Deprecate string.lower in favor of ll.ToLower

[tapple]

• Part of https://feedback.secondlife.com/slua-alpha/p/remove-duplicated-luau-methods-from-ll-table
• Reversal of part of Deprecate LSL String functions #74

[tapple]

string.lower uses 7 fewer bytes than ll.ToLower and runs twice as fast. Is that reason enough to keep it around? I dunno. Tested on Luau 2026-03-24.23471922914

for i = 1, 600 do local _ = ll.ToLower("hElLo WoRlD") end
print(os.clock(), ll.GetUsedMemory() - 2302) -- 0.0030    76

for i = 1, 1300 do local _ = ("hElLo WoRlD"):lower() end
print(os.clock(), ll.GetUsedMemory() - 2302) -- 0.0029    69

[HaroldCindy]

I can see some potential usecases for string.lower() if you expect to only be normalizing ascii text, but in general ll.ToLower() and ll.ToUpper() are preferred for internationalization reasons. That said, I don't know how easy it would be to deprecate in practice given that it's also accessible via the string metatable. Do the linters have support for that case?

[tapple]

I added this to the d.luau file:

declare extern type string with
  @[deprecated {use='ll.ToLower', reason='Unicode support.'}] function lower(self): string
end

It seems to work with variables, but not literals, and causes other problems with literals in strict mode:

--!strict
local _ = string.lower("HELLO")
local s: string = "HELLO"
local _ = tostring(NULL_KEY):lower()
local _ = URL_REQUEST_DENIED:lower()
local _ = s:lower()

$ ../../secondlife/luau-lsp/build/luau-lsp analyze --defs sl-vscode-plugin/slua_fixes.d.luau string_test.luau 
[INFO] Loading definitions file: @roblox - sl-vscode-plugin/slua_fixes.d.luau
[INFO] sourcemap is disabled - automatic updating on sourcemap changes disabled
string_test.luau(2,24): TypeError: Expected this to be 'string', but got 'string'
string_test.luau(3,1): TypeError: Expected this to be 'string', but got 'string'
string_test.luau(2,11): DeprecatedApi: Member 'string.lower' is deprecated, use 'll.ToLower' instead. Unicode support.
string_test.luau(5,11): DeprecatedApi: Member 'URL_REQUEST_DENIED.lower' is deprecated, use 'll.ToLower' instead. Unicode support.
string_test.luau(6,11): DeprecatedApi: Member 'lower' is deprecated, use 'll.ToLower' instead. Unicode support.

Issues:

1. False error Expected this to be 'string', but got 'string' on lines 2 and 3 (only with --!strict)
2. Missing DeprecatedApi error on lines 3 and 4

This works for regular extern types, just not string:

declare extern type uuid with
  @[deprecated {use="logic", reason="reason"}] function randomize(self): uuid
end

string seems to be magic, which isn't terribly surprising