Identifiers in the compiler

If you have read the few previous chapters, you now know that rustc uses many different intermediate representations to perform different kinds of analyses. However, like in every data structure, you need a way to traverse the structure and refer to other elements. In this chapter, you will find information on the different identifiers rustc uses for each intermediate representation.

In the AST

A NodeId is an identifier number that uniquely identifies an AST node within a crate. Every node in the AST has its own NodeId, including top-level items such as structs, but also individual statements and expressions.

However, because they are absolute within a crate, adding or removing a single node in the AST causes all the subsequent NodeIds to change. This renders NodeIds pretty much useless for incremental compilation, where you want as few things as possible to change.

NodeIds are used in all the rustc bits that operate directly on the AST, like macro expansion and name resolution.

In the HIR

The HIR uses a bunch of different identifiers that coexist and serve different purposes.

  • A DefId, as the name suggests, identifies a particular definition, or top-level item, in a given crate. It is composed of two parts: a CrateNum which identifies the crate the definition comes from, and a DefIndex which identifies the definition within the crate. Unlike HirIds, there isn't a DefId for every expression, which makes them more stable across compilations.

  • A LocalDefId is basically a DefId that is known to come from the current crate. This allows us to drop the CrateNum part, and use the type system to ensure that only local definitions are passed to functions that expect a local definition.

  • A HirId uniquely identifies a node in the HIR of the current crate. It is composed of two parts: an owner and a local_id that is unique within the owner. This combination makes for more stable values which are helpful for incremental compilation. Unlike DefIds, a HirId can refer to fine-grained entities like expressions, but stays local to the current crate.

  • A BodyId identifies a HIR Body in the current crate. It is currently only a wrapper around a HirId. For more info about HIR bodies, please refer to the HIR chapter.

These identifiers can be converted into one another through the HIR map. See the HIR chapter for more detailed information.

In the MIR

  • BasicBlock identifies a basic block. It points to an instance of BasicBlockData, which can be retrieved by indexing into Body.basic_blocks.

  • Local identifies a local variable in a function. Its associated data is in LocalDecl, which can be retrieved by indexing into Body.local_decls.

  • FieldIdx identifies a struct's, union's, or enum variant's field. It is used as a "projection" in Place.

  • SourceScope identifies a name scope in the original source code. Used for diagnostics and for debuginfo in debuggers. It points to an instance of SourceScopeData, which can be retrieved by indexing into Body.source_scopes.

  • Promoted identifies a promoted constant within another item (related to const evaluation). Note: it is unique only locally within the item, so it should be associated with a DefId. GlobalId will give you a more specific identifier.

  • GlobalId identifies a global variable: a const, a static, a const fn where all arguments are zero-sized types, or a promoted constant.

  • Location represents the location in the MIR of a statement or terminator. It identifies the block (using BasicBlock) and the index of the statement or terminator in the block.