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Compiler

Hew compiles through a pure-Rust pipeline: HIR and MIR lowering direct to LLVM IR.

Overview

The Hew compiler is written entirely in Rust. The frontend (hew-lexer, hew-parser, hew-types) handles lexing, parsing, and type checking. The typed program then lowers through two intermediate representations — HIR and MIR — before the codegen crate (hew-codegen-rs) generates LLVM IR directly. There is no C++ backend and no MLIR step.

Compilation pipeline

Source code passes through the Rust frontend for lexing, parsing, and type checking. The checker output lowers through HIR (high-level IR) and then MIR (mid-level IR) before codegen emits LLVM IR and drives compilation to a native binary:

Source → Lexer → Parser → Type Check → HIR → MIR → LLVM IR → Native

Ownership decisions — which values need dropping, what has moved — are resolved in MIR before codegen sees them. The codegen crate emits LLVM IR directly, carrying the already-resolved ownership facts rather than re-deriving them at code-generation time.

Intermediate representations

  • HIR — high-level IR: desugared and name-resolved AST, close to surface syntax but with all imports and scopes resolved.
  • MIR — mid-level IR: explicit control flow with ownership, drop placement, and suspension points fully resolved. The proving gate for codegen correctness.
  • LLVM IR — generated by hew-codegen-rs, linked and optimised by the system LLVM toolchain.

Runtime library

The Hew runtime provides the infrastructure that actors depend on at execution time:

  • M:N work-stealing scheduler — Lightweight actors run across a pool of worker threads.
  • Actor lifecycle — Mailboxes, actor groups, supervision, shutdown, and cancellation implement the language's lifecycle rules. See Actor Lifecycle.
  • Memory and resources — Actor-local allocation supports isolated state, while ownership and resource cleanup remain language-level guarantees. See Ownership and Affine Values.
  • I/O and time — Streams, networking, asynchronous waits, and timers let actors wait without tying up a worker thread.
  • Observability — Runtime events feed the Observe tools for live inspection and saved captures.

Current status

The pure-Rust pipeline powers hew check, hew build, hew run, and hew test. The same frontend crates (hew-lexer, hew-parser, hew-types) also power the language server (hew-lsp) for editor diagnostics, completion, hover, and semantic tokens.

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