compiler/rustc_middle/src/dep_graph/dep_node.rs - toolchain/rustc - Git at Google

 //! Nodes in the dependency graph.
 //!
 //! A node in the [dependency graph] is represented by a [`DepNode`].
 //! A `DepNode` consists of a [`DepKind`] (which
 //! specifies the kind of thing it represents, like a piece of HIR, MIR, etc.)
 //! and a [`Fingerprint`], a 128-bit hash value, the exact meaning of which
 //! depends on the node's `DepKind`. Together, the kind and the fingerprint
 //! fully identify a dependency node, even across multiple compilation sessions.
 //! In other words, the value of the fingerprint does not depend on anything
 //! that is specific to a given compilation session, like an unpredictable
 //! interning key (e.g., `NodeId`, `DefId`, `Symbol`) or the numeric value of a
 //! pointer. The concept behind this could be compared to how git commit hashes
 //! uniquely identify a given commit. The fingerprinting approach has
 //! a few advantages:
 //!
 //! * A `DepNode` can simply be serialized to disk and loaded in another session
 //!   without the need to do any "rebasing" (like we have to do for Spans and
 //!   NodeIds) or "retracing" (like we had to do for `DefId` in earlier
 //!   implementations of the dependency graph).
 //! * A `Fingerprint` is just a bunch of bits, which allows `DepNode` to
 //!   implement `Copy`, `Sync`, `Send`, `Freeze`, etc.
 //! * Since we just have a bit pattern, `DepNode` can be mapped from disk into
 //!   memory without any post-processing (e.g., "abomination-style" pointer
 //!   reconstruction).
 //! * Because a `DepNode` is self-contained, we can instantiate `DepNodes` that
 //!   refer to things that do not exist anymore. In previous implementations
 //!   `DepNode` contained a `DefId`. A `DepNode` referring to something that
 //!   had been removed between the previous and the current compilation session
 //!   could not be instantiated because the current compilation session
 //!   contained no `DefId` for thing that had been removed.
 //!
 //! `DepNode` definition happens in the `define_dep_nodes!()` macro. This macro
 //! defines the `DepKind` enum. Each `DepKind` has its own parameters that are
 //! needed at runtime in order to construct a valid `DepNode` fingerprint.
 //! However, only `CompileCodegenUnit` and `CompileMonoItem` are constructed
 //! explicitly (with `make_compile_codegen_unit` cq `make_compile_mono_item`).
 //!
 //! Because the macro sees what parameters a given `DepKind` requires, it can
 //! "infer" some properties for each kind of `DepNode`:
 //!
 //! * Whether a `DepNode` of a given kind has any parameters at all. Some
 //!   `DepNode`s could represent global concepts with only one value.
 //! * Whether it is possible, in principle, to reconstruct a query key from a
 //!   given `DepNode`. Many `DepKind`s only require a single `DefId` parameter,
 //!   in which case it is possible to map the node's fingerprint back to the
 //!   `DefId` it was computed from. In other cases, too much information gets
 //!   lost during fingerprint computation.
 //!
 //! `make_compile_codegen_unit` and `make_compile_mono_items`, together with
 //! `DepNode::new()`, ensures that only valid `DepNode` instances can be
 //! constructed. For example, the API does not allow for constructing
 //! parameterless `DepNode`s with anything other than a zeroed out fingerprint.
 //! More generally speaking, it relieves the user of the `DepNode` API of
 //! having to know how to compute the expected fingerprint for a given set of
 //! node parameters.
 //!
 //! [dependency graph]: https://rustc-dev-guide.rust-lang.org/query.html

 use crate::mir::mono::MonoItem;
 use crate::ty::TyCtxt;

 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
 use rustc_hir::definitions::DefPathHash;
 use rustc_hir::HirId;
 use rustc_query_system::dep_graph::FingerprintStyle;
 use rustc_span::symbol::Symbol;
 use std::hash::Hash;

 pub use rustc_query_system::dep_graph::{DepContext, DepNodeParams};

 /// This struct stores metadata about each DepKind.
 ///
 /// Information is retrieved by indexing the `DEP_KINDS` array using the integer value
 /// of the `DepKind`. Overall, this allows to implement `DepContext` using this manual
 /// jump table instead of large matches.
 pub struct DepKindStruct {
     /// Anonymous queries cannot be replayed from one compiler invocation to the next.
     /// When their result is needed, it is recomputed. They are useful for fine-grained
     /// dependency tracking, and caching within one compiler invocation.
     pub is_anon: bool,

     /// Eval-always queries do not track their dependencies, and are always recomputed, even if
     /// their inputs have not changed since the last compiler invocation. The result is still
     /// cached within one compiler invocation.
     pub is_eval_always: bool,

     /// Whether the query key can be recovered from the hashed fingerprint.
     /// See [DepNodeParams] trait for the behaviour of each key type.
     pub fingerprint_style: FingerprintStyle,

     /// The red/green evaluation system will try to mark a specific DepNode in the
     /// dependency graph as green by recursively trying to mark the dependencies of
     /// that `DepNode` as green. While doing so, it will sometimes encounter a `DepNode`
     /// where we don't know if it is red or green and we therefore actually have
     /// to recompute its value in order to find out. Since the only piece of
     /// information that we have at that point is the `DepNode` we are trying to
     /// re-evaluate, we need some way to re-run a query from just that. This is what
     /// `force_from_dep_node()` implements.
     ///
     /// In the general case, a `DepNode` consists of a `DepKind` and an opaque
     /// GUID/fingerprint that will uniquely identify the node. This GUID/fingerprint
     /// is usually constructed by computing a stable hash of the query-key that the
     /// `DepNode` corresponds to. Consequently, it is not in general possible to go
     /// back from hash to query-key (since hash functions are not reversible). For
     /// this reason `force_from_dep_node()` is expected to fail from time to time
     /// because we just cannot find out, from the `DepNode` alone, what the
     /// corresponding query-key is and therefore cannot re-run the query.
     ///
     /// The system deals with this case letting `try_mark_green` fail which forces
     /// the root query to be re-evaluated.
     ///
     /// Now, if `force_from_dep_node()` would always fail, it would be pretty useless.
     /// Fortunately, we can use some contextual information that will allow us to
     /// reconstruct query-keys for certain kinds of `DepNode`s. In particular, we
     /// enforce by construction that the GUID/fingerprint of certain `DepNode`s is a
     /// valid `DefPathHash`. Since we also always build a huge table that maps every
     /// `DefPathHash` in the current codebase to the corresponding `DefId`, we have
     /// everything we need to re-run the query.
     ///
     /// Take the `mir_promoted` query as an example. Like many other queries, it
     /// just has a single parameter: the `DefId` of the item it will compute the
     /// validated MIR for. Now, when we call `force_from_dep_node()` on a `DepNode`
     /// with kind `MirValidated`, we know that the GUID/fingerprint of the `DepNode`
     /// is actually a `DefPathHash`, and can therefore just look up the corresponding
     /// `DefId` in `tcx.def_path_hash_to_def_id`.
     pub force_from_dep_node: Option<fn(tcx: TyCtxt<'_>, dep_node: DepNode) -> bool>,

     /// Invoke a query to put the on-disk cached value in memory.
     pub try_load_from_on_disk_cache: Option<fn(TyCtxt<'_>, DepNode)>,
 }

 impl DepKind {
     #[inline(always)]
     pub fn fingerprint_style(self, tcx: TyCtxt<'_>) -> FingerprintStyle {
         // Only fetch the DepKindStruct once.
         let data = tcx.query_kind(self);
         if data.is_anon {
             return FingerprintStyle::Opaque;
         }
         data.fingerprint_style
     }
 }

 macro_rules! define_dep_nodes {
     (<$tcx:tt>
     $(
         [$($attrs:tt)*]
         $variant:ident $(( $tuple_arg_ty:ty $(,)? ))*
       ,)*
     ) => (
         #[macro_export]
         macro_rules! make_dep_kind_array {
             ($mod:ident) => {[ $($mod::$variant()),* ]};
         }

         /// This enum serves as an index into arrays built by `make_dep_kind_array`.
         #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
         #[allow(non_camel_case_types)]
         pub enum DepKind {
             $($variant),*
         }

         fn dep_kind_from_label_string(label: &str) -> Result<DepKind, ()> {
             match label {
                 $(stringify!($variant) => Ok(DepKind::$variant),)*
                 _ => Err(()),
             }
         }

         /// Contains variant => str representations for constructing
         /// DepNode groups for tests.
         #[allow(dead_code, non_upper_case_globals)]
         pub mod label_strs {
            $(
                 pub const $variant: &str = stringify!($variant);
             )*
         }
     );
 }

 rustc_dep_node_append!([define_dep_nodes!][ <'tcx>
     // We use this for most things when incr. comp. is turned off.
     [] Null,

     [anon] TraitSelect,

     // WARNING: if `Symbol` is changed, make sure you update `make_compile_codegen_unit` below.
     [] CompileCodegenUnit(Symbol),

     // WARNING: if `MonoItem` is changed, make sure you update `make_compile_mono_item` below.
     // Only used by rustc_codegen_cranelift
     [] CompileMonoItem(MonoItem),
 ]);

 // WARNING: `construct` is generic and does not know that `CompileCodegenUnit` takes `Symbol`s as keys.
 // Be very careful changing this type signature!
 crate fn make_compile_codegen_unit(tcx: TyCtxt<'_>, name: Symbol) -> DepNode {
     DepNode::construct(tcx, DepKind::CompileCodegenUnit, &name)
 }

 // WARNING: `construct` is generic and does not know that `CompileMonoItem` takes `MonoItem`s as keys.
 // Be very careful changing this type signature!
 crate fn make_compile_mono_item<'tcx>(tcx: TyCtxt<'tcx>, mono_item: &MonoItem<'tcx>) -> DepNode {
     DepNode::construct(tcx, DepKind::CompileMonoItem, mono_item)
 }

 pub type DepNode = rustc_query_system::dep_graph::DepNode<DepKind>;

 // We keep a lot of `DepNode`s in memory during compilation. It's not
 // required that their size stay the same, but we don't want to change
 // it inadvertently. This assert just ensures we're aware of any change.
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 static_assert_size!(DepNode, 18);

 #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
 static_assert_size!(DepNode, 24);

 pub trait DepNodeExt: Sized {
     /// Construct a DepNode from the given DepKind and DefPathHash. This
     /// method will assert that the given DepKind actually requires a
     /// single DefId/DefPathHash parameter.
     fn from_def_path_hash(tcx: TyCtxt<'_>, def_path_hash: DefPathHash, kind: DepKind) -> Self;

     /// Extracts the DefId corresponding to this DepNode. This will work
     /// if two conditions are met:
     ///
     /// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
     /// 2. the item that the DefPath refers to exists in the current tcx.
     ///
     /// Condition (1) is determined by the DepKind variant of the
     /// DepNode. Condition (2) might not be fulfilled if a DepNode
     /// refers to something from the previous compilation session that
     /// has been removed.
     fn extract_def_id(&self, tcx: TyCtxt<'_>) -> Option<DefId>;

     /// Used in testing
     fn from_label_string(
         tcx: TyCtxt<'_>,
         label: &str,
         def_path_hash: DefPathHash,
     ) -> Result<Self, ()>;

     /// Used in testing
     fn has_label_string(label: &str) -> bool;
 }

 impl DepNodeExt for DepNode {
     /// Construct a DepNode from the given DepKind and DefPathHash. This
     /// method will assert that the given DepKind actually requires a
     /// single DefId/DefPathHash parameter.
     fn from_def_path_hash(tcx: TyCtxt<'_>, def_path_hash: DefPathHash, kind: DepKind) -> DepNode {
         debug_assert!(kind.fingerprint_style(tcx) == FingerprintStyle::DefPathHash);
         DepNode { kind, hash: def_path_hash.0.into() }
     }

     /// Extracts the DefId corresponding to this DepNode. This will work
     /// if two conditions are met:
     ///
     /// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
     /// 2. the item that the DefPath refers to exists in the current tcx.
     ///
     /// Condition (1) is determined by the DepKind variant of the
     /// DepNode. Condition (2) might not be fulfilled if a DepNode
     /// refers to something from the previous compilation session that
     /// has been removed.
     fn extract_def_id<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Option<DefId> {
         if self.kind.fingerprint_style(tcx) == FingerprintStyle::DefPathHash {
             Some(tcx.def_path_hash_to_def_id(DefPathHash(self.hash.into()), &mut || {
                 panic!("Failed to extract DefId: {:?} {}", self.kind, self.hash)
             }))
         } else {
             None
         }
     }

     /// Used in testing
     fn from_label_string(
         tcx: TyCtxt<'_>,
         label: &str,
         def_path_hash: DefPathHash,
     ) -> Result<DepNode, ()> {
         let kind = dep_kind_from_label_string(label)?;

         match kind.fingerprint_style(tcx) {
             FingerprintStyle::Opaque => Err(()),
             FingerprintStyle::Unit => Ok(DepNode::new_no_params(tcx, kind)),
             FingerprintStyle::DefPathHash => {
                 Ok(DepNode::from_def_path_hash(tcx, def_path_hash, kind))
             }
         }
     }

     /// Used in testing
     fn has_label_string(label: &str) -> bool {
         dep_kind_from_label_string(label).is_ok()
     }
 }

 impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for () {
     #[inline(always)]
     fn fingerprint_style() -> FingerprintStyle {
         FingerprintStyle::Unit
     }

     #[inline(always)]
     fn to_fingerprint(&self, _: TyCtxt<'tcx>) -> Fingerprint {
         Fingerprint::ZERO
     }

     #[inline(always)]
     fn recover(_: TyCtxt<'tcx>, _: &DepNode) -> Option<Self> {
         Some(())
     }
 }

 impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for DefId {
     #[inline(always)]
     fn fingerprint_style() -> FingerprintStyle {
         FingerprintStyle::DefPathHash
     }

     #[inline(always)]
     fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
         tcx.def_path_hash(*self).0
     }

     #[inline(always)]
     fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
         tcx.def_path_str(*self)
     }

     #[inline(always)]
     fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
         dep_node.extract_def_id(tcx)
     }
 }

 impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for LocalDefId {
     #[inline(always)]
     fn fingerprint_style() -> FingerprintStyle {
         FingerprintStyle::DefPathHash
     }

     #[inline(always)]
     fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
         self.to_def_id().to_fingerprint(tcx)
     }

     #[inline(always)]
     fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
         self.to_def_id().to_debug_str(tcx)
     }

     #[inline(always)]
     fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
         dep_node.extract_def_id(tcx).map(|id| id.expect_local())
     }
 }

 impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for CrateNum {
     #[inline(always)]
     fn fingerprint_style() -> FingerprintStyle {
         FingerprintStyle::DefPathHash
     }

     #[inline(always)]
     fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
         let def_id = self.as_def_id();
         def_id.to_fingerprint(tcx)
     }

     #[inline(always)]
     fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
         tcx.crate_name(*self).to_string()
     }

     #[inline(always)]
     fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
         dep_node.extract_def_id(tcx).map(|id| id.krate)
     }
 }

 impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for (DefId, DefId) {
     #[inline(always)]
     fn fingerprint_style() -> FingerprintStyle {
         FingerprintStyle::Opaque
     }

     // We actually would not need to specialize the implementation of this
     // method but it's faster to combine the hashes than to instantiate a full
     // hashing context and stable-hashing state.
     #[inline(always)]
     fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
         let (def_id_0, def_id_1) = *self;

         let def_path_hash_0 = tcx.def_path_hash(def_id_0);
         let def_path_hash_1 = tcx.def_path_hash(def_id_1);

         def_path_hash_0.0.combine(def_path_hash_1.0)
     }

     #[inline(always)]
     fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
         let (def_id_0, def_id_1) = *self;

         format!("({}, {})", tcx.def_path_debug_str(def_id_0), tcx.def_path_debug_str(def_id_1))
     }
 }

 impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for HirId {
     #[inline(always)]
     fn fingerprint_style() -> FingerprintStyle {
         FingerprintStyle::Opaque
     }

     // We actually would not need to specialize the implementation of this
     // method but it's faster to combine the hashes than to instantiate a full
     // hashing context and stable-hashing state.
     #[inline(always)]
     fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
         let HirId { owner, local_id } = *self;

         let def_path_hash = tcx.def_path_hash(owner.to_def_id());
         let local_id = Fingerprint::from_smaller_hash(local_id.as_u32().into());

         def_path_hash.0.combine(local_id)
     }
 }
	//! Nodes in the dependency graph.
	//!
	//! A node in the [dependency graph] is represented by a [`DepNode`].
	//! A `DepNode` consists of a [`DepKind`] (which
	//! specifies the kind of thing it represents, like a piece of HIR, MIR, etc.)
	//! and a [`Fingerprint`], a 128-bit hash value, the exact meaning of which
	//! depends on the node's `DepKind`. Together, the kind and the fingerprint
	//! fully identify a dependency node, even across multiple compilation sessions.
	//! In other words, the value of the fingerprint does not depend on anything
	//! that is specific to a given compilation session, like an unpredictable
	//! interning key (e.g., `NodeId`, `DefId`, `Symbol`) or the numeric value of a
	//! pointer. The concept behind this could be compared to how git commit hashes
	//! uniquely identify a given commit. The fingerprinting approach has
	//! a few advantages:
	//!
	//! * A `DepNode` can simply be serialized to disk and loaded in another session
	//! without the need to do any "rebasing" (like we have to do for Spans and
	//! NodeIds) or "retracing" (like we had to do for `DefId` in earlier
	//! implementations of the dependency graph).
	//! * A `Fingerprint` is just a bunch of bits, which allows `DepNode` to
	//! implement `Copy`, `Sync`, `Send`, `Freeze`, etc.
	//! * Since we just have a bit pattern, `DepNode` can be mapped from disk into
	//! memory without any post-processing (e.g., "abomination-style" pointer
	//! reconstruction).
	//! * Because a `DepNode` is self-contained, we can instantiate `DepNodes` that
	//! refer to things that do not exist anymore. In previous implementations
	//! `DepNode` contained a `DefId`. A `DepNode` referring to something that
	//! had been removed between the previous and the current compilation session
	//! could not be instantiated because the current compilation session
	//! contained no `DefId` for thing that had been removed.
	//!
	//! `DepNode` definition happens in the `define_dep_nodes!()` macro. This macro
	//! defines the `DepKind` enum. Each `DepKind` has its own parameters that are
	//! needed at runtime in order to construct a valid `DepNode` fingerprint.
	//! However, only `CompileCodegenUnit` and `CompileMonoItem` are constructed
	//! explicitly (with `make_compile_codegen_unit` cq `make_compile_mono_item`).
	//!
	//! Because the macro sees what parameters a given `DepKind` requires, it can
	//! "infer" some properties for each kind of `DepNode`:
	//!
	//! * Whether a `DepNode` of a given kind has any parameters at all. Some
	//! `DepNode`s could represent global concepts with only one value.
	//! * Whether it is possible, in principle, to reconstruct a query key from a
	//! given `DepNode`. Many `DepKind`s only require a single `DefId` parameter,
	//! in which case it is possible to map the node's fingerprint back to the
	//! `DefId` it was computed from. In other cases, too much information gets
	//! lost during fingerprint computation.
	//!
	//! `make_compile_codegen_unit` and `make_compile_mono_items`, together with
	//! `DepNode::new()`, ensures that only valid `DepNode` instances can be
	//! constructed. For example, the API does not allow for constructing
	//! parameterless `DepNode`s with anything other than a zeroed out fingerprint.
	//! More generally speaking, it relieves the user of the `DepNode` API of
	//! having to know how to compute the expected fingerprint for a given set of
	//! node parameters.
	//!
	//! [dependency graph]: https://rustc-dev-guide.rust-lang.org/query.html

	use crate::mir::mono::MonoItem;
	use crate::ty::TyCtxt;

	use rustc_data_structures::fingerprint::Fingerprint;
	use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
	use rustc_hir::definitions::DefPathHash;
	use rustc_hir::HirId;
	use rustc_query_system::dep_graph::FingerprintStyle;
	use rustc_span::symbol::Symbol;
	use std::hash::Hash;

	pub use rustc_query_system::dep_graph::{DepContext, DepNodeParams};

	/// This struct stores metadata about each DepKind.
	///
	/// Information is retrieved by indexing the `DEP_KINDS` array using the integer value
	/// of the `DepKind`. Overall, this allows to implement `DepContext` using this manual
	/// jump table instead of large matches.
	pub struct DepKindStruct {
	/// Anonymous queries cannot be replayed from one compiler invocation to the next.
	/// When their result is needed, it is recomputed. They are useful for fine-grained
	/// dependency tracking, and caching within one compiler invocation.
	pub is_anon: bool,

	/// Eval-always queries do not track their dependencies, and are always recomputed, even if
	/// their inputs have not changed since the last compiler invocation. The result is still
	/// cached within one compiler invocation.
	pub is_eval_always: bool,

	/// Whether the query key can be recovered from the hashed fingerprint.
	/// See [DepNodeParams] trait for the behaviour of each key type.
	pub fingerprint_style: FingerprintStyle,

	/// The red/green evaluation system will try to mark a specific DepNode in the
	/// dependency graph as green by recursively trying to mark the dependencies of
	/// that `DepNode` as green. While doing so, it will sometimes encounter a `DepNode`
	/// where we don't know if it is red or green and we therefore actually have
	/// to recompute its value in order to find out. Since the only piece of
	/// information that we have at that point is the `DepNode` we are trying to
	/// re-evaluate, we need some way to re-run a query from just that. This is what
	/// `force_from_dep_node()` implements.
	///
	/// In the general case, a `DepNode` consists of a `DepKind` and an opaque
	/// GUID/fingerprint that will uniquely identify the node. This GUID/fingerprint
	/// is usually constructed by computing a stable hash of the query-key that the
	/// `DepNode` corresponds to. Consequently, it is not in general possible to go
	/// back from hash to query-key (since hash functions are not reversible). For
	/// this reason `force_from_dep_node()` is expected to fail from time to time
	/// because we just cannot find out, from the `DepNode` alone, what the
	/// corresponding query-key is and therefore cannot re-run the query.
	///
	/// The system deals with this case letting `try_mark_green` fail which forces
	/// the root query to be re-evaluated.
	///
	/// Now, if `force_from_dep_node()` would always fail, it would be pretty useless.
	/// Fortunately, we can use some contextual information that will allow us to
	/// reconstruct query-keys for certain kinds of `DepNode`s. In particular, we
	/// enforce by construction that the GUID/fingerprint of certain `DepNode`s is a
	/// valid `DefPathHash`. Since we also always build a huge table that maps every
	/// `DefPathHash` in the current codebase to the corresponding `DefId`, we have
	/// everything we need to re-run the query.
	///
	/// Take the `mir_promoted` query as an example. Like many other queries, it
	/// just has a single parameter: the `DefId` of the item it will compute the
	/// validated MIR for. Now, when we call `force_from_dep_node()` on a `DepNode`
	/// with kind `MirValidated`, we know that the GUID/fingerprint of the `DepNode`
	/// is actually a `DefPathHash`, and can therefore just look up the corresponding
	/// `DefId` in `tcx.def_path_hash_to_def_id`.
	pub force_from_dep_node: Option<fn(tcx: TyCtxt<'_>, dep_node: DepNode) -> bool>,

	/// Invoke a query to put the on-disk cached value in memory.
	pub try_load_from_on_disk_cache: Option<fn(TyCtxt<'_>, DepNode)>,
	}

	impl DepKind {
	#[inline(always)]
	pub fn fingerprint_style(self, tcx: TyCtxt<'_>) -> FingerprintStyle {
	// Only fetch the DepKindStruct once.
	let data = tcx.query_kind(self);
	if data.is_anon {
	return FingerprintStyle::Opaque;
	}
	data.fingerprint_style
	}
	}

	macro_rules! define_dep_nodes {
	(<$tcx:tt>
	$(
	[$($attrs:tt)*]
	$variant:ident $(( $tuple_arg_ty:ty $(,)? ))*
	,)*
	) => (
	#[macro_export]
	macro_rules! make_dep_kind_array {
	($mod:ident) => {[ $($mod::$variant()),* ]};
	}

	/// This enum serves as an index into arrays built by `make_dep_kind_array`.
	#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
	#[allow(non_camel_case_types)]
	pub enum DepKind {
	$($variant),*
	}

	fn dep_kind_from_label_string(label: &str) -> Result<DepKind, ()> {
	match label {
	$(stringify!($variant) => Ok(DepKind::$variant),)*
	_ => Err(()),
	}
	}

	/// Contains variant => str representations for constructing
	/// DepNode groups for tests.
	#[allow(dead_code, non_upper_case_globals)]
	pub mod label_strs {
	$(
	pub const $variant: &str = stringify!($variant);
	)*
	}
	);
	}

	rustc_dep_node_append!([define_dep_nodes!][ <'tcx>
	// We use this for most things when incr. comp. is turned off.
	[] Null,

	[anon] TraitSelect,

	// WARNING: if `Symbol` is changed, make sure you update `make_compile_codegen_unit` below.
	[] CompileCodegenUnit(Symbol),

	// WARNING: if `MonoItem` is changed, make sure you update `make_compile_mono_item` below.
	// Only used by rustc_codegen_cranelift
	[] CompileMonoItem(MonoItem),
	]);

	// WARNING: `construct` is generic and does not know that `CompileCodegenUnit` takes `Symbol`s as keys.
	// Be very careful changing this type signature!
	crate fn make_compile_codegen_unit(tcx: TyCtxt<'_>, name: Symbol) -> DepNode {
	DepNode::construct(tcx, DepKind::CompileCodegenUnit, &name)
	}

	// WARNING: `construct` is generic and does not know that `CompileMonoItem` takes `MonoItem`s as keys.
	// Be very careful changing this type signature!
	crate fn make_compile_mono_item<'tcx>(tcx: TyCtxt<'tcx>, mono_item: &MonoItem<'tcx>) -> DepNode {
	DepNode::construct(tcx, DepKind::CompileMonoItem, mono_item)
	}

	pub type DepNode = rustc_query_system::dep_graph::DepNode<DepKind>;

	// We keep a lot of `DepNode`s in memory during compilation. It's not
	// required that their size stay the same, but we don't want to change
	// it inadvertently. This assert just ensures we're aware of any change.
	#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
	static_assert_size!(DepNode, 18);

	#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
	static_assert_size!(DepNode, 24);

	pub trait DepNodeExt: Sized {
	/// Construct a DepNode from the given DepKind and DefPathHash. This
	/// method will assert that the given DepKind actually requires a
	/// single DefId/DefPathHash parameter.
	fn from_def_path_hash(tcx: TyCtxt<'_>, def_path_hash: DefPathHash, kind: DepKind) -> Self;

	/// Extracts the DefId corresponding to this DepNode. This will work
	/// if two conditions are met:
	///
	/// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
	/// 2. the item that the DefPath refers to exists in the current tcx.
	///
	/// Condition (1) is determined by the DepKind variant of the
	/// DepNode. Condition (2) might not be fulfilled if a DepNode
	/// refers to something from the previous compilation session that
	/// has been removed.
	fn extract_def_id(&self, tcx: TyCtxt<'_>) -> Option<DefId>;

	/// Used in testing
	fn from_label_string(
	tcx: TyCtxt<'_>,
	label: &str,
	def_path_hash: DefPathHash,
	) -> Result<Self, ()>;

	/// Used in testing
	fn has_label_string(label: &str) -> bool;
	}

	impl DepNodeExt for DepNode {
	/// Construct a DepNode from the given DepKind and DefPathHash. This
	/// method will assert that the given DepKind actually requires a
	/// single DefId/DefPathHash parameter.
	fn from_def_path_hash(tcx: TyCtxt<'_>, def_path_hash: DefPathHash, kind: DepKind) -> DepNode {
	debug_assert!(kind.fingerprint_style(tcx) == FingerprintStyle::DefPathHash);
	DepNode { kind, hash: def_path_hash.0.into() }
	}

	/// Extracts the DefId corresponding to this DepNode. This will work
	/// if two conditions are met:
	///
	/// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
	/// 2. the item that the DefPath refers to exists in the current tcx.
	///
	/// Condition (1) is determined by the DepKind variant of the
	/// DepNode. Condition (2) might not be fulfilled if a DepNode
	/// refers to something from the previous compilation session that
	/// has been removed.
	fn extract_def_id<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Option<DefId> {
	if self.kind.fingerprint_style(tcx) == FingerprintStyle::DefPathHash {
	Some(tcx.def_path_hash_to_def_id(DefPathHash(self.hash.into()), &mut \|\| {
	panic!("Failed to extract DefId: {:?} {}", self.kind, self.hash)
	}))
	} else {
	None
	}
	}

	/// Used in testing
	fn from_label_string(
	tcx: TyCtxt<'_>,
	label: &str,
	def_path_hash: DefPathHash,
	) -> Result<DepNode, ()> {
	let kind = dep_kind_from_label_string(label)?;

	match kind.fingerprint_style(tcx) {
	FingerprintStyle::Opaque => Err(()),
	FingerprintStyle::Unit => Ok(DepNode::new_no_params(tcx, kind)),
	FingerprintStyle::DefPathHash => {
	Ok(DepNode::from_def_path_hash(tcx, def_path_hash, kind))
	}
	}
	}

	/// Used in testing
	fn has_label_string(label: &str) -> bool {
	dep_kind_from_label_string(label).is_ok()
	}
	}

	impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for () {
	#[inline(always)]
	fn fingerprint_style() -> FingerprintStyle {
	FingerprintStyle::Unit
	}

	#[inline(always)]
	fn to_fingerprint(&self, _: TyCtxt<'tcx>) -> Fingerprint {
	Fingerprint::ZERO
	}

	#[inline(always)]
	fn recover(_: TyCtxt<'tcx>, _: &DepNode) -> Option<Self> {
	Some(())
	}
	}

	impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for DefId {
	#[inline(always)]
	fn fingerprint_style() -> FingerprintStyle {
	FingerprintStyle::DefPathHash
	}

	#[inline(always)]
	fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
	tcx.def_path_hash(*self).0
	}

	#[inline(always)]
	fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
	tcx.def_path_str(*self)
	}

	#[inline(always)]
	fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
	dep_node.extract_def_id(tcx)
	}
	}

	impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for LocalDefId {
	#[inline(always)]
	fn fingerprint_style() -> FingerprintStyle {
	FingerprintStyle::DefPathHash
	}

	#[inline(always)]
	fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
	self.to_def_id().to_fingerprint(tcx)
	}

	#[inline(always)]
	fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
	self.to_def_id().to_debug_str(tcx)
	}

	#[inline(always)]
	fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
	dep_node.extract_def_id(tcx).map(\|id\| id.expect_local())
	}
	}

	impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for CrateNum {
	#[inline(always)]
	fn fingerprint_style() -> FingerprintStyle {
	FingerprintStyle::DefPathHash
	}

	#[inline(always)]
	fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
	let def_id = self.as_def_id();
	def_id.to_fingerprint(tcx)
	}

	#[inline(always)]
	fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
	tcx.crate_name(*self).to_string()
	}

	#[inline(always)]
	fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
	dep_node.extract_def_id(tcx).map(\|id\| id.krate)
	}
	}

	impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for (DefId, DefId) {
	#[inline(always)]
	fn fingerprint_style() -> FingerprintStyle {
	FingerprintStyle::Opaque
	}

	// We actually would not need to specialize the implementation of this
	// method but it's faster to combine the hashes than to instantiate a full
	// hashing context and stable-hashing state.
	#[inline(always)]
	fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
	let (def_id_0, def_id_1) = *self;

	let def_path_hash_0 = tcx.def_path_hash(def_id_0);
	let def_path_hash_1 = tcx.def_path_hash(def_id_1);

	def_path_hash_0.0.combine(def_path_hash_1.0)
	}

	#[inline(always)]
	fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
	let (def_id_0, def_id_1) = *self;

	format!("({}, {})", tcx.def_path_debug_str(def_id_0), tcx.def_path_debug_str(def_id_1))
	}
	}

	impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for HirId {
	#[inline(always)]
	fn fingerprint_style() -> FingerprintStyle {
	FingerprintStyle::Opaque
	}

	// We actually would not need to specialize the implementation of this
	// method but it's faster to combine the hashes than to instantiate a full
	// hashing context and stable-hashing state.
	#[inline(always)]
	fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
	let HirId { owner, local_id } = *self;

	let def_path_hash = tcx.def_path_hash(owner.to_def_id());
	let local_id = Fingerprint::from_smaller_hash(local_id.as_u32().into());

	def_path_hash.0.combine(local_id)
	}
	}