luau/docs/_posts/2021-09-30-luau-recap-september-2021.md

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single	Luau Recap: September 2021

Luau is our new language that you can read more about at https://luau-lang.org.

[Cross-posted to the Roblox Developer Forum.]

Editor features

We have made some improvements to the Luau-powered autocomplete beta feature in Roblox Studio:

• We no longer give autocomplete suggestions for client-only APIs in server-side scripts, or vice versa.
• For table literals with known shape, we provide autocomplete suggestions for properties.
• We provide autocomplete suggestions for Player.PlayerGui.
• Keywords such as then and else are autocompleted better.
• Autocompletion is disabled inside a comment span (a comment starting --[[).

Typechecking improvements

The big news this month is that generic functions are back!

write-up of generics goes here

In other typechecking news:

• The Luau constraint resolver can now refine the operands of equality expressions.
• We fixed some crashes caused by UAF during type inference.
• We do a better job of tracking updates when script is moved inside the data model.
• We fixed one of the ways that recursive types could cause free types to leak.
• We improved the way that return statements interact with mutually recursive function declarations.
• We improved parser recovery from code which looks like a function call (but isn't) e.g.

local x = y
(expr)[smth] = z

• We consistently report parse errors before type errors.
• We display more types as *unknown* rather than as an internal type name like error####.
• Luau now infers the result of Instance:Clone() much more accurately.
• Luau type guard refinements now support more arbitrary cases, for instance typeof(foo) ~= "Instance" eliminates anything not a subclass of Instance.

Performance improvements

• Vector3.new constructor has been optimized and is now ~2x faster
• A previously implemented optimization for table size prediction has been enhanced to predict final table size when setmetatable is used, such as local self = setmetatable({}, Klass)
• Method calls for user-specified objects have been optimized and are now 2-4% faster
• debug.traceback is now 1.5x faster, although debug.info is likely still superior for performance-conscious code
• Creating table literals with explicit numeric indices, such as { [1] = 42 }, is now noticeably faster, although list-style construction is still recommended.

Other improvements

• The existing 'TableLiteral' lint now flags cases when table literals have duplicate numeric indices, such as { [1] = 1, [1] = 2 }