Lint to track "Panics" section of rustdoc?

[asajeffrey]

At the moment, we have an ad-hoc script which greps for function names that look like they'll cause panic. https://github.com/servo/servo/blob/master/etc/ci/check_no_panic.sh

A somewhat more principled approach would be a lint that checks functions for the Panics rustdoc section, and warns if it finds a function that a) calls a function with a Panics section of its docs, and b) doesn't have its own Panics section. With an appropriate allow for cases where we're really really sure the panic can't happen.

[asajeffrey]

On irc, @lqd points out https://github.com/dtolnay/no-panic: https://mozilla.logbot.info/servo/20190305#c16044765

[asajeffrey]

The no-panic crate works after monomorphization, so can deal with generic functions (at the cost of working at link time rather than compile time).