WIP: Add support for StringBuilder.Chars for Rust

[MangelMaxime]

Hello @ncave ,

While trying to add support for StringBuilder.Charts to Rust I run into an issue where fable-library-rust cannot be compiled.

Checking fable_library_rust v0.1.0 (/Users/mmangel/Workspaces/Github/fable-compiler/Fable/temp/fable-library-rust)
error[E0599]: no method named `Clear` found for reference `&module_5bc0559d::System::Text::StringBuilder` in the current scope
   --> src/./System.Text.rs:210:37
    |
210 | ...                   (_self_.Clear()).Append_Z372E4D23(bufferText_1);
    |                               ^^^^^ method not found in `&StringBuilder`
    |
    = help: items from traits can only be used if the trait is implemented and in scope
note: `ICollection_1` defines an item `Clear`, perhaps you need to implement it
   --> src/./Interfaces.rs:245:17
    |
245 |                 pub trait ICollection_1<T: Clone + 'static> {
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0599`.
error: could not compile `fable_library_rust` (lib) due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `fable_library_rust` (lib test) due to previous error

While looking at the generated source code I discovered that member have self: &Lrc<Self> generated but getter/setter have &self.

Manually changing

pub fn set_Chars_413E0D0A(self, index: i32, value: char) {

to

pub fn set_Chars_413E0D0A(self: &Lrc<Self>, index: i32, value: char) {

seems to make the error go away at least in the IDE.

I found that the line responsible for generating &Lrc<Self> is

Fable/src/Fable.Transforms/Rust/Fable2Rust.fs

Lines 3352 to 3374 in f96d458

	let makeTypedParam (com: IRustCompiler) ctx (ident: Fable.Ident) returnType =
	if ident.IsThisArgument then
	// is this a fluent API?
	match ident.Type, shouldBeRefCountWrapped com ctx ident.Type with
	| Fable.DeclaredType(entRef, genArgs), Some ptrType when ident.Type = returnType ->
	// for fluent APIs, set the type of thisArg to (self: &Lrc<Self>)
	let ty = mkImplSelfTy ()
	let ty =
	match ptrType with
	| Lrc -> ty |> makeFluentTy com ctx
	| Rc -> ty |> makeRcTy com ctx
	| Arc -> ty |> makeArcTy com ctx
	| Box -> ty |> makeBoxTy com ctx
	|> mkRefTy None
	mkTypedSelfParam ty false false
	| _ -> mkImplSelfParam false false
	elif ctx.IsLambda && ident.Type = Fable.Any then
	mkInferredParam ident.Name false false
	else
	let ty = transformType com ctx ident.Type
	mkTypedParam ident.Name ty false false

But it doesn't work in the current situation because the ident.Type and returnType are different. I am not sure what is the path of action to take from here.

[ncave]

@MangelMaxime Thanks, I'll take a look.

[ncave]

Superseded by #3761.