feat(rust)!: use path in generated export! macro instead of ident

[tqwewe]

Changes the generated export! macro to accept a $ty:path instead of $ty:ident for more flexibility.

Requires the with_types_in separator to be changed to , with_types_in, since a path cannot be followed by an ident in macros.

Closes #1564

[alexcrichton]

Thanks! Personally though I think that this'd need to be bikeshed a bit since the syntax here is pretty odd. I'm not sure if the restrictions on tokens after path are different than ty, so that may also be an option? I'd like to have this flow a bit more naturally though so if reading a macro invocation it's a bit more obvious as opposed to just having a stray comma somewhere in there.

I'll also note that there's a workaround today (I think) but I also agree it'd be best to not need a workaround, so that's just for a temporary state of affairs.

[tqwewe]

Yeah I actually initially used the syntax $ty => $path_to_types_root instead of $ty, with_types_in $path_to_types_root, but wanted to stick as close to the existing API as I could.

I agree the API is a bit weird. Not sure of any better alternatives than just =>.

I'm not sure if the restrictions on tokens after path are different than ty

I think I had the same issue when I initially tried ty.

[alexcrichton]

Perhaps we could take a leaf out of Wasmtime's book? Something like:

generate!(MyType);
generate!(()); // uses `$t:ty` for this
generate!({
    ty: MyType,
    with_types_in: some::other::module,
});