Skip to content

Different.md

Latest commit

 

History

History
103 lines (61 loc) · 3.97 KB

Different.md

File metadata and controls

103 lines (61 loc) · 3.97 KB

Behavior Differences: luars vs C Lua 5.5

This document records all known behavioral differences between luars (Rust implementation) and the official C Lua 5.5 reference interpreter.

1. No C API / testC support

luars is a pure Rust implementation with no C API (lua_State*, lua_pushinteger, etc.). The official test suite's T (testC) library is unavailable, causing the following test files/sections to be fully or partially skipped:

Test file Scope Reason
api.lua whole file C API tests
memerr.lua whole file C memory-error injection
code.lua whole file opcode-level tests via T.listcode
main.lua whole file interactive interpreter / CLI arg tests
db.lua whole file full debug-library tests (hook-dependent)
coroutine.lua 2 sections (~L595, ~L1062) C API coroutine tests
events.lua 1 section (~L331) userdata event tests
errors.lua 1 section (~L94) C function error-message tests
gc.lua 1 section (~L401) userdata GC tests
strings.lua 2 sections (~L474, ~L534) pushfstring / external-string tests
nextvar.lua 2 sections (~L146, ~L665) table-size / table-lib-on-non-tables tests

2. Rust host API string view

At the Lua level, luars now stores strings as raw byte strings, so arbitrary bytes are preserved just like in C Lua.

The remaining difference is on the Rust host side:

  • LuaValue::as_str() only returns Some(&str) when the underlying bytes are valid UTF-8.
  • LuaValue::as_bytes() should be used when exact byte preservation matters.
  • LuaState::create_bytes() can be used to create Lua strings from arbitrary bytes without forcing UTF-8.

This is an embedding/API difference, not a Lua-language semantic difference.

3. No locale-dependent number parsing

Number parsing always uses . as the decimal separator and is unaffected by os.setlocale.

Affected: literals.lua (~L300–L325) — locale test block skipped via if false then.

4. No C module loading

  • package.loadlib always returns "loadlib not implemented".
  • The package.cpath searcher (searcher 3) always fails; .so / .dll modules cannot be loaded.

Affected: attrib.lua (~L265, ~L335) — C library / external-string tests skipped.

5. debug library limitations

The debug library is partially implemented. debug.setuservalue and debug.getuservalue are not implemented. Because we are different userdata implementations.

6. Bytecode format

luars uses its own bytecode format, incompatible with C Lua binary chunks. string.dump output can only be loaded by luars itself. Upvalue names are not included in the dump.

The on-disk format still distinguishes UTF-8 string constants from raw byte-string constants so luars can round-trip non-UTF-8 data through string.dump and load, even though the runtime no longer has a separate binary value tag.

Affected: calls.lua (~L484) — binary-chunk header tests skipped.

7. io.popen not implemented

io.popen always raises "io.popen not yet implemented". Tests in files.lua are guarded by pcall(io.popen, ...) and gracefully skipped.

8. GC implementation differences

  • __gc finalizers do not save/restore L->allowhook or set CIST_FIN.
  • Stack shrinking during GC is not implemented.

9. Coroutine limitations

  • Multi-value .. (OP_CONCAT) across a yield does not resume correctly — missing finishOp continuation for CONCAT. Test in coroutine.lua (~L982) skipped.
  • Stack-overflow limit differs from C Lua's 1 MiB policy. Test in coroutine.lua (~L839) skipped.

10. os.date DST flag

os.date does not compute the isdst (daylight saving time) field correctly; it defaults to false.

11. Long-string address reuse

C Lua may reuse the same address for identical long-string constants (useful with const tags). luars allocates separate objects.

Affected: literals.lua (~L213) — long-string reuse test skipped.