I don't think it's a good idea to completely ignore errors in stderr. That makes solving the problems impossible as the user does not notice them in the first place.

Fair point. The error output will now be sent to the Messages buffer.

Since I commented the original version, I continue but feel free to ignore me.

• I don't use cargo.el with tramp, but process-file works on remote files whereas call-process doesn't.
• buffer-string is a bit simpler than (with-output-to-string (with-current-buffer standard-output ....
• It is not the fault of this PR, but the output of cargo inserted into a buffer, which then converted into a string, which json-read-from-string inserts into a buffer. This is a bit inefficient.

By the way, what kind of warnings / situation inspired this PR?