The rust-zstd package has both a dependency and a build-dependency on librust-zstd-safe-2.0.3+experimental-dev, which does not exist anywhere in Debian. Presumably it would be built by a rust-zstd-safe package, but no such package exists, including in the Debian NEW queue.
Specifically looking through the mailing list archives.
rust-zstd-sys, rust-zstd-safe and rust-zstd were all uploaded to NEW in mid January 2020. In late Febuary 2020 the ftpmasters processed the uploads, zstd was accepted but zstd-sys and zstd-safe were rejected. zstd-sys and zstd-safe were re-uploaded to new in march 2020 and rejected again in may 2020. zstd-sys was once-again uploaded to NEW in october 2020 and this time was accepted. At around the same time a source-only upload of zstd was also made, but zstd-safe was not touched. The route to fixing rust-zstd, thus involves fixing the ftpmaster's objection to the previous upload, checking the package for any other such issues and if-so fixing them and then re-uploading it. The reject mail can be found at https://alioth-lists.debian.net/pipermail/pkg-rust-maintainers/2020-May/012388.html
