Debian, Rust, and librsvg

It seems to me that the point Matthew seems to be making has eluded you.

Debian stretch is the current Debian stable.

New packages are basically never accepted into Debian stable. (It is possible to imagine some exotic scenarios where they might be, but I imagine them being along the lines of "upstream for package Q currently in Debian stable has fixed a critical security vulnerability by a method involving the use of package P not currently available in stable and the Debian maintainers of Q do not have the labour resource available to develop their own fix".)

If you care enough to ensure that your program will work with 2- to 4-year-old versions of its dynamic library dependencies so that you don't have to drag in eleventy-one updated library versions that might actually break a stable system because the SONAME has been bumped, then once you've got it into unstable you can get it into the -backports repo for stable or possibly even oldstable - but that's officially not part of Debian.