_SEPARATOR)
-        )
-    }
-}
-
-/// Represents a successor from a branching BasicCoverageBlock (such as the arms of a `SwitchInt`)
-/// as either the successor BCB itself, if it has only one incoming edge, or the successor _plus_
-/// the specific branching BCB, representing the edge between the two. The latter case
-/// distinguishes this incoming edge from other incoming edges to the same `target_bcb`.
-#[derive(Clone, Copy, PartialEq, Eq)]
-pub(super) struct BcbBranch {
-    pub edge_from_bcb: Option<BasicCoverageBlock>,
-    pub target_bcb: BasicCoverageBlock,
-}
-
-impl BcbBranch {
-    pub fn from_to(
-        from_bcb: BasicCoverageBlock,
-        to_bcb: BasicCoverageBlock,
-        basic_coverage_blocks: &CoverageGraph,
-    ) -> Self {
-        let edge_from_bcb = if basic_coverage_blocks.predecessors[to_bcb].len() > 1 {
-            Some(from_bcb)
-        } else {
-            None
-        };
-        Self { edge_from_bcb, target_bcb: to_bcb }
-    }
-
-    pub fn counter<'a>(
-        &self,
-        basic_coverage_blocks: &'a CoverageGraph,
-    ) -> Option<&'a CoverageKind> {
-        if let Some(from_bcb) = self.edge_from_bcb {
-            basic_coverage_blocks[self.target_bcb].edge_counter_from(from_bcb)
-        } else {
-            basic_coverage_blocks[self.target_bcb].counter()
-        }
-    }
-
-    pub fn is_only_path_to_target(&self) -> bool {
-        self.edge_from_bcb.is_none()
-    }
-}
-
-impl std::fmt::Debug for BcbBranch {
-    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        if let Some(from_bcb) = self.edge_from_bcb {
-            write!(fmt, "{:?}->{:?}", from_bcb, self.target_bcb)
-        } else {
-            write!(fmt, "{:?}", self.target_bcb)
-        }
-    }
-}
-
-// Returns the `Terminator`s non-unwind successors.
-// FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and
-// `catch_unwind()` handlers.
-fn bcb_filtered_successors<'a, 'tcx>(
-    body: &'tcx &'a mir::Body<'tcx>,
-    term_kind: &'tcx TerminatorKind<'tcx>,
-) -> Box<dyn Iterator<Item = &'a BasicBlock> + 'a> {
-    let mut successors = term_kind.successors();
-    Box::new(
-        match &term_kind {
-            // SwitchInt successors are never unwind, and all of them should be traversed.
-            TerminatorKind::SwitchInt { .. } => successors,
-            // For all other kinds, return only the first successor, if any, and ignore unwinds.
-            // NOTE: `chain(&[])` is required to coerce the `option::iter` (from
-            // `next().into_iter()`) into the `mir::Successors` aliased type.
-            _ => successors.next().into_iter().chain(&[]),
-        }
-        .filter(move |&&successor| {
-            body[successor].terminator().kind != TerminatorKind::Unreachable
-        }),
-    )
-}
-
-/// Maintains separate worklists for each loop in the BasicCoverageBlock CFG, plus one for the
-/// CoverageGraph outside all loops. This supports traversing the BCB CFG in a way that
-/// ensures a loop is completely traversed before processing Blocks after the end of the loop.
-#[derive(Debug)]
-pub(super) struct TraversalContext {
-    /// From one or more backedges returning to a loop header.
-    pub loop_backedges: Option<(Vec<BasicCoverageBlock>, BasicCoverageBlock)>,
-
-    /// worklist, to be traversed, of CoverageGraph in the loop with the given loop
-    /// backedges, such that the loop is the inner inner-most loop containing these
-    /// CoverageGraph
-    pub worklist: Vec<BasicCoverageBlock>,
-}
-
-pub(super) struct TraverseCoverageGraphWithLoops {
-    pub backedges: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
-    pub context_stack: Vec<TraversalContext>,
-    visited: BitSet<BasicCoverageBlock>,
-}
-
-impl TraverseCoverageGraphWithLoops {
-    pub fn new(basic_coverage_blocks: &CoverageGraph) -> Self {
-        let start_bcb = basic_coverage_blocks.start_node();
-        let backedges = find_loop_backedges(basic_coverage_blocks);
-        let context_stack =
-            vec![TraversalContext { loop_backedges: None, worklist: vec![start_bcb] }];
-        // `context_stack` starts with a `TraversalContext` for the main function context (beginning
-        // with the `start` BasicCoverageBlock of the function). New worklists are pushed to the top
-        // of the stack as loops are entered, and popped off of the stack when a loop's worklist is
-        // exhausted.
-        let visited = BitSet::new_empty(basic_coverage_blocks.num_nodes());
-        Self { backedges, context_stack, visited }
-    }
-
-    pub fn next(&mut self, basic_coverage_blocks: &CoverageGraph) -> Option<BasicCoverageBlock> {
-        debug!(
-            "TraverseCoverageGraphWithLoops::next - context_stack: {:?}",
-            self.context_stack.iter().rev().collect::<Vec<_>>()
-        );
-        while let Some(next_bcb) = {
-            // Strip contexts with empty worklists from the top of the stack
-            while self.context_stack.last().map_or(false, |context| context.worklist.is_empty()) {
-                self.context_stack.pop();
-            }
-            // Pop the next bcb off of the current context_stack. If none, all BCBs were visited.
-            self.context_stack.last_mut().map_or(None, |context| context.worklist.pop())
-        } {
-            if !self.visited.insert(next_bcb) {
-                debug!("Already visited: {:?}", next_bcb);
-                continue;
-            }
-            debug!("Visiting {:?}", next_bcb);
-            if self.backedges[next_bcb].len() > 0 {
-                debug!("{:?} is a loop header! Start a new TraversalContext...", next_bcb);
-                self.context_stack.push(TraversalContext {
-                    loop_backedges: Some((self.backedges[next_bcb].clone(), next_bcb)),
-                    worklist: Vec::new(),
-                });
-            }
-            self.extend_worklist(basic_coverage_blocks, next_bcb);
-            return Some(next_bcb);
-        }
-        None
-    }
-
-    pub fn extend_worklist(
-        &mut self,
-        basic_coverage_blocks: &CoverageGraph,
-        bcb: BasicCoverageBlock,
-    ) {
-        let successors = &basic_coverage_blocks.successors[bcb];
-        debug!("{:?} has {} successors:", bcb, successors.len());
-        for &successor in successors {
-            if successor == bcb {
-                debug!(
-                    "{:?} has itself as its own successor. (Note, the compiled code will \
-                    generate an infinite loop.)",
-                    bcb
-                );
-                // Don't re-add this successor to the worklist. We are already processing it.
-                break;
-            }
-            for context in self.context_stack.iter_mut().rev() {
-                // Add successors of the current BCB to the appropriate context. Successors that
-                // stay within a loop are added to the BCBs context worklist. Successors that
-                // exit the loop (they are not dominated by the loop header) must be reachable
-                // from other BCBs outside the loop, and they will be added to a different
-                // worklist.
-                //
-                // Branching blocks (with more than one successor) must be processed before
-                // blocks with only one successor, to prevent unnecessarily complicating
-                // `Expression`s by creating a Counter in a `BasicCoverageBlock` that the
-                // branching block would have given an `Expression` (or vice versa).
-                let (some_successor_to_add, some_loop_header) =
-                    if let Some((_, loop_header)) = context.loop_backedges {
-                        if basic_coverage_blocks.is_dominated_by(successor, loop_header) {
-                            (Some(successor), Some(loop_header))
-                        } else {
-                            (None, None)
-                        }
-                    } else {
-                        (Some(successor), None)
-                    };
-                if let Some(successor_to_add) = some_successor_to_add {
-                    if basic_coverage_blocks.successors[successor_to_add].len() > 1 {
-                        debug!(
-                            "{:?} successor is branching. Prioritize it at the beginning of \
-                            the {}",
-                            successor_to_add,
-                            if let Some(loop_header) = some_loop_header {
-                                format!("worklist for the loop headed by {:?}", loop_header)
-                            } else {
-                                String::from("non-loop worklist")
-                            },
-                        );
-                        context.worklist.insert(0, successor_to_add);
-                    } else {
-                        debug!(
-                            "{:?} successor is non-branching. Defer it to the end of the {}",
-                            successor_to_add,
-                            if let Some(loop_header) = some_loop_header {
-                                format!("worklist for the loop headed by {:?}", loop_header)
-                            } else {
-                                String::from("non-loop worklist")
-                            },
-                        );
-                        context.worklist.push(successor_to_add);
-                    }
-                    break;
-                }
-            }
-        }
-    }
-
-    pub fn is_complete(&self) -> bool {
-        self.visited.count() == self.visited.domain_size()
-    }
-
-    pub fn unvisited(&self) -> Vec<BasicCoverageBlock> {
-        let mut unvisited_set: BitSet<BasicCoverageBlock> =
-            BitSet::new_filled(self.visited.domain_size());
-        unvisited_set.subtract(&self.visited);
-        unvisited_set.iter().collect::<Vec<_>>()
-    }
-}
-
-pub(super) fn find_loop_backedges(
-    basic_coverage_blocks: &CoverageGraph,
-) -> IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>> {
-    let num_bcbs = basic_coverage_blocks.num_nodes();
-    let mut backedges = IndexVec::from_elem_n(Vec::<BasicCoverageBlock>::new(), num_bcbs);
-
-    // Identify loops by their backedges.
-    //
-    // The computational complexity is bounded by: n(s) x d where `n` is the number of
-    // `BasicCoverageBlock` nodes (the simplified/reduced representation of the CFG derived from the
-    // MIR); `s` is the average number of successors per node (which is most likely less than 2, and
-    // independent of the size of the function, so it can be treated as a constant);
-    // and `d` is the average number of dominators per node.
-    //
-    // The average number of dominators depends on the size and complexity of the function, and
-    // nodes near the start of the function's control flow graph typically have less dominators
-    // than nodes near the end of the CFG. Without doing a detailed mathematical analysis, I
-    // think the resulting complexity has the characteristics of O(n log n).
-    //
-    // The overall complexity appears to be comparable to many other MIR transform algorithms, and I
-    // don't expect that this function is creating a performance hot spot, but if this becomes an
-    // issue, there may be ways to optimize the `is_dominated_by` algorithm (as indicated by an
-    // existing `FIXME` comment in that code), or possibly ways to optimize it's usage here, perhaps
-    // by keeping track of results for visited `BasicCoverageBlock`s if they can be used to short
-    // circuit downstream `is_dominated_by` checks.
-    //
-    // For now, that kind of optimization seems unnecessarily complicated.
-    for (bcb, _) in basic_coverage_blocks.iter_enumerated() {
-        for &successor in &basic_coverage_blocks.successors[bcb] {
-            if basic_coverage_blocks.is_dominated_by(bcb, successor) {
-                let loop_header = successor;
-                let backedge_from_bcb = bcb;
-                debug!(
-                    "Found BCB backedge: {:?} -> loop_header: {:?}",
-                    backedge_from_bcb, loop_header
-                );
-                backedges[loop_header].push(backedge_from_bcb);
-            }
-        }
-    }
-    backedges
-}
-
-pub struct ShortCircuitPreorder<
-    'a,
-    'tcx,
-    F: Fn(
-        &'tcx &'a mir::Body<'tcx>,
-        &'tcx TerminatorKind<'tcx>,
-    ) -> Box<dyn Iterator<Item = &'a BasicBlock> + 'a>,
-> {
-    body: &'tcx &'a mir::Body<'tcx>,
-    visited: BitSet<BasicBlock>,
-    worklist: Vec<BasicBlock>,
-    filtered_successors: F,
-}
-
-impl<
-    'a,
-    'tcx,
-    F: Fn(
-        &'tcx &'a mir::Body<'tcx>,
-        &'tcx TerminatorKind<'tcx>,
-    ) -> Box<dyn Iterator<Item = &'a BasicBlock> + 'a>,
-> ShortCircuitPreorder<'a, 'tcx, F>
-{
-    pub fn new(
-        body: &'tcx &'a mir::Body<'tcx>,
-        filtered_successors: F,
-    ) -> ShortCircuitPreorder<'a, 'tcx, F> {
-        let worklist = vec![mir::START_BLOCK];
-
-        ShortCircuitPreorder {
-            body,
-            visited: BitSet::new_empty(body.basic_blocks().len()),
-            worklist,
-            filtered_successors,
-        }
-    }
-}
-
-impl<
-    'a: 'tcx,
-    'tcx,
-    F: Fn(
-        &'tcx &'a mir::Body<'tcx>,
-        &'tcx TerminatorKind<'tcx>,
-    ) -> Box<dyn Iterator<Item = &'a BasicBlock> + 'a>,
-> Iterator for ShortCircuitPreorder<'a, 'tcx, F>
-{
-    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
-
-    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
-        while let Some(idx) = self.worklist.pop() {
-            if !self.visited.insert(idx) {
-                continue;
-            }
-
-            let data = &self.body[idx];
-
-            if let Some(ref term) = data.terminator {
-                self.worklist.extend((self.filtered_successors)(&self.body, &term.kind));
-            }
-
-            return Some((idx, data));
-        }
-
-        None
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let size = self.body.basic_blocks().len() - self.visited.count();
-        (size, Some(size))
-    }
-}