# 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 [`NodeId`]s to change. This renders [`NodeId`]s pretty much useless for incremental compilation, where you want as few things as possible to change. [`NodeId`]s are used in all the `rustc` bits that operate directly on the AST, like macro expansion and name resolution. [`NodeId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html ## 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 [`HirId`]s, 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 [`DefId`]s, a [`HirId`] can refer to [fine-grained entities][Node] 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][hir-bodies]. These identifiers can be converted into one another through the [HIR map][map]. See the [HIR chapter][hir-map] for more detailed information. [`DefId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html [`LocalDefId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.LocalDefId.html [`HirId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir_id/struct.HirId.html [`BodyId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.BodyId.html [`CrateNum`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.CrateNum.html [`DefIndex`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefIndex.html [`Body`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.Body.html [Node]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.Node.html [hir-map]: ./hir.md#the-hir-map [hir-bodies]: ./hir.md#hir-bodies [map]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/hir/map/struct.Map.html ## 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()`] (note that you must call a function; the field is private). - [`Local`] identifies a local variable in a function. Its associated data is in [`LocalDecl`], which can be retrieved by indexing into [`Body.local_decls`]. - [`Field`] 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. [`BasicBlock`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.BasicBlock.html [`BasicBlockData`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.BasicBlockData.html [`Body::basic_blocks()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Body.html#method.basic_blocks [`Local`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Local.html [`LocalDecl`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.LocalDecl.html [`Body.local_decls`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Body.html#structfield.local_decls [`Field`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Field.html [`Place`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Place.html [`SourceScope`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.SourceScope.html [`SourceScopeData`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.SourceScopeData.html [`Body.source_scopes`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Body.html#structfield.source_scopes [`Promoted`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Promoted.html [`GlobalId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/struct.GlobalId.html [`Location`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Location.html [zero-sized types]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts