Glossary

A short reference for project-specific terms. The authoritative list lives in the rustc-dev-guide glossary; this page is a curated subset most contributors will hit early.

Compiler stages and pipelines

Term	Meaning
Stage 0	The pre-built downloaded compiler used to bootstrap. Pinned by src/stage0.
Stage 1	The compiler built by stage 0. Used for local development.
Stage 2	The compiler built by stage 1. Distributed in releases.
Sysroot	The directory containing a compiler's lib/, bin/, and rustlib/ — the standard library it links against.
Snapshot / beta	The previous stable Rust release used as stage 0; bumped by src/tools/bump-stage0.
Subtree / submodule	External-repo tools pulled in. Subtrees (clippy, miri, rustfmt, rustc-dev-guide) live in this repo via git subtree; submodules (cargo, rust-analyzer) are pinned commits.

Intermediate representations

Term	Meaning
Token tree	The output of the lexer (rustc_lexer) plus light grouping from the parser.
AST	Abstract Syntax Tree — produced by rustc_parse, defined in rustc_ast. Source-faithful, includes unexpanded macros.
HIR	High-level IR — produced by rustc_ast_lowering from the AST after macro expansion and name resolution. Stable target for type checking. Defined in rustc_hir.
THIR	Typed HIR — type-annotated tree built from HIR, used briefly during MIR construction. Defined in rustc_middle::thir.
MIR	Mid-level IR — control-flow-graph based IR used for borrow checking, optimization, and codegen. Defined in rustc_middle::mir.
LLVM IR	The output of the LLVM codegen backend, fed to LLVM proper. The Cranelift and GCC backends produce their own IRs instead.

Type system

Term	Meaning
TyCtxt	The type context — the central handle through which most compiler queries flow. Lives in rustc_middle::ty.
DefId	Stable identifier for a definition (function, type, module). Carries both crate index and intra-crate index.
HirId	Identifier for a node in HIR.
Substs / GenericArgs	The arguments to a generic — type, lifetime, and const args bundled together.
Trait obligation	A "this type implements this trait" claim that the trait solver tries to prove.
Trait solver	The engine in rustc_trait_selection (and the experimental rustc_next_trait_solver) that proves obligations.
NLL	Non-Lexical Lifetimes — the borrow checker's region inference (now the default; the term survives in tests under tests/nll/).
Polonius	An experimental Datalog-based reformulation of NLL.

Compilation infrastructure

Term	Meaning
Query	A demand-driven computation registered in rustc_middle::query and executed by rustc_query_impl. The basis of incremental compilation.
Incr. comp.	Incremental compilation — the on-disk cache of query results stored in target/.../incremental/.
Crate metadata (.rmeta)	Serialized HIR/types/etc. for a dependency crate, read by rustc_metadata.
Codegen unit (CGU)	A unit of LLVM-level work; rustc partitions monomorphized items into CGUs that are compiled in parallel.
Monomorphization	The process of instantiating each generic at every concrete type it's used with, performed by rustc_monomorphize.
CTFE	Compile-Time Function Evaluation — the MIR interpreter in rustc_const_eval. Powers const fn, const, and patterns.
Miri	The same MIR interpreter used as a standalone undefined-behavior checker (src/tools/miri/).

Project workflow

Term	Meaning
bors / homu / rust-bors	The merge queue bot. Only it merges to main. Triggered by @bors r+.
Try build	A no-merge full CI run on a PR (@bors try), used to gather artifacts for perf runs.
Crater	The ecosystem-impact tool that compiles every crate on crates.io with a candidate compiler.
rustc-perf	The performance tracker; CI runs benchmarks against try builds.
Beta crater run / beta backport	A nominated PR for inclusion in the beta release. Driven by labels in triagebot.toml.
FCP	Final Comment Period — the team-driven approval phase before stabilizing or accepting a feature.
Tidy	The repo-wide lint (src/tools/tidy/). Catches license-header issues, alphabetical-order violations, oversized files, banned patterns, etc.
Compiletest	The test harness (src/tools/compiletest/) that drives the tests/ directory.
UI test	A test under tests/ui/ that compares compiler stdout/stderr against expected .stderr / .stdout files.
Run-make test	A test under tests/run-make/ driven by a Rust support library (src/tools/run-make-support/).
--bless	Tell compiletest to overwrite the expected output files with the current actual output.

Editions and release channels

Term	Meaning
Edition	Opt-in language updates (2015, 2018, 2021, 2024, …). Crate-level setting.
Channel	nightly (every day from main), beta (frozen 6 weeks), stable (frozen 6 weeks). The cadence is the "release train".
Stage0 channel	The channel of the downloaded bootstrap compiler — usually beta. Stored in src/ci/channel.
Tier	Platform support tier. Tier 1 has full automated test coverage and binary distribution; tier 2 has builds; tier 3 is best-effort.