rustc-dev-guide/src/glossary.md

Glossary

The compiler uses a number of...idiosyncratic abbreviations and things. This glossary attempts to list them and give you a few pointers for understanding them better.

Term	Meaning
AST	the abstract syntax tree produced by the syntax crate; reflects user syntax very closely.
codegen unit	when we produce LLVM IR, we group the Rust code into a number of codegen units. Each of these units is processed by LLVM independently from one another, enabling parallelism. They are also the unit of incremental re-use.
cx	we tend to use "cx" as an abbrevation for context. See also tcx, infcx, etc.
DefId	an index identifying a definition (see librustc/hir/def_id.rs). Uniquely identifies a DefPath.
HIR	the High-level IR, created by lowering and desugaring the AST. See librustc/hir.
HirId	identifies a particular node in the HIR by combining a def-id with an "intra-definition offset".
'gcx	the lifetime of the global arena (see librustc/ty).
generics	the set of generic type parameters defined on a type or item
ICE	internal compiler error. When the compiler crashes.
infcx	the inference context (see librustc/infer)
MIR	the Mid-level IR that is created after type-checking for use by borrowck and trans. Defined in the src/librustc/mir/ module, but much of the code that manipulates it is found in src/librustc_mir.
obligation	something that must be proven by the trait system; see librustc/traits.
local crate	the crate currently being compiled.
node-id or NodeId	an index identifying a particular node in the AST or HIR; gradually being phased out and replaced with HirId.
query	perhaps some sub-computation during compilation; see librustc/maps.
provider	the function that executes a query; see librustc/maps.
sess	the compiler session, which stores global data used throughout compilation
side tables	because the AST and HIR are immutable once created, we often carry extra information about them in the form of hashtables, indexed by the id of a particular node.
span	a location in the user's source code, used for error reporting primarily. These are like a file-name/line-number/column tuple on steroids: they carry a start/end point, and also track macro expansions and compiler desugaring. All while being packed into a few bytes (really, it's an index into a table). See the Span datatype for more.
substs	the substitutions for a given generic type or item (e.g., the i32, u32 in HashMap<i32, u32>)
tcx	the "typing context", main data structure of the compiler (see librustc/ty).
trans	the code to translate MIR into LLVM IR.
trait reference	a trait and values for its type parameters (see librustc/ty).
ty	the internal representation of a type (see librustc/ty).