Module rustc_hir::def_id

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  • A DefId identifies a particular definition, by combining a crate index and a def index.
  • A DefIndex is an index into the hir-map for a crate, identifying a particular definition. It should really be considered an interned shorthand for a particular DefPath.
  • A DefPathHash is a fixed-size representation of a DefPath that is stable across crate and compilation session boundaries. It consists of two separate 64-bit hashes. The first uniquely identifies the crate this DefPathHash originates from (see StableCrateId), and the second uniquely identifies the corresponding DefPath within that crate. Together they form a unique identifier within an entire crate graph.
  • A LocalDefId is equivalent to a DefId with krate == LOCAL_CRATE. Since we encode this information in the type, we can ensure at compile time that no DefIds from upstream crates get thrown into the mix. There are quite a few cases where we know that only DefIds from the local crate are expected; a DefId from a different crate would signify a bug somewhere. This is when LocalDefId comes in handy.
  • A StableCrateId is a 64-bit hash of a crate name, together with all -Cmetadata arguments, and some other data. It is to CrateNum what DefPathHash is to DefId. It is stable across compilation sessions.


  • The crate root is always assigned index 0 by the AST Map code, thanks to NodeCollector::new.
  • Item definitions in the currently-compiled crate would have the CrateNum LOCAL_CRATE in their DefId.



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