Luau Recap: July 2023

Our team is still spending a lot of time working on upcoming replacement for our type inference engine as well as working on native code generation to improve runtime performance.

However, we also worked on unrelated improvements during this time that are summarized here.

[Cross-posted to the Roblox Developer Forum.]

Analysis improvements

Indexing table intersections using x["prop"] syntax has been fixed and no longer reports a false positive error:

type T = { foo: string } & { bar: number }
local x: T = { foo = "1", bar = 2 }

local y = x["bar"] -- This is no longer an error

Generic T... type is now convertible to ...any variadic parameter.

This solves issues people had with variadic functions and variadic argument:

local function foo(...: any)

local function bar<T...>(...: T...)
    foo(...) -- This is no longer an error

We have also improved our general typechecking performance by ~17% and by additional ~30% in modules with complex types.

Other fixes include:

  • Fixed issue with type T? not being convertible to T | T or T? which could’ve generated confusing errors
  • Return type of is now inferred as DateTypeResult when argument is “t” or “!t”

Runtime improvements

Out-of-memory exception handling has been improved. xpcall handlers will now actually be called with a “not enough memory” string and whatever string/object they return will be correctly propagated.

Other runtime improvements we’ve made:

  • Performance of table.sort was improved further. It now guarantees N*log(N) time complexity in the worst case
  • Performance of table.concat was improved by ~5-7%
  • Performance of math.noise was improved by ~30%
  • Inlining of functions is now possible even when they used to compute their own arguments
  • Improved logic for determining whether inlining a function or unrolling a loop is profitable

Autocomplete improvements

An issue with exported types not being suggested is now fixed.

Debugger improvements

We have fixed the search for the closest executable breakpoint line.

Previously, breakpoints might have been skipped in else blocks at the end of a function. This simplified example shows the issue:

local function foo(isIt)
    if isIt then
        -- When 'true' block exits the function, breakpoint couldn't be placed here


A very special thanks to all of our open source contributors: