RFC: Type Guards

[Bottersnike]

Rendered

Adds type guards of the form

function isFoo(x): x is Foo
    return x.type == "foo"
end

which can be used in control statements and assertions.

[deviaze]

Could we get this as a sort of typecast as well/instead? This could allow user-defined narrowing in a more natural inline manner.

For example:

type Entry = File | Directory | Symlink

local entry = getentry()
if str.endswith(entry.path, ".luau") :: entry is File then
    print(entry:read()) -- a method only on `File`
end

In this case we want to assume the entry is a File if it ends with .luau, but that isn't something inherent to the function str.endswith (which is just a substring check), but inherent to the logic of our conditional itself.

[Bottersnike]

In terms of as well, I think that would be cool, though potentially worth a separate RFC. In terms of instead, you then lose many of the benefits of having dedicated guard functions, so I don't think I'd want to not have those.