dh-cargo forces packages to patch build.rs when a static library is
built. (I think in some cases, this can happen even if a static
library is built only by build tests.)

In the patch, the packagers needs to indicate "whether the license(s)
of the included static libs require source distribution alongside
binaries". There isn't really an explanation for which common licenses
might require this. I didn't see a bug or discussion about why "source
distribution alongside binaries" is something that Debian needs to
worry about since we already distribute source code for everything.

I see some indications that some people think this applies to Rust
code distributed under the GPL but I don't think the GPL requires this
interpretation.

This feature was added in
https://salsa.debian.org/rust-team/dh-cargo/-/commit/36790ab787

This request for clarification is related to the much more general
https://bugs.debian.org/924751

Thank you,
Jeremy Bícha