String is not a good example of a mutable struct

[oxinabox]

In the docs on mapping julia types to C types,
it gives String as an example of a mutable struct

Which is not really a clear example.

https://github.com/JuliaLang/julia/blob/391f2dd2b5ac67da4a136673b1923d966b606439/doc/src/manual/calling-c-and-fortran-code.md#type-correspondences

[StefanKarpinski]

Yes, that is really a bad choice for that example.

[oxinabox]

There are very few good examples in Base,
We use very few mutable structs that are not type parametric,
or otherwise weird.

filter(names(Base)) do name
	T = try
		eval(name)
	catch
		return false
	end
	
	isconcretetype(T) &&
	T.mutable &&
	fieldcount(T) > 0 && # knocks out String
	nameof(T) == name && # Knock-out type-aliases
	!any(F <: Ptr for F in fieldtype.(T, fieldnames(T))) # Nothing Pointer based
end

Gives back:

4-element Array{Symbol,1}:
 :BitSet
 :Condition
 :Pipe
 :ReentrantLock

So it really looks like BitSet is our best option.

[KristofferC]

#29475