Add a chapter on all the identifiers used through `rustc`

src/SUMMARY.md

@@ -85,6 +85,7 @@
     - [THIR and MIR construction](./mir/construction.md)
     - [MIR visitor and traversal](./mir/visitor.md)
     - [MIR passes: getting the MIR for a function](./mir/passes.md)
+- [Identifiers in the Compiler](./identifiers.md)
 - [Closure expansion](./closure.md)
 
 # Analysis

src/identifiers.md

@@ -0,0 +1,63 @@
+# Identifiers in the Compiler
+
+If you have read the few previous chapters, you now know that the `rustc` uses
+many different intermediate representations to perform different kinds of analysis.
+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 in 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 grate. 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 [`NodeId`]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 currenty 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/enum.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
+
+**TODO**