0011. Use LuaJIT as the embedded runtime and sole compile target

Date: 2026-10-10

Status

Accepted. Supersedes ADR 0001.

Context

ADR 0001 chose Luau as the embedded language. The compiler since grew a second concern: which language neoc emits. Luau as runtime plus a typed Luau emit target plus a plain Lua 5.1 base left two output dialects and a downgrade step between them. Authors and agents could pick the wrong one.

Facts:

• The Lua 5.1 emitter (neoc-codegen-lua) already existed and defines the lowering in lang/DYNAMIC.md.
• LuaJIT runs Lua 5.1 directly and is bindable from Rust via mlua luajit.
• LuaJIT is a mature, widely embedded runtime.

Decision

neoc embeds LuaJIT through mlua's luajit feature. lua (Lua 5.1, .lua) is the only compile target. One runtime, one target.

The sandbox amends ADR 0002 for LuaJIT; 0002 holds the strip list.

Conformance run-trials execute in-process on the same LuaJIT sandbox.

Consequences

• One runtime and one target: nothing to choose, so nothing to choose wrongly.
• No typed output. Declared neoc types are enforced by runtime checks (lang/DYNAMIC.md §9), not by an emitted type layer.
• Numbers are doubles only, unchanged from ADR 0001. There is no native i64; integers beyond 2^53 lose precision.
• No buffer type. Binary data is Lua strings.
• ffi, jit, and debug are unavailable to scripts; the library set is an allow-list; base globals are a deny-list (STRIPPED_GLOBALS).
• Gated on mlua upstream's continued LuaJIT support.
• vendored builds LuaJIT into the binary: no system Lua dependency; a C toolchain is required at build time.