Compiler

The compiler/ directory contains the rustc compiler itself, split across roughly seventy rustc_* crates. This index gives a fly-over of what each crate is for and how they fit into the pipeline. Detailed coverage of the most important areas lives in the dedicated pages linked below.

Pages in this section

• Driver and interface — how compilation starts and is orchestrated
• Frontend: parse, expand, resolve, lower — source → HIR
• Type system and trait solving — rustc_middle::ty, TyCtxt, trait selection
• HIR analysis and type checking — rustc_hir_analysis, rustc_hir_typeck
• MIR pipeline — building, borrow checking, dataflow, transforms, const-eval
• Codegen backends — LLVM, Cranelift, GCC; SSA layer; metadata; monomorphization
• Diagnostics and errors — rustc_errors, Fluent messages, error codes
• Query system and incremental — rustc_query_system, rustc_query_impl, rustc_incremental
• Other compiler crates — utility, infrastructure, and small support crates

Pipeline at a glance

graph LR
    Src["source.rs"] --> Lex[rustc_lexer]
    Lex --> Parse[rustc_parse]
    Parse --> Expand[rustc_expand + rustc_builtin_macros]
    Expand --> Resolve[rustc_resolve]
    Resolve --> Lower[rustc_ast_lowering]
    Lower --> HIR[(HIR)]
    HIR --> HirAna[rustc_hir_analysis]
    HirAna --> HirCk[rustc_hir_typeck]
    HirCk --> MIRBuild[rustc_mir_build]
    MIRBuild --> MIR[(MIR)]
    MIR --> Borrowck[rustc_borrowck]
    MIR --> Const[rustc_const_eval]
    Borrowck --> MIROpt[rustc_mir_transform]
    MIROpt --> Mono[rustc_monomorphize]
    Mono --> CodegenSSA[rustc_codegen_ssa]
    CodegenSSA --> LLVM[rustc_codegen_llvm]
    CodegenSSA --> Cranelift[rustc_codegen_cranelift]
    CodegenSSA --> GCC[rustc_codegen_gcc]

The query system threads through the entire diagram: most edges in the graph above are query invocations, and rustc_middle is the central hub that holds all the data they read and write.

Crate map by phase

The following table groups all rustc_* crates by what they do.

Driver and infrastructure

Crate	Role
rustc	The thin main binary; calls rustc_driver::main()
rustc_driver	Public driver API
rustc_driver_impl	Implementation: command-line dispatch, --print modes, batch compilation
rustc_interface	Phase orchestration; "run a compilation" abstraction
rustc_session	Session, command-line options, target selection
rustc_log	tracing setup
rustc_data_structures	FxHashMap, IndexVec, BitSet, sync primitives
rustc_arena	Typed arena allocators
rustc_index	IndexVec and newtyped indices
rustc_macros	proc-macros for diagnostic derives, Encodable, query keys
rustc_serialize	Internal binary serialization (incremental cache, .rmeta)
rustc_fs_util	Filesystem helpers
rustc_thread_pool	rustc's fork of Rayon (with deadlock detection)
rustc_graphviz	Graphviz emission for debug dumps
rustc_hashes	Hashing primitives (FxHash, etc.)
rustc_baked_icu_data	Pre-compiled ICU locale data for diagnostics

Frontend

Crate	Role
rustc_lexer	Tokenizer
rustc_ast	AST data types
rustc_ast_ir	Shared AST utilities
rustc_ast_pretty	AST pretty-printer
rustc_parse	Parser
rustc_parse_format	Parser for the format! mini-language
rustc_attr_parsing	Attribute parsing
rustc_expand	Macro expansion engine
rustc_builtin_macros	format_args!, derive(...), panic!, cfg!, etc.
rustc_proc_macro	Server side of the proc-macro bridge
rustc_resolve	Name resolution
rustc_ast_lowering	AST → HIR
rustc_ast_passes	Post-parse AST validation

HIR and analysis

Crate	Role
rustc_hir	HIR data types
rustc_hir_id	Stable HIR identifiers
rustc_hir_pretty	HIR pretty-printer
rustc_hir_analysis	Coherence, well-formedness, item-level checks
rustc_hir_typeck	Type checking and inference for function bodies
rustc_passes	A grab-bag of HIR-level passes (lang-item check, diagnostic items, dead code, etc.)
rustc_privacy	Visibility checking
rustc_lint	Lint pass framework + built-in lints
rustc_lint_defs	Lint definitions (separate to break a cycle)
rustc_pattern_analysis	Exhaustiveness checking for match

Type system

Crate	Role
rustc_middle	The central data structures (TyCtxt, Ty, Body, …)
rustc_type_ir	Generic type-system traits used by rustc_middle and rustc_next_trait_solver
rustc_type_ir_macros	proc-macros for rustc_type_ir
rustc_infer	Type inference engine
rustc_trait_selection	Classic trait solver
rustc_next_trait_solver	Next-generation trait solver (behind -Znext-solver)
rustc_traits	Glue exposing trait queries
rustc_ty_utils	Type-related query providers
rustc_transmute	transmute validity checking

MIR pipeline

Crate	Role
rustc_mir_build	HIR → THIR → MIR construction; pattern lowering
rustc_mir_dataflow	Generic dataflow framework
rustc_borrowck	Borrow checker (NLL)
rustc_mir_transform	MIR optimizations and lints
rustc_const_eval	MIR interpreter; powers const fn and Miri
rustc_monomorphize	Generic instantiation, codegen-unit partitioning

Codegen and metadata

Crate	Role
rustc_codegen_ssa	Backend-agnostic codegen (linking, debuginfo, target spec, …)
rustc_codegen_llvm	LLVM backend
rustc_codegen_cranelift	Cranelift backend (separate workspace)
rustc_codegen_gcc	GCC (libgccjit) backend (separate workspace)
rustc_llvm	FFI bindings to in-tree LLVM (src/llvm-project/)
rustc_metadata	.rmeta / .rlib reading and writing
rustc_symbol_mangling	Stable symbol mangling (legacy and v0)
rustc_target	Built-in target triples and target spec types
rustc_abi	Target-agnostic ABI / layout types
rustc_sanitizers	ASan / TSan / MSan / others

Errors

Crate	Role
rustc_errors	Diagnostic infrastructure (Diag, DiagCtxt, Subdiagnostic)
rustc_error_messages	Fluent loading and translation
rustc_error_codes	Long-form error code descriptions (E0XXX)
rustc_span	Span, Symbol, source-map handling

Incremental and queries

Crate	Role
rustc_query_impl	Query implementation (separated for compile-time isolation)
rustc_incremental	On-disk incremental compilation cache

Public APIs (experimental)

Crate	Role
rustc_public	Stable Mid-level IR API (formerly StableMIR)
rustc_public_bridge	Bridge between rustc internals and the stable API
rustc_windows_rc	Windows resource compiler integration

Reading order

If you're new to the compiler, the rustc-dev-guide recommends reading in pipeline order: lexer → parser → resolver → AST lowering → HIR → type checking → MIR build → borrow check → MIR opt → codegen. The pages in this section are organized to support that progression — start with Driver and interface, then Frontend, then Type checking and Type system, then MIR, then Codegen.