Module rustc_query_system::dep_graph::dep_node

This module defines the DepNode type which the compiler uses to represent nodes in the dependency graph. 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 and 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 rustc_middle with the define_dep_nodes!() macro. This macro defines the DepKind enum and a corresponding DepConstructor enum. The DepConstructor enum links a DepKind to the parameters that are needed at runtime in order to construct a valid DepNode fingerprint.

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 DepNodes could represent global concepts with only one value.
• Whether it is possible, in principle, to reconstruct a query key from a given DepNode. Many DepKinds 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.

Structs

• DepKind
  This serves as an index into arrays built by make_dep_kind_array.
• DepKindStruct
  This struct stores metadata about each DepKind.
• DepNode
• WorkProductId
  A “work product” corresponds to a .o (or other) file that we save in between runs. These IDs do not have a DefId but rather some independent path or string that persists between runs without the need to be mapped or unmapped. (This ensures we can serialize them even in the absence of a tcx.)

Statics

• DEP_KIND_DEBUG
• DEP_NODE_DEBUG

Traits

• DepNodeParams

Functions

• default_dep_kind_debug
• default_dep_node_debug