FFlags

Luau has a slew of fast flags (FFlags) which can be configured from code. FFlags are scattered about Luau. If you come across one that you would like to toggle, you can do so with the LUAU_FASTFLAG and LUAU_FASTINT macros. These macros inject an extern variable pointing to the desired fast flag.

Example

Boolean Flag

In Luau version 0.711, Luau added some new math constants, such as tau and phi. In order for your embedded version of Luau to add these constants to the math library, you need to turn on the LuauNewMathConstantsRuntime flag. This needs to be done prior to opening up the math library.

// The Common header includes the necessary macros:
#include <Luau/Common.h>

// Get access to the appropriate fast flag:
LUAU_FASTFLAG(LuauNewMathConstantsRuntime);

void setup() {
    // Set the flag on:
    FFlag::LuauNewMathConstantsRuntime.value = true;

    // ...open Luau state and libraries
}

Integer flag

Alongside fast flags, there are also fast ints. An example of a fast int is LuauRecursionLimit. As of writing this, the limit is set to 1000. Let's set it to 800 instead.

LUAU_FASTINT(LuauRecursionLimit);

void setup() {
    FInt::LuauRecursionLimit.value = 800;
}

Custom Flags

Using the LUAU_FASTFLAGVARIABLE and LUAU_FASTINTVARIABLE macros, you can create your own fast flags.

LUAU_FASTFLAGVARIABLE(MyFeature, true);
LUAU_FASTINTVARIABLE(SomeLimit, 12);

void foo() {
    if (FFlag::MyFeature) {
        // Do something
    }

    doSomethingWithLimit(FInt::SomeLimit);
}

Other code can then use LUAU_FASTFLAG(MyFeature) and LUAU_FASTINT(SomeLimit) to get a reference to the flags elsewhere.