Glossary

Rustc-specific terms used across the rustc research docs.

CGU (Codegen Unit)

A partition of a crate’s code that is compiled independently by the backend (LLVM). rustc splits a crate into CGUs to enable parallel compilation and incremental reuse. Each CGU maps to a set of symbols (functions, statics) and produces its own artifact files.

DefId

A numeric identifier for a definition (function, struct, trait, etc.) within a specific compilation session. DefId values are session-local - the same definition may get a different DefId in the next session if the source changes. They must never be persisted directly to disk.

DefPath

A path-based, stable representation of a definition - e.g. std::collections::HashMap. Unlike DefId, DefPath is derived from the source structure and does not shift when unrelated code is added or removed.

DefPathHash

A 128-bit hash of a DefPath. It is Copy, cheap to compare, and stable across sessions. This is the practical unit of identity used when persisting anything keyed by definition to disk. On load, the compiler maps DefPathHash back to the current session’s DefId via a hash table lookup.

DepNode

A node in the dependency graph. Represents one unit of computation (a query invocation). Each DepNode carries a DepKind (which query) and a key fingerprint (which input). Edges between nodes encode “this computation read the result of that computation”.

Fingerprint

A 128-bit hash (u64, u64) produced by StableHasher. Used in two roles per dep node:

• Key fingerprint: identifies which node this is (a stable hash of the query key, e.g. a DefPathHash).
• Value fingerprint: a hash of the query’s return value, used for change detection across sessions.

HirId

An identifier for a HIR node that is not a definition-level item. Conceptually a pair of (DefPath, LocalId) - the DefPath identifies the owning item, and the LocalId identifies something within it. Stable across sessions as long as the owning item’s path does not change.

SVH (Stable Version Hash)

A hash of the crate’s content used to name finalized session directories. A session directory named s-{timestamp}-{SVH} is considered finalized and valid. This lets the compiler quickly identify the most recent valid cache for a given crate.

Work Product

A record of the compiled artifact files produced for one CGU in a session. Stored in work-products.bin. Contains the CGU name and a map of file type to file path (e.g. "o" -> "/path/to/foo.o"). On an incremental rebuild, if a CGU’s dep-node is green and the symbol hash matches, its work product files are reused directly instead of recompiling.

Source: graph.rs:1110-1135