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Allow extra C++ flags and include dirs via the environment - #8
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The vendored Luau sources are compiled by this crate on behalf of mlua-sys, so a consumer several levels up has no way to influence the build. Projects needing a compiler flag for correctness rather than taste currently have to vendor the whole crate to get one in. Read LUAU_CXXFLAGS (whitespace separated) and LUAU_INCLUDE_DIRS (path separated), and declare both with rerun-if-env-changed so changing them forces a rebuild rather than silently reusing an artifact compiled with the old settings. The motivating case is a deterministic simulation: it needs -ffp-contract=off so the modulo expansion cannot fuse into a multiply-add on some architectures and not others, and a force-included header redirecting the libm functions IEEE 754 leaves unspecified.
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Hey there! Long time no chat, I've picked back up using mlua as part of porting a game I was working on from being written entirely in Lua on top of Love2D to instead be a hand-written Rust project that provides modding capabilities via mlua.
My challenge is that the game needs to be deterministic including user-created mods written in Lua. I've managed to handle most determinism by shadowing
math.*, but operators like^and^=can't be shadowed in the same way. To solve this, I have to force-include a header that redirects the libm functions IEEE 754 leaves unspecified to my own implementations, which catches^and^=at runtime. The problem is there's no way to pass a compiler flag, so this opens that up for me.AI helped me figure this out, as a heads up. To my understanding, I just need to read
LUAU_CXXFLAGS(whitespace separated) andLUAU_INCLUDE_DIRS(path separated), and declare both withrerun-if-env-changedso changing them forces a rebuild rather than silently reusing an artifact compiled with the old settings.Also, for clarity, I made this PR even though
ccalready picks upCXXFLAGSbecause nothing declarescargo:rerun-if-env-changedfor it, so changing the value doesn't trigger a rebuild, which means you silently get an artifact compiled with the old flags while the build reports success.