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diff --git a/compiler/rustc_data_structures/src/graph/implementation/mod.rs b/compiler/rustc_data_structures/src/graph/implementation/mod.rs
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+++ b/compiler/rustc_data_structures/src/graph/implementation/mod.rs
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+//! A graph module for use in dataflow, region resolution, and elsewhere.
+//!
+//! # Interface details
+//!
+//! You customize the graph by specifying a "node data" type `N` and an
+//! "edge data" type `E`. You can then later gain access (mutable or
+//! immutable) to these "user-data" bits. Currently, you can only add
+//! nodes or edges to the graph. You cannot remove or modify them once
+//! added. This could be changed if we have a need.
+//!
+//! # Implementation details
+//!
+//! The main tricky thing about this code is the way that edges are
+//! stored. The edges are stored in a central array, but they are also
+//! threaded onto two linked lists for each node, one for incoming edges
+//! and one for outgoing edges. Note that every edge is a member of some
+//! incoming list and some outgoing list. Basically you can load the
+//! first index of the linked list from the node data structures (the
+//! field `first_edge`) and then, for each edge, load the next index from
+//! the field `next_edge`). Each of those fields is an array that should
+//! be indexed by the direction (see the type `Direction`).
+
+use crate::snapshot_vec::{SnapshotVec, SnapshotVecDelegate};
+use rustc_index::bit_set::BitSet;
+use std::fmt::Debug;
+
+#[cfg(test)]
+mod tests;
+
+pub struct Graph<N, E> {
+    nodes: SnapshotVec<Node<N>>,
+    edges: SnapshotVec<Edge<E>>,
+}
+
+pub struct Node<N> {
+    first_edge: [EdgeIndex; 2], // see module comment
+    pub data: N,
+}
+
+#[derive(Debug)]
+pub struct Edge<E> {
+    next_edge: [EdgeIndex; 2], // see module comment
+    source: NodeIndex,
+    target: NodeIndex,
+    pub data: E,
+}
+
+impl<N> SnapshotVecDelegate for Node<N> {
+    type Value = Node<N>;
+    type Undo = ();
+
+    fn reverse(_: &mut Vec<Node<N>>, _: ()) {}
+}
+
+impl<N> SnapshotVecDelegate for Edge<N> {
+    type Value = Edge<N>;
+    type Undo = ();
+
+    fn reverse(_: &mut Vec<Edge<N>>, _: ()) {}
+}
+
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub struct NodeIndex(pub usize);
+
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub struct EdgeIndex(pub usize);
+
+pub const INVALID_EDGE_INDEX: EdgeIndex = EdgeIndex(usize::MAX);
+
+// Use a private field here to guarantee no more instances are created:
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Direction {
+    repr: usize,
+}
+
+pub const OUTGOING: Direction = Direction { repr: 0 };
+
+pub const INCOMING: Direction = Direction { repr: 1 };
+
+impl NodeIndex {
+    /// Returns unique ID (unique with respect to the graph holding associated node).
+    pub fn node_id(self) -> usize {
+        self.0
+    }
+}
+
+impl<N: Debug, E: Debug> Graph<N, E> {
+    pub fn new() -> Graph<N, E> {
+        Graph { nodes: SnapshotVec::new(), edges: SnapshotVec::new() }
+    }
+
+    pub fn with_capacity(nodes: usize, edges: usize) -> Graph<N, E> {
+        Graph { nodes: SnapshotVec::with_capacity(nodes), edges: SnapshotVec::with_capacity(edges) }
+    }
+
+    // # Simple accessors
+
+    #[inline]
+    pub fn all_nodes(&self) -> &[Node<N>] {
+        &self.nodes
+    }
+
+    #[inline]
+    pub fn len_nodes(&self) -> usize {
+        self.nodes.len()
+    }
+
+    #[inline]
+    pub fn all_edges(&self) -> &[Edge<E>] {
+        &self.edges
+    }
+
+    #[inline]
+    pub fn len_edges(&self) -> usize {
+        self.edges.len()
+    }
+
+    // # Node construction
+
+    pub fn next_node_index(&self) -> NodeIndex {
+        NodeIndex(self.nodes.len())
+    }
+
+    pub fn add_node(&mut self, data: N) -> NodeIndex {
+        let idx = self.next_node_index();
+        self.nodes.push(Node { first_edge: [INVALID_EDGE_INDEX, INVALID_EDGE_INDEX], data });
+        idx
+    }
+
+    pub fn mut_node_data(&mut self, idx: NodeIndex) -> &mut N {
+        &mut self.nodes[idx.0].data
+    }
+
+    pub fn node_data(&self, idx: NodeIndex) -> &N {
+        &self.nodes[idx.0].data
+    }
+
+    pub fn node(&self, idx: NodeIndex) -> &Node<N> {
+        &self.nodes[idx.0]
+    }
+
+    // # Edge construction and queries
+
+    pub fn next_edge_index(&self) -> EdgeIndex {
+        EdgeIndex(self.edges.len())
+    }
+
+    pub fn add_edge(&mut self, source: NodeIndex, target: NodeIndex, data: E) -> EdgeIndex {
+        debug!("graph: add_edge({:?}, {:?}, {:?})", source, target, data);
+
+        let idx = self.next_edge_index();
+
+        // read current first of the list of edges from each node
+        let source_first = self.nodes[source.0].first_edge[OUTGOING.repr];
+        let target_first = self.nodes[target.0].first_edge[INCOMING.repr];
+
+        // create the new edge, with the previous firsts from each node
+        // as the next pointers
+        self.edges.push(Edge { next_edge: [source_first, target_first], source, target, data });
+
+        // adjust the firsts for each node target be the next object.
+        self.nodes[source.0].first_edge[OUTGOING.repr] = idx;
+        self.nodes[target.0].first_edge[INCOMING.repr] = idx;
+
+        idx
+    }
+
+    pub fn edge(&self, idx: EdgeIndex) -> &Edge<E> {
+        &self.edges[idx.0]
+    }
+
+    // # Iterating over nodes, edges
+
+    pub fn enumerated_nodes(&self) -> impl Iterator<Item = (NodeIndex, &Node<N>)> {
+        self.nodes.iter().enumerate().map(|(idx, n)| (NodeIndex(idx), n))
+    }
+
+    pub fn enumerated_edges(&self) -> impl Iterator<Item = (EdgeIndex, &Edge<E>)> {
+        self.edges.iter().enumerate().map(|(idx, e)| (EdgeIndex(idx), e))
+    }
+
+    pub fn each_node<'a>(&'a self, mut f: impl FnMut(NodeIndex, &'a Node<N>) -> bool) -> bool {
+        //! Iterates over all edges defined in the graph.
+        self.enumerated_nodes().all(|(node_idx, node)| f(node_idx, node))
+    }
+
+    pub fn each_edge<'a>(&'a self, mut f: impl FnMut(EdgeIndex, &'a Edge<E>) -> bool) -> bool {
+        //! Iterates over all edges defined in the graph
+        self.enumerated_edges().all(|(edge_idx, edge)| f(edge_idx, edge))
+    }
+
+    pub fn outgoing_edges(&self, source: NodeIndex) -> AdjacentEdges<'_, N, E> {
+        self.adjacent_edges(source, OUTGOING)
+    }
+
+    pub fn incoming_edges(&self, source: NodeIndex) -> AdjacentEdges<'_, N, E> {
+        self.adjacent_edges(source, INCOMING)
+    }
+
+    pub fn adjacent_edges(
+        &self,
+        source: NodeIndex,
+        direction: Direction,
+    ) -> AdjacentEdges<'_, N, E> {
+        let first_edge = self.node(source).first_edge[direction.repr];
+        AdjacentEdges { graph: self, direction, next: first_edge }
+    }
+
+    pub fn successor_nodes<'a>(
+        &'a self,
+        source: NodeIndex,
+    ) -> impl Iterator<Item = NodeIndex> + 'a {
+        self.outgoing_edges(source).targets()
+    }
+
+    pub fn predecessor_nodes<'a>(
+        &'a self,
+        target: NodeIndex,
+    ) -> impl Iterator<Item = NodeIndex> + 'a {
+        self.incoming_edges(target).sources()
+    }
+
+    pub fn depth_traverse(
+        &self,
+        start: NodeIndex,
+        direction: Direction,
+    ) -> DepthFirstTraversal<'_, N, E> {
+        DepthFirstTraversal::with_start_node(self, start, direction)
+    }
+
+    pub fn nodes_in_postorder(
+        &self,
+        direction: Direction,
+        entry_node: NodeIndex,
+    ) -> Vec<NodeIndex> {
+        let mut visited = BitSet::new_empty(self.len_nodes());
+        let mut stack = vec![];
+        let mut result = Vec::with_capacity(self.len_nodes());
+        let mut push_node = |stack: &mut Vec<_>, node: NodeIndex| {
+            if visited.insert(node.0) {
+                stack.push((node, self.adjacent_edges(node, direction)));
+            }
+        };
+
+        for node in
+            Some(entry_node).into_iter().chain(self.enumerated_nodes().map(|(node, _)| node))
+        {
+            push_node(&mut stack, node);
+            while let Some((node, mut iter)) = stack.pop() {
+                if let Some((_, child)) = iter.next() {
+                    let target = child.source_or_target(direction);
+                    // the current node needs more processing, so
+                    // add it back to the stack
+                    stack.push((node, iter));
+                    // and then push the new node
+                    push_node(&mut stack, target);
+                } else {
+                    result.push(node);
+                }
+            }
+        }
+
+        assert_eq!(result.len(), self.len_nodes());
+        result
+    }
+}
+
+// # Iterators
+
+pub struct AdjacentEdges<'g, N, E> {
+    graph: &'g Graph<N, E>,
+    direction: Direction,
+    next: EdgeIndex,
+}
+
+impl<'g, N: Debug, E: Debug> AdjacentEdges<'g, N, E> {
+    fn targets(self) -> impl Iterator<Item = NodeIndex> + 'g {
+        self.map(|(_, edge)| edge.target)
+    }
+
+    fn sources(self) -> impl Iterator<Item = NodeIndex> + 'g {
+        self.map(|(_, edge)| edge.source)
+    }
+}
+
+impl<'g, N: Debug, E: Debug> Iterator for AdjacentEdges<'g, N, E> {
+    type Item = (EdgeIndex, &'g Edge<E>);
+
+    fn next(&mut self) -> Option<(EdgeIndex, &'g Edge<E>)> {
+        let edge_index = self.next;
+        if edge_index == INVALID_EDGE_INDEX {
+            return None;
+        }
+
+        let edge = self.graph.edge(edge_index);
+        self.next = edge.next_edge[self.direction.repr];
+        Some((edge_index, edge))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // At most, all the edges in the graph.
+        (0, Some(self.graph.len_edges()))
+    }
+}
+
+pub struct DepthFirstTraversal<'g, N, E> {
+    graph: &'g Graph<N, E>,
+    stack: Vec<NodeIndex>,
+    visited: BitSet<usize>,
+    direction: Direction,
+}
+
+impl<'g, N: Debug, E: Debug> DepthFirstTraversal<'g, N, E> {
+    pub fn with_start_node(
+        graph: &'g Graph<N, E>,
+        start_node: NodeIndex,
+        direction: Direction,
+    ) -> Self {
+        let mut visited = BitSet::new_empty(graph.len_nodes());
+        visited.insert(start_node.node_id());
+        DepthFirstTraversal { graph, stack: vec![start_node], visited, direction }
+    }
+
+    fn visit(&mut self, node: NodeIndex) {
+        if self.visited.insert(node.node_id()) {
+            self.stack.push(node);
+        }
+    }
+}
+
+impl<'g, N: Debug, E: Debug> Iterator for DepthFirstTraversal<'g, N, E> {
+    type Item = NodeIndex;
+
+    fn next(&mut self) -> Option<NodeIndex> {
+        let next = self.stack.pop();
+        if let Some(idx) = next {
+            for (_, edge) in self.graph.adjacent_edges(idx, self.direction) {
+                let target = edge.source_or_target(self.direction);
+                self.visit(target);
+            }
+        }
+        next
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // We will visit every node in the graph exactly once.
+        let remaining = self.graph.len_nodes() - self.visited.count();
+        (remaining, Some(remaining))
+    }
+}
+
+impl<'g, N: Debug, E: Debug> ExactSizeIterator for DepthFirstTraversal<'g, N, E> {}
+
+impl<E> Edge<E> {
+    pub fn source(&self) -> NodeIndex {
+        self.source
+    }
+
+    pub fn target(&self) -> NodeIndex {
+        self.target
+    }
+
+    pub fn source_or_target(&self, direction: Direction) -> NodeIndex {
+        if direction == OUTGOING { self.target } else { self.source }
+    }
+}