Please include build dependencies in build instructions

[glaubitz]

Building authenticator-rs requires installing several other packages such as libudev-dev, mercurial, gyp and ninja-build (package names from Debian).

It's not easy to understand from the error messages when these packages are missing. It would be great if the necessary build dependencies could be explicitly mentioned in the README.md so that users don't have to waste their time with try-and-error.

[micolous]

All of those dependencies are transitive dependencies of authenticator-rs. While they aren't mentioned in this crate's documentation, they are mentioned in the documentation of crates or packages it depends on (ie: where the error is thrown from).

Feedback about error messages should be directed to the respective crates' maintainers. :)

udev

On Debian, libudev-dev is a dependency of the libudev-sys crate. This is mentioned in its README and in libudev's README.

This is only needed on Linux, and if it's missing, its build scripts return an error message from pkg-config that's messy (a Debug-formatted string) and indicates the problem, but not the resolution.

NSS

The other dependencies you've mentioned are transitive dependencies of nss-rs, which is the default crypto backend. authenticator-rs has other crypto backends, which have different system dependencies.

nss-rs will try to use pkg-config to find a system NSS, which is available in Debian's libnss3-dev package. If the system version is new enough, you don't need any of those other dependencies.

If the system version of NSS is too old, the nss-rs build script will error out:

  nss-rs has NSS version requirement >=3.126, found 3.98; set NSS_DIR to a newer checkout, or NSS_NO_PKG_CONFIG=1 to build NSS from source

If pkg-config can't find a suitable system NSS, nss-rs will (silently) fall back to vendoring NSS at hg tip (which needs mercurial, but this will change in mozilla/nss-rs#239). gyp and ninja-build are needed to build NSS from source on some platforms (as noted in its README), but it's error messages for when they're missing aren't clear, and silent auto-vendoring is bad for security.

Another change may be to make authenticator-rs not provide a default crypto backend, as NSS is difficult to build on some platforms (due to gyp), but it's still needed for Firefox. This would reduce the number of unexpected system dependencies, but would break API compatibility with existing users.