Dlang's betterC would be a good option for just a better-than-C language but looks like it doesn't support D's standard library [1]. That'd mean being stuck with plain C container libraries. :/

Personally, I was impressed by an approach taken by the authors of Pixie (a fast 2d library like Cairo) [2]. They created a "library wrapper" generator for Nim code that supports creating libraries Python, Node, C, and Nim itself that they call Genny [3]. Haven't tried it but being able to use Nim and it's ref based gc but still export to nice API's in other languages is fantastic. Pixie's aim is to be a Cairo alternative so it makes sense they'd need this. I hope the approach takes off. Usually writing any cross language API's is a lossy operation.

Here's a sample of the API definition:

    exportObject Matrix3:
      constructor:
         matrix3
      procs:
         mul(Matrix3, Matrix3)

1: https://dlang.org/spec/betterc.html 2: https://github.com/treeform/pixie 3: https://github.com/treeform/genny

(edited formatting)