rustc_middle/dep_graph/

graph.rs

use std::assert_matches;
use std::fmt::Debug;
use std::hash::Hash;
use std::sync::Arc;
use std::sync::atomic::{AtomicU32, Ordering};

use rustc_data_structures::fingerprint::{Fingerprint, PackedFingerprint};
use rustc_data_structures::fx::FxHashSet;
use rustc_data_structures::profiling::QueryInvocationId;
use rustc_data_structures::sharded::{self, ShardedHashMap};
use rustc_data_structures::stable_hash::{StableHash, StableHasher};
use rustc_data_structures::sync::{AtomicU64, Lock};
use rustc_data_structures::unord::UnordMap;
use rustc_errors::DiagInner;
use rustc_index::IndexVec;
use rustc_macros::{Decodable, Encodable};
use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
use rustc_session::Session;
use rustc_span::Symbol;
use tracing::instrument;
#[cfg(debug_assertions)]
use {super::debug::EdgeFilter, std::env};

use super::retained::RetainedDepGraph;
use super::serialized::{GraphEncoder, SerializedDepGraph, SerializedDepNodeIndex};
use super::{DepKind, DepNode, WorkProductId, read_deps, with_deps};
use crate::dep_graph::edges::EdgesVec;
use crate::ich::StableHashState;
use crate::ty::TyCtxt;
use crate::verify_ich::incremental_verify_ich;

/// Tracks 'side effects' for a particular query.
/// This struct is saved to disk along with the query result,
/// and loaded from disk if we mark the query as green.
/// This allows us to 'replay' changes to global state
/// that would otherwise only occur if we actually
/// executed the query method.
///
/// Each side effect gets an unique dep node index which is added
/// as a dependency of the query which had the effect.
#[derive(Debug, Encodable, Decodable)]
pub enum QuerySideEffect {
    /// Stores a diagnostic emitted during query execution.
    /// This diagnostic will be re-emitted if we mark
    /// the query as green, as that query will have the side
    /// effect dep node as a dependency.
    Diagnostic(DiagInner),
    /// Records the feature used during query execution.
    /// This feature will be inserted into `sess.used_features`
    /// if we mark the query as green, as that query will have
    /// the side effect dep node as a dependency.
    CheckFeature { symbol: Symbol },
}

#[derive(Clone)]
pub struct DepGraph {
    data: Option<Arc<DepGraphData>>,

    /// This field is used for assigning DepNodeIndices when running in
    /// non-incremental mode. Even in non-incremental mode we make sure that
    /// each task has a `DepNodeIndex` that uniquely identifies it. This unique
    /// ID is used for self-profiling.
    virtual_dep_node_index: Arc<AtomicU32>,
}

rustc_index::newtype_index! {
    pub struct DepNodeIndex {}
}

// We store a large collection of these in `prev_index_to_index` during
// non-full incremental builds, and want to ensure that the element size
// doesn't inadvertently increase.
rustc_data_structures::static_assert_size!(Option<DepNodeIndex>, 4);

impl DepNodeIndex {
    const SINGLETON_ZERO_DEPS_ANON_NODE: DepNodeIndex = DepNodeIndex::ZERO;
    pub const FOREVER_RED_NODE: DepNodeIndex = DepNodeIndex::from_u32(1);
}

impl From<DepNodeIndex> for QueryInvocationId {
    #[inline(always)]
    fn from(dep_node_index: DepNodeIndex) -> Self {
        QueryInvocationId(dep_node_index.as_u32())
    }
}

pub(crate) struct MarkFrame<'a> {
    index: SerializedDepNodeIndex,
    parent: Option<&'a MarkFrame<'a>>,
}

#[derive(Debug)]
pub(super) enum DepNodeColor {
    Green(DepNodeIndex),
    Red,
    Unknown,
}

pub struct DepGraphData {
    /// The new encoding of the dependency graph, optimized for red/green
    /// tracking. The `current` field is the dependency graph of only the
    /// current compilation session: We don't merge the previous dep-graph into
    /// current one anymore, but we do reference shared data to save space.
    current: CurrentDepGraph,

    /// The dep-graph from the previous compilation session. It contains all
    /// nodes and edges as well as all fingerprints of nodes that have them.
    previous: Arc<SerializedDepGraph>,

    colors: DepNodeColorMap,

    /// When we load, there may be `.o` files, cached MIR, or other such
    /// things available to us. If we find that they are not dirty, we
    /// load the path to the file storing those work-products here into
    /// this map. We can later look for and extract that data.
    previous_work_products: WorkProductMap,

    /// Used by incremental compilation tests to assert that
    /// a particular query result was decoded from disk
    /// (not just marked green)
    debug_loaded_from_disk: Lock<FxHashSet<DepNode>>,
}

pub fn hash_result<R>(hcx: &mut StableHashState<'_>, result: &R) -> Fingerprint
where
    R: StableHash,
{
    let mut stable_hasher = StableHasher::new();
    result.stable_hash(hcx, &mut stable_hasher);
    stable_hasher.finish()
}

impl DepGraph {
    pub fn new(
        session: &Session,
        prev_graph: Arc<SerializedDepGraph>,
        prev_work_products: WorkProductMap,
        encoder: FileEncoder,
    ) -> DepGraph {
        let prev_graph_node_count = prev_graph.node_count();

        let current =
            CurrentDepGraph::new(session, prev_graph_node_count, encoder, Arc::clone(&prev_graph));

        let colors = DepNodeColorMap::new(prev_graph_node_count);

        // Instantiate a node with zero dependencies only once for anonymous queries.
        let _green_node_index = current.alloc_new_node(
            DepNode { kind: DepKind::AnonZeroDeps, key_fingerprint: current.anon_id_seed.into() },
            EdgesVec::new(),
            Fingerprint::ZERO,
        );
        assert_eq!(_green_node_index, DepNodeIndex::SINGLETON_ZERO_DEPS_ANON_NODE);

        // Create a single always-red node, with no dependencies of its own.
        // Other nodes can use the always-red node as a fake dependency, to
        // ensure that their dependency list will never be all-green.
        let red_node_index = current.alloc_new_node(
            DepNode { kind: DepKind::Red, key_fingerprint: Fingerprint::ZERO.into() },
            EdgesVec::new(),
            Fingerprint::ZERO,
        );
        assert_eq!(red_node_index, DepNodeIndex::FOREVER_RED_NODE);
        if prev_graph_node_count > 0 {
            let prev_index =
                const { SerializedDepNodeIndex::from_u32(DepNodeIndex::FOREVER_RED_NODE.as_u32()) };
            let result = colors.try_set_color(prev_index, DesiredColor::Red);
            assert_matches!(result, TrySetColorResult::Success);
        }

        DepGraph {
            data: Some(Arc::new(DepGraphData {
                previous_work_products: prev_work_products,
                current,
                previous: prev_graph,
                colors,
                debug_loaded_from_disk: Default::default(),
            })),
            virtual_dep_node_index: Arc::new(AtomicU32::new(0)),
        }
    }

    pub fn new_disabled() -> DepGraph {
        DepGraph { data: None, virtual_dep_node_index: Arc::new(AtomicU32::new(0)) }
    }

    #[inline]
    pub fn data(&self) -> Option<&DepGraphData> {
        self.data.as_deref()
    }

    /// Returns `true` if we are actually building the full dep-graph, and `false` otherwise.
    #[inline]
    pub fn is_fully_enabled(&self) -> bool {
        self.data.is_some()
    }

    pub fn with_retained_dep_graph(&self, f: impl Fn(&RetainedDepGraph)) {
        if let Some(data) = &self.data {
            data.current.encoder.with_retained_dep_graph(f)
        }
    }

    pub fn assert_ignored(&self) {
        if let Some(..) = self.data {
            read_deps(|task_deps| {
                assert_matches!(
                    task_deps,
                    TaskDepsRef::Ignore,
                    "expected no task dependency tracking"
                );
            })
        }
    }

    pub fn assert_eval_always(&self) {
        if self.data.is_some() {
            read_deps(|deps| {
                assert_matches!(deps, TaskDepsRef::EvalAlways, "expected eval always context")
            });
        }
    }

    pub fn with_ignore<OP, R>(&self, op: OP) -> R
    where
        OP: FnOnce() -> R,
    {
        with_deps(TaskDepsRef::Ignore, op)
    }

    /// Used to wrap the deserialization of a query result from disk,
    /// This method enforces that no new `DepNodes` are created during
    /// query result deserialization.
    ///
    /// Enforcing this makes the query dep graph simpler - all nodes
    /// must be created during the query execution, and should be
    /// created from inside the 'body' of a query (the implementation
    /// provided by a particular compiler crate).
    ///
    /// Consider the case of three queries `A`, `B`, and `C`, where
    /// `A` invokes `B` and `B` invokes `C`:
    ///
    /// `A -> B -> C`
    ///
    /// Suppose that decoding the result of query `B` required re-computing
    /// the query `C`. If we did not create a fresh `TaskDeps` when
    /// decoding `B`, we would still be using the `TaskDeps` for query `A`
    /// (if we needed to re-execute `A`). This would cause us to create
    /// a new edge `A -> C`. If this edge did not previously
    /// exist in the `DepGraph`, then we could end up with a different
    /// `DepGraph` at the end of compilation, even if there were no
    /// meaningful changes to the overall program (e.g. a newline was added).
    /// In addition, this edge might cause a subsequent compilation run
    /// to try to force `C` before marking other necessary nodes green. If
    /// `C` did not exist in the new compilation session, then we could
    /// get an ICE. Normally, we would have tried (and failed) to mark
    /// some other query green (e.g. `item_children`) which was used
    /// to obtain `C`, which would prevent us from ever trying to force
    /// a nonexistent `D`.
    ///
    /// It might be possible to enforce that all `DepNode`s read during
    /// deserialization already exist in the previous `DepGraph`. In
    /// the above example, we would invoke `D` during the deserialization
    /// of `B`. Since we correctly create a new `TaskDeps` from the decoding
    /// of `B`, this would result in an edge `B -> D`. If that edge already
    /// existed (with the same `DepPathHash`es), then it should be correct
    /// to allow the invocation of the query to proceed during deserialization
    /// of a query result. We would merely assert that the dep-graph fragment
    /// that would have been added by invoking `C` while decoding `B`
    /// is equivalent to the dep-graph fragment that we already instantiated for B
    /// (at the point where we successfully marked B as green).
    ///
    /// However, this would require additional complexity
    /// in the query infrastructure, and is not currently needed by the
    /// decoding of any query results. Should the need arise in the future,
    /// we should consider extending the query system with this functionality.
    pub fn with_query_deserialization<OP, R>(&self, op: OP) -> R
    where
        OP: FnOnce() -> R,
    {
        with_deps(TaskDepsRef::Forbid, op)
    }

    #[inline(always)]
    pub fn with_task<'tcx, OP, R>(
        &self,
        dep_node: DepNode,
        tcx: TyCtxt<'tcx>,
        op: OP,
        hash_result: Option<fn(&mut StableHashState<'_>, &R) -> Fingerprint>,
    ) -> (R, DepNodeIndex)
    where
        OP: FnOnce() -> R,
    {
        match self.data() {
            Some(data) => data.with_task(dep_node, tcx, op, hash_result),
            None => (op(), self.next_virtual_depnode_index()),
        }
    }

    pub fn with_anon_task<'tcx, OP, R>(
        &self,
        tcx: TyCtxt<'tcx>,
        dep_kind: DepKind,
        op: OP,
    ) -> (R, DepNodeIndex)
    where
        OP: FnOnce() -> R,
    {
        match self.data() {
            Some(data) => {
                let (result, index) = data.with_anon_task_inner(tcx, dep_kind, op);
                self.read_index(index);
                (result, index)
            }
            None => (op(), self.next_virtual_depnode_index()),
        }
    }
}

impl DepGraphData {
    #[inline(always)]
    pub fn with_task<'tcx, OP, R>(
        &self,
        dep_node: DepNode,
        tcx: TyCtxt<'tcx>,
        op: OP,
        hash_result: Option<fn(&mut StableHashState<'_>, &R) -> Fingerprint>,
    ) -> (R, DepNodeIndex)
    where
        OP: FnOnce() -> R,
    {
        // If the following assertion triggers, it can have two reasons:
        // 1. Something is wrong with DepNode creation, either here or
        //    in `DepGraph::try_mark_green()`.
        // 2. Two distinct query keys get mapped to the same `DepNode`
        //    (see for example #48923).
        self.assert_dep_node_not_yet_allocated_in_current_session(tcx.sess, &dep_node, || {
            format!("forcing query with already existing `DepNode`: {dep_node:?}")
        });

        let (result, edges) = if tcx.is_eval_always(dep_node.kind) {
            (with_deps(TaskDepsRef::EvalAlways, op), EdgesVec::new())
        } else {
            let task_deps = Lock::new(TaskDeps::new(
                #[cfg(debug_assertions)]
                Some(dep_node),
                0,
            ));
            (with_deps(TaskDepsRef::Allow(&task_deps), op), task_deps.into_inner().reads)
        };

        let dep_node_index =
            self.hash_result_and_alloc_node(tcx, dep_node, edges, &result, hash_result);

        (result, dep_node_index)
    }

    /// Executes something within an "anonymous" task, that is, a task the
    /// `DepNode` of which is determined by the list of inputs it read from.
    ///
    /// NOTE: this does not actually count as a read of the DepNode here.
    /// Using the result of this task without reading the DepNode will result
    /// in untracked dependencies which may lead to ICEs as nodes are
    /// incorrectly marked green.
    ///
    /// FIXME: This could perhaps return a `WithDepNode` to ensure that the
    /// user of this function actually performs the read.
    fn with_anon_task_inner<'tcx, OP, R>(
        &self,
        tcx: TyCtxt<'tcx>,
        dep_kind: DepKind,
        op: OP,
    ) -> (R, DepNodeIndex)
    where
        OP: FnOnce() -> R,
    {
        debug_assert!(!tcx.is_eval_always(dep_kind));

        // Large numbers of reads are common enough here that pre-sizing `read_set`
        // to 128 actually helps perf on some benchmarks.
        let task_deps = Lock::new(TaskDeps::new(
            #[cfg(debug_assertions)]
            None,
            128,
        ));
        let result = with_deps(TaskDepsRef::Allow(&task_deps), op);
        let task_deps = task_deps.into_inner();
        let reads = task_deps.reads;

        let dep_node_index = match reads.len() {
            0 => {
                // Because the dep-node id of anon nodes is computed from the sets of its
                // dependencies we already know what the ID of this dependency-less node is
                // going to be (i.e. equal to the precomputed
                // `SINGLETON_DEPENDENCYLESS_ANON_NODE`). As a consequence we can skip creating
                // a `StableHasher` and sending the node through interning.
                DepNodeIndex::SINGLETON_ZERO_DEPS_ANON_NODE
            }
            1 => {
                // When there is only one dependency, don't bother creating a node.
                reads[0]
            }
            _ => {
                // The dep node indices are hashed here instead of hashing the dep nodes of the
                // dependencies. These indices may refer to different nodes per session, but this
                // isn't a problem here because we that ensure the final dep node hash is per
                // session only by combining it with the per session `anon_id_seed`. This hash only
                // need to map the dependencies to a single value on a per session basis.
                let mut hasher = StableHasher::new();
                reads.hash(&mut hasher);

                let target_dep_node = DepNode {
                    kind: dep_kind,
                    // Fingerprint::combine() is faster than sending Fingerprint
                    // through the StableHasher (at least as long as StableHasher
                    // is so slow).
                    key_fingerprint: self.current.anon_id_seed.combine(hasher.finish()).into(),
                };

                // The DepNodes generated by the process above are not unique. 2 queries could
                // have exactly the same dependencies. However, deserialization does not handle
                // duplicated nodes, so we do the deduplication here directly.
                //
                // As anonymous nodes are a small quantity compared to the full dep-graph, the
                // memory impact of this `anon_node_to_index` map remains tolerable, and helps
                // us avoid useless growth of the graph with almost-equivalent nodes.
                self.current.anon_node_to_index.get_or_insert_with(target_dep_node, || {
                    self.current.alloc_new_node(target_dep_node, reads, Fingerprint::ZERO)
                })
            }
        };

        (result, dep_node_index)
    }

    /// Intern the new `DepNode` with the dependencies up-to-now.
    fn hash_result_and_alloc_node<'tcx, R>(
        &self,
        tcx: TyCtxt<'tcx>,
        node: DepNode,
        edges: EdgesVec,
        result: &R,
        hash_result: Option<fn(&mut StableHashState<'_>, &R) -> Fingerprint>,
    ) -> DepNodeIndex {
        let hashing_timer = tcx.prof.incr_result_hashing();
        let current_fingerprint = hash_result.map(|hash_result| {
            tcx.with_stable_hashing_context(|mut hcx| hash_result(&mut hcx, result))
        });
        let dep_node_index = self.alloc_and_color_node(node, edges, current_fingerprint);
        hashing_timer.finish_with_query_invocation_id(dep_node_index.into());
        dep_node_index
    }
}

impl DepGraph {
    #[inline]
    pub fn read_index(&self, dep_node_index: DepNodeIndex) {
        if let Some(ref data) = self.data {
            read_deps(|task_deps| {
                let mut task_deps = match task_deps {
                    TaskDepsRef::Allow(deps) => deps.lock(),
                    TaskDepsRef::EvalAlways => {
                        // We don't need to record dependencies of eval_always
                        // queries. They are re-evaluated unconditionally anyway.
                        return;
                    }
                    TaskDepsRef::Ignore => return,
                    TaskDepsRef::Forbid => {
                        // Reading is forbidden in this context. ICE with a useful error message.
                        panic_on_forbidden_read(data, dep_node_index)
                    }
                };
                let task_deps = &mut *task_deps;

                if cfg!(debug_assertions) {
                    data.current.total_read_count.fetch_add(1, Ordering::Relaxed);
                }

                // Has `dep_node_index` been seen before? Use either a linear scan or a hashset
                // lookup to determine this. See `TaskDeps::read_set` for details.
                let new_read = if task_deps.reads.len() <= TaskDeps::LINEAR_SCAN_MAX {
                    !task_deps.reads.contains(&dep_node_index)
                } else {
                    task_deps.read_set.insert(dep_node_index)
                };
                if new_read {
                    task_deps.reads.push(dep_node_index);
                    if task_deps.reads.len() == TaskDeps::LINEAR_SCAN_MAX + 1 {
                        // Fill `read_set` with what we have so far. Future lookups will use it.
                        task_deps.read_set.extend(task_deps.reads.iter().copied());
                    }

                    #[cfg(debug_assertions)]
                    {
                        if let Some(target) = task_deps.node
                            && let Some(ref forbidden_edge) = data.current.forbidden_edge
                        {
                            let src = forbidden_edge.index_to_node.lock()[&dep_node_index];
                            if forbidden_edge.test(&src, &target) {
                                panic!("forbidden edge {:?} -> {:?} created", src, target)
                            }
                        }
                    }
                } else if cfg!(debug_assertions) {
                    data.current.total_duplicate_read_count.fetch_add(1, Ordering::Relaxed);
                }
            })
        }
    }

    /// This encodes a side effect by creating a node with an unique index and associating
    /// it with the node, for use in the next session.
    #[inline]
    pub fn record_diagnostic<'tcx>(&self, tcx: TyCtxt<'tcx>, diagnostic: &DiagInner) {
        if let Some(ref data) = self.data {
            read_deps(|task_deps| match task_deps {
                TaskDepsRef::EvalAlways | TaskDepsRef::Ignore => return,
                TaskDepsRef::Forbid | TaskDepsRef::Allow(..) => {
                    let dep_node_index = data
                        .encode_side_effect(tcx, QuerySideEffect::Diagnostic(diagnostic.clone()));
                    self.read_index(dep_node_index);
                }
            })
        }
    }
    /// This forces a side effect node green by running its side effect. `prev_index` would
    /// refer to a node created used `encode_side_effect` in the previous session.
    #[inline]
    pub fn force_side_effect<'tcx>(&self, tcx: TyCtxt<'tcx>, prev_index: SerializedDepNodeIndex) {
        if let Some(ref data) = self.data {
            data.force_side_effect(tcx, prev_index);
        }
    }

    #[inline]
    pub fn encode_side_effect<'tcx>(
        &self,
        tcx: TyCtxt<'tcx>,
        side_effect: QuerySideEffect,
    ) -> DepNodeIndex {
        if let Some(ref data) = self.data {
            data.encode_side_effect(tcx, side_effect)
        } else {
            self.next_virtual_depnode_index()
        }
    }

    /// Create a node when we force-feed a value into the query cache.
    /// This is used to remove cycles during type-checking const generic parameters.
    ///
    /// As usual in the query system, we consider the current state of the calling query
    /// only depends on the list of dependencies up to now. As a consequence, the value
    /// that this query gives us can only depend on those dependencies too. Therefore,
    /// it is sound to use the current dependency set for the created node.
    ///
    /// During replay, the order of the nodes is relevant in the dependency graph.
    /// So the unchanged replay will mark the caller query before trying to mark this one.
    /// If there is a change to report, the caller query will be re-executed before this one.
    ///
    /// FIXME: If the code is changed enough for this node to be marked before requiring the
    /// caller's node, we suppose that those changes will be enough to mark this node red and
    /// force a recomputation using the "normal" way.
    pub fn with_feed_task<'tcx, R>(
        &self,
        node: DepNode,
        tcx: TyCtxt<'tcx>,
        result: &R,
        hash_result: Option<fn(&mut StableHashState<'_>, &R) -> Fingerprint>,
        format_value_fn: fn(&R) -> String,
    ) -> DepNodeIndex {
        if let Some(data) = self.data.as_ref() {
            // The caller query has more dependencies than the node we are creating. We may
            // encounter a case where this created node is marked as green, but the caller query is
            // subsequently marked as red or recomputed. In this case, we will end up feeding a
            // value to an existing node.
            //
            // For sanity, we still check that the loaded stable hash and the new one match.
            if let Some(prev_index) = data.previous.node_to_index_opt(&node) {
                let dep_node_index = data.colors.current(prev_index);
                if let Some(dep_node_index) = dep_node_index {
                    incremental_verify_ich(
                        tcx,
                        data,
                        result,
                        prev_index,
                        hash_result,
                        format_value_fn,
                    );

                    #[cfg(debug_assertions)]
                    if hash_result.is_some() {
                        data.current.record_edge(
                            dep_node_index,
                            node,
                            data.prev_value_fingerprint_of(prev_index),
                        );
                    }

                    return dep_node_index;
                }
            }

            let mut edges = EdgesVec::new();
            read_deps(|task_deps| match task_deps {
                TaskDepsRef::Allow(deps) => edges.extend(deps.lock().reads.iter().copied()),
                TaskDepsRef::EvalAlways => {
                    edges.push(DepNodeIndex::FOREVER_RED_NODE);
                }
                TaskDepsRef::Ignore => {}
                TaskDepsRef::Forbid => {
                    panic!("Cannot summarize when dependencies are not recorded.")
                }
            });

            data.hash_result_and_alloc_node(tcx, node, edges, result, hash_result)
        } else {
            // Incremental compilation is turned off. We just execute the task
            // without tracking. We still provide a dep-node index that uniquely
            // identifies the task so that we have a cheap way of referring to
            // the query for self-profiling.
            self.next_virtual_depnode_index()
        }
    }
}

impl DepGraphData {
    fn assert_dep_node_not_yet_allocated_in_current_session<S: std::fmt::Display>(
        &self,
        sess: &Session,
        dep_node: &DepNode,
        msg: impl FnOnce() -> S,
    ) {
        if let Some(prev_index) = self.previous.node_to_index_opt(dep_node) {
            let color = self.colors.get(prev_index);
            let ok = match color {
                DepNodeColor::Unknown => true,
                DepNodeColor::Red => false,
                DepNodeColor::Green(..) => sess.threads().is_some(), // Other threads may mark this green
            };
            if !ok {
                panic!("{}", msg())
            }
        }
    }

    fn node_color(&self, dep_node: &DepNode) -> DepNodeColor {
        if let Some(prev_index) = self.previous.node_to_index_opt(dep_node) {
            self.colors.get(prev_index)
        } else {
            // This is a node that did not exist in the previous compilation session.
            DepNodeColor::Unknown
        }
    }

    /// Returns true if the given node has been marked as green during the
    /// current compilation session. Used in various assertions
    #[inline]
    pub fn is_index_green(&self, prev_index: SerializedDepNodeIndex) -> bool {
        matches!(self.colors.get(prev_index), DepNodeColor::Green(_))
    }

    #[inline]
    pub fn prev_value_fingerprint_of(&self, prev_index: SerializedDepNodeIndex) -> Fingerprint {
        self.previous.value_fingerprint_for_index(prev_index)
    }

    #[inline]
    pub(crate) fn prev_node_of(&self, prev_index: SerializedDepNodeIndex) -> &DepNode {
        self.previous.index_to_node(prev_index)
    }

    pub fn mark_debug_loaded_from_disk(&self, dep_node: DepNode) {
        self.debug_loaded_from_disk.lock().insert(dep_node);
    }

    /// This encodes a side effect by creating a node with an unique index and associating
    /// it with the node, for use in the next session.
    #[inline]
    fn encode_side_effect<'tcx>(
        &self,
        tcx: TyCtxt<'tcx>,
        side_effect: QuerySideEffect,
    ) -> DepNodeIndex {
        // Use `send_new` so we get an unique index, even though the dep node is not.
        let dep_node_index = self.current.encoder.send_new(
            DepNode {
                kind: DepKind::SideEffect,
                key_fingerprint: PackedFingerprint::from(Fingerprint::ZERO),
            },
            Fingerprint::ZERO,
            // We want the side effect node to always be red so it will be forced and run the
            // side effect.
            std::iter::once(DepNodeIndex::FOREVER_RED_NODE).collect(),
        );
        tcx.query_system.side_effects.borrow_mut().insert(dep_node_index, side_effect);
        dep_node_index
    }

    /// This forces a side effect node green by running its side effect. `prev_index` would
    /// refer to a node created used `encode_side_effect` in the previous session.
    #[inline]
    fn force_side_effect<'tcx>(&self, tcx: TyCtxt<'tcx>, prev_index: SerializedDepNodeIndex) {
        with_deps(TaskDepsRef::Ignore, || {
            let side_effect = tcx
                .query_system
                .on_disk_cache
                .as_ref()
                .unwrap()
                .load_side_effect(tcx, prev_index)
                .unwrap();

            // Use `send_and_color` as `promote_node_and_deps_to_current` expects all
            // green dependencies. `send_and_color` will also prevent multiple nodes
            // being encoded for concurrent calls.
            let dep_node_index = self.current.encoder.send_and_color(
                prev_index,
                &self.colors,
                DepNode {
                    kind: DepKind::SideEffect,
                    key_fingerprint: PackedFingerprint::from(Fingerprint::ZERO),
                },
                Fingerprint::ZERO,
                std::iter::once(DepNodeIndex::FOREVER_RED_NODE).collect(),
                true,
            );

            match &side_effect {
                QuerySideEffect::Diagnostic(diagnostic) => {
                    tcx.dcx().emit_diagnostic(diagnostic.clone());
                }
                QuerySideEffect::CheckFeature { symbol } => {
                    tcx.sess.used_features.lock().insert(*symbol, dep_node_index.as_u32());
                }
            }

            // This will just overwrite the same value for concurrent calls.
            tcx.query_system.side_effects.borrow_mut().insert(dep_node_index, side_effect);
        })
    }

    fn alloc_and_color_node(
        &self,
        key: DepNode,
        edges: EdgesVec,
        value_fingerprint: Option<Fingerprint>,
    ) -> DepNodeIndex {
        if let Some(prev_index) = self.previous.node_to_index_opt(&key) {
            // Determine the color and index of the new `DepNode`.
            let is_green = if let Some(value_fingerprint) = value_fingerprint {
                if value_fingerprint == self.previous.value_fingerprint_for_index(prev_index) {
                    // This is a green node: it existed in the previous compilation,
                    // its query was re-executed, and it has the same result as before.
                    true
                } else {
                    // This is a red node: it existed in the previous compilation, its query
                    // was re-executed, but it has a different result from before.
                    false
                }
            } else {
                // This is a red node, effectively: it existed in the previous compilation
                // session, its query was re-executed, but it doesn't compute a result hash
                // (i.e. it represents a `no_hash` query), so we have no way of determining
                // whether or not the result was the same as before.
                false
            };

            let value_fingerprint = value_fingerprint.unwrap_or(Fingerprint::ZERO);

            let dep_node_index = self.current.encoder.send_and_color(
                prev_index,
                &self.colors,
                key,
                value_fingerprint,
                edges,
                is_green,
            );

            #[cfg(debug_assertions)]
            self.current.record_edge(dep_node_index, key, value_fingerprint);

            dep_node_index
        } else {
            self.current.alloc_new_node(key, edges, value_fingerprint.unwrap_or(Fingerprint::ZERO))
        }
    }

    fn promote_node_and_deps_to_current(
        &self,
        prev_index: SerializedDepNodeIndex,
    ) -> Option<DepNodeIndex> {
        let dep_node_index = self.current.encoder.send_promoted(prev_index, &self.colors);

        #[cfg(debug_assertions)]
        if let Some(dep_node_index) = dep_node_index {
            self.current.record_edge(
                dep_node_index,
                *self.previous.index_to_node(prev_index),
                self.previous.value_fingerprint_for_index(prev_index),
            );
        }

        dep_node_index
    }
}

impl DepGraph {
    /// Checks whether a previous work product exists for `v` and, if
    /// so, return the path that leads to it. Used to skip doing work.
    pub fn previous_work_product(&self, v: &WorkProductId) -> Option<WorkProduct> {
        self.data.as_ref().and_then(|data| data.previous_work_products.get(v).cloned())
    }

    /// Access the map of work-products created during the cached run. Only
    /// used during saving of the dep-graph.
    pub fn previous_work_products(&self) -> &WorkProductMap {
        &self.data.as_ref().unwrap().previous_work_products
    }

    pub fn debug_was_loaded_from_disk(&self, dep_node: DepNode) -> bool {
        self.data.as_ref().unwrap().debug_loaded_from_disk.lock().contains(&dep_node)
    }

    pub fn debug_dep_kind_was_loaded_from_disk(&self, dep_kind: DepKind) -> bool {
        // We only check if we have a dep node corresponding to the given dep kind.
        #[allow(rustc::potential_query_instability)]
        self.data
            .as_ref()
            .unwrap()
            .debug_loaded_from_disk
            .lock()
            .iter()
            .any(|node| node.kind == dep_kind)
    }

    fn node_color(&self, dep_node: &DepNode) -> DepNodeColor {
        if let Some(ref data) = self.data {
            return data.node_color(dep_node);
        }

        DepNodeColor::Unknown
    }

    pub fn try_mark_green<'tcx>(
        &self,
        tcx: TyCtxt<'tcx>,
        dep_node: &DepNode,
    ) -> Option<(SerializedDepNodeIndex, DepNodeIndex)> {
        self.data()?.try_mark_green(tcx, dep_node)
    }
}

impl DepGraphData {
    /// Try to mark a node index for the node dep_node.
    ///
    /// A node will have an index, when it's already been marked green, or when we can mark it
    /// green. This function will mark the current task as a reader of the specified node, when
    /// a node index can be found for that node.
    pub fn try_mark_green<'tcx>(
        &self,
        tcx: TyCtxt<'tcx>,
        dep_node: &DepNode,
    ) -> Option<(SerializedDepNodeIndex, DepNodeIndex)> {
        debug_assert!(!tcx.is_eval_always(dep_node.kind));

        // Return None if the dep node didn't exist in the previous session
        let prev_index = self.previous.node_to_index_opt(dep_node)?;

        debug_assert_eq!(self.previous.index_to_node(prev_index), dep_node);

        match self.colors.get(prev_index) {
            DepNodeColor::Green(dep_node_index) => Some((prev_index, dep_node_index)),
            DepNodeColor::Red => None,
            DepNodeColor::Unknown => {
                // This DepNode and the corresponding query invocation existed
                // in the previous compilation session too, so we can try to
                // mark it as green by recursively marking all of its
                // dependencies green.
                self.try_mark_previous_green(tcx, prev_index, None)
                    .map(|dep_node_index| (prev_index, dep_node_index))
            }
        }
    }

    /// Try to mark a dep-node which existed in the previous compilation session as green.
    #[instrument(skip(self, tcx, prev_dep_node_index, frame), level = "debug")]
    fn try_mark_previous_green<'tcx>(
        &self,
        tcx: TyCtxt<'tcx>,
        prev_dep_node_index: SerializedDepNodeIndex,
        frame: Option<&MarkFrame<'_>>,
    ) -> Option<DepNodeIndex> {
        let frame = MarkFrame { index: prev_dep_node_index, parent: frame };

        // We never try to mark eval_always nodes as green
        debug_assert!(!tcx.is_eval_always(self.previous.index_to_node(prev_dep_node_index).kind));

        for parent_dep_node_index in self.previous.edge_targets_from(prev_dep_node_index) {
            match self.colors.get(parent_dep_node_index) {
                // This dependency has been marked as green before, we are still ok and can
                // continue checking the remaining dependencies.
                DepNodeColor::Green(_) => continue,

                // This dependency's result is different to the previous compilation session. We
                // cannot mark this dep_node as green, so stop checking.
                DepNodeColor::Red => return None,

                // We still need to determine this dependency's colour.
                DepNodeColor::Unknown => {}
            }

            let parent_dep_node = self.previous.index_to_node(parent_dep_node_index);

            // If this dependency isn't eval_always, try to mark it green recursively.
            if !tcx.is_eval_always(parent_dep_node.kind)
                && self.try_mark_previous_green(tcx, parent_dep_node_index, Some(&frame)).is_some()
            {
                continue;
            }

            // We failed to mark it green, so we try to force the query.
            if !tcx.try_force_from_dep_node(*parent_dep_node, parent_dep_node_index, &frame) {
                return None;
            }

            match self.colors.get(parent_dep_node_index) {
                DepNodeColor::Green(_) => continue,
                DepNodeColor::Red => return None,
                DepNodeColor::Unknown => {}
            }

            if tcx.dcx().has_errors_or_delayed_bugs().is_none() {
                panic!("try_mark_previous_green() - forcing failed to set a color");
            }

            // If the query we just forced has resulted in some kind of compilation error, we
            // cannot rely on the dep-node color having been properly updated. This means that the
            // query system has reached an invalid state. We let the compiler continue (by
            // returning `None`) so it can emit error messages and wind down, but rely on the fact
            // that this invalid state will not be persisted to the incremental compilation cache
            // because of compilation errors being present.
            return None;
        }

        // If we got here without hitting a `return` that means that all
        // dependencies of this DepNode could be marked as green. Therefore we
        // can also mark this DepNode as green.

        // There may be multiple threads trying to mark the same dep node green concurrently.

        // We allocating an entry for the node in the current dependency graph and
        // adding all the appropriate edges imported from the previous graph.
        //
        // `no_hash` nodes may fail this promotion due to already being conservatively colored red.
        let dep_node_index = self.promote_node_and_deps_to_current(prev_dep_node_index)?;

        // ... and finally storing a "Green" entry in the color map.
        // Multiple threads can all write the same color here.

        Some(dep_node_index)
    }
}

impl DepGraph {
    /// Returns true if the given node has been marked as red during the
    /// current compilation session. Used in various assertions
    pub fn is_red(&self, dep_node: &DepNode) -> bool {
        matches!(self.node_color(dep_node), DepNodeColor::Red)
    }

    /// Returns true if the given node has been marked as green during the
    /// current compilation session. Used in various assertions
    pub fn is_green(&self, dep_node: &DepNode) -> bool {
        matches!(self.node_color(dep_node), DepNodeColor::Green(_))
    }

    pub fn assert_dep_node_not_yet_allocated_in_current_session<S: std::fmt::Display>(
        &self,
        sess: &Session,
        dep_node: &DepNode,
        msg: impl FnOnce() -> S,
    ) {
        if let Some(data) = &self.data {
            data.assert_dep_node_not_yet_allocated_in_current_session(sess, dep_node, msg)
        }
    }

    /// This method loads all on-disk cacheable query results into memory, so
    /// they can be written out to the new cache file again. Most query results
    /// will already be in memory but in the case where we marked something as
    /// green but then did not need the value, that value will never have been
    /// loaded from disk.
    ///
    /// This method will only load queries that will end up in the disk cache.
    /// Other queries will not be executed.
    pub fn exec_cache_promotions<'tcx>(&self, tcx: TyCtxt<'tcx>) {
        let _prof_timer = tcx.prof.generic_activity("incr_comp_query_cache_promotion");

        let data = self.data.as_ref().unwrap();
        for prev_index in data.colors.values.indices() {
            match data.colors.get(prev_index) {
                DepNodeColor::Green(_) => {
                    let dep_node = data.previous.index_to_node(prev_index);
                    if let Some(promote_fn) =
                        tcx.dep_kind_vtable(dep_node.kind).promote_from_disk_fn
                    {
                        promote_fn(tcx, *dep_node)
                    };
                }
                DepNodeColor::Unknown | DepNodeColor::Red => {
                    // We can skip red nodes because a node can only be marked
                    // as red if the query result was recomputed and thus is
                    // already in memory.
                }
            }
        }
    }

    pub(crate) fn finish_encoding(&self) -> FileEncodeResult {
        if let Some(data) = &self.data { data.current.encoder.finish(&data.current) } else { Ok(0) }
    }

    pub fn next_virtual_depnode_index(&self) -> DepNodeIndex {
        debug_assert!(self.data.is_none());
        let index = self.virtual_dep_node_index.fetch_add(1, Ordering::Relaxed);
        DepNodeIndex::from_u32(index)
    }
}

/// A "work product" is an intermediate result that we save into the
/// incremental directory for later re-use. The primary example are
/// the object files that we save for each partition at code
/// generation time.
///
/// Each work product is associated with a dep-node, representing the
/// process that produced the work-product. If that dep-node is found
/// to be dirty when we load up, then we will delete the work-product
/// at load time. If the work-product is found to be clean, then we
/// will keep a record in the `previous_work_products` list.
///
/// In addition, work products have an associated hash. This hash is
/// an extra hash that can be used to decide if the work-product from
/// a previous compilation can be re-used (in addition to the dirty
/// edges check).
///
/// As the primary example, consider the object files we generate for
/// each partition. In the first run, we create partitions based on
/// the symbols that need to be compiled. For each partition P, we
/// hash the symbols in P and create a `WorkProduct` record associated
/// with `DepNode::CodegenUnit(P)`; the hash is the set of symbols
/// in P.
///
/// The next time we compile, if the `DepNode::CodegenUnit(P)` is
/// judged to be clean (which means none of the things we read to
/// generate the partition were found to be dirty), it will be loaded
/// into previous work products. We will then regenerate the set of
/// symbols in the partition P and hash them (note that new symbols
/// may be added -- for example, new monomorphizations -- even if
/// nothing in P changed!). We will compare that hash against the
/// previous hash. If it matches up, we can reuse the object file.
#[derive(Clone, Debug, Encodable, Decodable)]
pub struct WorkProduct {
    pub cgu_name: String,
    /// Saved files associated with this CGU. In each key/value pair, the value is the path to the
    /// saved file and the key is some identifier for the type of file being saved.
    ///
    /// By convention, file extensions are currently used as identifiers, i.e. the key "o" maps to
    /// the object file's path, and "dwo" to the dwarf object file's path.
    pub saved_files: UnordMap<String, String>,
}

pub type WorkProductMap = UnordMap<WorkProductId, WorkProduct>;

// Index type for `DepNodeData`'s edges.
rustc_index::newtype_index! {
    struct EdgeIndex {}
}

/// `CurrentDepGraph` stores the dependency graph for the current session. It
/// will be populated as we run queries or tasks. We never remove nodes from the
/// graph: they are only added.
///
/// The nodes in it are identified by a `DepNodeIndex`. We avoid keeping the nodes
/// in memory. This is important, because these graph structures are some of the
/// largest in the compiler.
///
/// For this reason, we avoid storing `DepNode`s more than once as map
/// keys. The `anon_node_to_index` map only contains nodes of anonymous queries not in the previous
/// graph, and we map nodes in the previous graph to indices via a two-step
/// mapping. `SerializedDepGraph` maps from `DepNode` to `SerializedDepNodeIndex`,
/// and the `prev_index_to_index` vector (which is more compact and faster than
/// using a map) maps from `SerializedDepNodeIndex` to `DepNodeIndex`.
///
/// This struct uses three locks internally. The `data`, `anon_node_to_index`,
/// and `prev_index_to_index` fields are locked separately. Operations that take
/// a `DepNodeIndex` typically just access the `data` field.
///
/// We only need to manipulate at most two locks simultaneously:
/// `anon_node_to_index` and `data`, or `prev_index_to_index` and `data`. When
/// manipulating both, we acquire `anon_node_to_index` or `prev_index_to_index`
/// first, and `data` second.
pub(super) struct CurrentDepGraph {
    encoder: GraphEncoder,
    anon_node_to_index: ShardedHashMap<DepNode, DepNodeIndex>,

    /// This is used to verify that value fingerprints do not change between the
    /// creation of a node and its recomputation.
    #[cfg(debug_assertions)]
    value_fingerprints: Lock<IndexVec<DepNodeIndex, Option<Fingerprint>>>,

    /// Used to trap when a specific edge is added to the graph.
    /// This is used for debug purposes and is only active with `debug_assertions`.
    #[cfg(debug_assertions)]
    forbidden_edge: Option<EdgeFilter>,

    /// Anonymous `DepNode`s are nodes whose IDs we compute from the list of
    /// their edges. This has the beneficial side-effect that multiple anonymous
    /// nodes can be coalesced into one without changing the semantics of the
    /// dependency graph. However, the merging of nodes can lead to a subtle
    /// problem during red-green marking: The color of an anonymous node from
    /// the current session might "shadow" the color of the node with the same
    /// ID from the previous session. In order to side-step this problem, we make
    /// sure that anonymous `NodeId`s allocated in different sessions don't overlap.
    /// This is implemented by mixing a session-key into the ID fingerprint of
    /// each anon node. The session-key is a hash of the number of previous sessions.
    anon_id_seed: Fingerprint,

    /// These are simple counters that are for profiling and
    /// debugging and only active with `debug_assertions`.
    pub(super) total_read_count: AtomicU64,
    pub(super) total_duplicate_read_count: AtomicU64,
}

impl CurrentDepGraph {
    fn new(
        session: &Session,
        prev_graph_node_count: usize,
        encoder: FileEncoder,
        previous: Arc<SerializedDepGraph>,
    ) -> Self {
        let mut stable_hasher = StableHasher::new();
        previous.session_count().hash(&mut stable_hasher);
        let anon_id_seed = stable_hasher.finish();

        #[cfg(debug_assertions)]
        let forbidden_edge = match env::var("RUST_FORBID_DEP_GRAPH_EDGE") {
            Ok(s) => match EdgeFilter::new(&s) {
                Ok(f) => Some(f),
                Err(err) => panic!("RUST_FORBID_DEP_GRAPH_EDGE invalid: {}", err),
            },
            Err(_) => None,
        };

        let new_node_count_estimate = 102 * prev_graph_node_count / 100 + 200;

        CurrentDepGraph {
            encoder: GraphEncoder::new(session, encoder, prev_graph_node_count, previous),
            anon_node_to_index: ShardedHashMap::with_capacity(
                // FIXME: The count estimate is off as anon nodes are only a portion of the nodes.
                new_node_count_estimate / sharded::shards(),
            ),
            anon_id_seed,
            #[cfg(debug_assertions)]
            forbidden_edge,
            #[cfg(debug_assertions)]
            value_fingerprints: Lock::new(IndexVec::from_elem_n(None, new_node_count_estimate)),
            total_read_count: AtomicU64::new(0),
            total_duplicate_read_count: AtomicU64::new(0),
        }
    }

    #[cfg(debug_assertions)]
    fn record_edge(
        &self,
        dep_node_index: DepNodeIndex,
        key: DepNode,
        value_fingerprint: Fingerprint,
    ) {
        if let Some(forbidden_edge) = &self.forbidden_edge {
            forbidden_edge.index_to_node.lock().insert(dep_node_index, key);
        }
        let prior_value_fingerprint = *self
            .value_fingerprints
            .lock()
            .get_or_insert_with(dep_node_index, || value_fingerprint);
        assert_eq!(prior_value_fingerprint, value_fingerprint, "Unstable fingerprints for {key:?}");
    }

    /// Writes the node to the current dep-graph and allocates a `DepNodeIndex` for it.
    /// Assumes that this is a node that has no equivalent in the previous dep-graph.
    #[inline(always)]
    fn alloc_new_node(
        &self,
        key: DepNode,
        edges: EdgesVec,
        value_fingerprint: Fingerprint,
    ) -> DepNodeIndex {
        let dep_node_index = self.encoder.send_new(key, value_fingerprint, edges);

        #[cfg(debug_assertions)]
        self.record_edge(dep_node_index, key, value_fingerprint);

        dep_node_index
    }
}

#[derive(Debug, Clone, Copy)]
pub enum TaskDepsRef<'a> {
    /// New dependencies can be added to the
    /// `TaskDeps`. This is used when executing a 'normal' query
    /// (no `eval_always` modifier)
    Allow(&'a Lock<TaskDeps>),
    /// This is used when executing an `eval_always` query. We don't
    /// need to track dependencies for a query that's always
    /// re-executed -- but we need to know that this is an `eval_always`
    /// query in order to emit dependencies to `DepNodeIndex::FOREVER_RED_NODE`
    /// when directly feeding other queries.
    EvalAlways,
    /// New dependencies are ignored. This is also used for `dep_graph.with_ignore`.
    Ignore,
    /// Any attempt to add new dependencies will cause a panic.
    /// This is used when decoding a query result from disk,
    /// to ensure that the decoding process doesn't itself
    /// require the execution of any queries.
    Forbid,
}

#[derive(Debug)]
pub struct TaskDeps {
    #[cfg(debug_assertions)]
    node: Option<DepNode>,

    /// A vector of `DepNodeIndex`, basically. Contains no duplicates.
    reads: EdgesVec,

    /// When adding a new edge to `reads` in `DepGraph::read_index` we must determine if the edge
    /// has been seen before. We just do a linear scan of `reads` if its length is less than or
    /// equal to `LINEAR_SCAN_MAX`. Otherwise, we use this hashset for better performance. Note:
    /// `reads` is always the canonical edges representation; this field is just to speed up the
    /// seen-before test.
    read_set: FxHashSet<DepNodeIndex>,
}

impl TaskDeps {
    /// See `TaskDeps::read_set` above.
    const LINEAR_SCAN_MAX: usize = 16;

    #[inline]
    fn new(#[cfg(debug_assertions)] node: Option<DepNode>, read_set_capacity: usize) -> Self {
        TaskDeps {
            #[cfg(debug_assertions)]
            node,
            reads: EdgesVec::new(),
            read_set: FxHashSet::with_capacity_and_hasher(read_set_capacity, Default::default()),
        }
    }
}

// A data structure that stores Option<DepNodeColor> values as a contiguous
// array, using one u32 per entry.
pub(super) struct DepNodeColorMap {
    values: IndexVec<SerializedDepNodeIndex, AtomicU32>,
}

// All values below `COMPRESSED_RED` are green.
const COMPRESSED_RED: u32 = u32::MAX - 1;
const COMPRESSED_UNKNOWN: u32 = u32::MAX;

impl DepNodeColorMap {
    fn new(size: usize) -> DepNodeColorMap {
        debug_assert!(COMPRESSED_RED > DepNodeIndex::MAX_AS_U32);
        DepNodeColorMap { values: (0..size).map(|_| AtomicU32::new(COMPRESSED_UNKNOWN)).collect() }
    }

    #[inline]
    pub(super) fn current(&self, index: SerializedDepNodeIndex) -> Option<DepNodeIndex> {
        let value = self.values[index].load(Ordering::Relaxed);
        if value <= DepNodeIndex::MAX_AS_U32 { Some(DepNodeIndex::from_u32(value)) } else { None }
    }

    /// Atomically sets the color of a previous-session dep node to either green
    /// or red, if it has not already been colored.
    ///
    /// If the node already has a color, the new color is ignored, and the
    /// return value indicates the existing color.
    #[inline(always)]
    pub(super) fn try_set_color(
        &self,
        prev_index: SerializedDepNodeIndex,
        color: DesiredColor,
    ) -> TrySetColorResult {
        match self.values[prev_index].compare_exchange(
            COMPRESSED_UNKNOWN,
            match color {
                DesiredColor::Red => COMPRESSED_RED,
                DesiredColor::Green { index } => index.as_u32(),
            },
            Ordering::Relaxed,
            Ordering::Relaxed,
        ) {
            Ok(_) => TrySetColorResult::Success,
            Err(COMPRESSED_RED) => TrySetColorResult::AlreadyRed,
            Err(index) => TrySetColorResult::AlreadyGreen { index: DepNodeIndex::from_u32(index) },
        }
    }

    #[inline]
    pub(super) fn get(&self, index: SerializedDepNodeIndex) -> DepNodeColor {
        let value = self.values[index].load(Ordering::Acquire);
        // Green is by far the most common case. Check for that first so we can succeed with a
        // single comparison.
        if value < COMPRESSED_RED {
            DepNodeColor::Green(DepNodeIndex::from_u32(value))
        } else if value == COMPRESSED_RED {
            DepNodeColor::Red
        } else {
            debug_assert_eq!(value, COMPRESSED_UNKNOWN);
            DepNodeColor::Unknown
        }
    }
}

/// The color that [`DepNodeColorMap::try_set_color`] should try to apply to a node.
#[derive(Clone, Copy, Debug)]
pub(super) enum DesiredColor {
    /// Try to mark the node red.
    Red,
    /// Try to mark the node green, associating it with a current-session node index.
    Green { index: DepNodeIndex },
}

/// Return value of [`DepNodeColorMap::try_set_color`], indicating success or failure,
/// and (on failure) what the existing color is.
#[derive(Clone, Copy, Debug)]
pub(super) enum TrySetColorResult {
    /// The [`DesiredColor`] was freshly applied to the node.
    Success,
    /// Coloring failed because the node was already marked red.
    AlreadyRed,
    /// Coloring failed because the node was already marked green,
    /// and corresponds to node `index` in the current-session dep graph.
    AlreadyGreen { index: DepNodeIndex },
}

#[inline(never)]
#[cold]
pub(crate) fn print_markframe_trace(graph: &DepGraph, frame: &MarkFrame<'_>) {
    let data = graph.data.as_ref().unwrap();

    eprintln!("there was a panic while trying to force a dep node");
    eprintln!("try_mark_green dep node stack:");

    let mut i = 0;
    let mut current = Some(frame);
    while let Some(frame) = current {
        let node = data.previous.index_to_node(frame.index);
        eprintln!("#{i} {node:?}");
        current = frame.parent;
        i += 1;
    }

    eprintln!("end of try_mark_green dep node stack");
}

#[cold]
#[inline(never)]
fn panic_on_forbidden_read(data: &DepGraphData, dep_node_index: DepNodeIndex) -> ! {
    // We have to do an expensive reverse-lookup of the DepNode that
    // corresponds to `dep_node_index`, but that's OK since we are about
    // to ICE anyway.
    let mut dep_node = None;

    // First try to find the dep node among those that already existed in the
    // previous session and has been marked green
    for prev_index in data.colors.values.indices() {
        if data.colors.current(prev_index) == Some(dep_node_index) {
            dep_node = Some(*data.previous.index_to_node(prev_index));
            break;
        }
    }

    let dep_node = dep_node.map_or_else(
        || format!("with index {:?}", dep_node_index),
        |dep_node| format!("`{:?}`", dep_node),
    );

    panic!(
        "Error: trying to record dependency on DepNode {dep_node} in a \
         context that does not allow it (e.g. during query deserialization). \
         The most common case of recording a dependency on a DepNode `foo` is \
         when the corresponding query `foo` is invoked. Invoking queries is not \
         allowed as part of loading something from the incremental on-disk cache. \
         See <https://github.com/rust-lang/rust/pull/91919>."
    )
}

impl<'tcx> TyCtxt<'tcx> {
    /// Return whether this kind always require evaluation.
    #[inline(always)]
    fn is_eval_always(self, kind: DepKind) -> bool {
        self.dep_kind_vtable(kind).is_eval_always
    }
}