WIP: make deriving Debug optional

[TheVova]

addresses #334 by allowing users to opt-out of having prost derive Debug for its structs.

1. prost_build::Config gains a no_derive_dbg(bool) method for users to use. this inserts a #[prost(no_dbg)] attribute on the generated struct.
2. prost-derive checks for that attribute and skips deriving Debug if it is present.
3. The bound on Message change from Debug + Send + Sync to just Send + Sync.
   The default behavior is opt-out, so it breaks nothing for current users.
   WIP because still need to adjust docs, but wanted to see if this is acceptable.

[TheVova]

Ok, so this breaks some tests atm because of the hidden assumption that Message implies Debug, which probably means it will break some user code that relies on this property. One solution would be to restore the Debug bound to Message and just rely that every type is gonna have either prost-debug, #[derive(Debug)] or a manual impl Debug done for it.
alternatively, could also make this PR more granular and apply to single messages rather than all...
this then still doesnt allow deriving Message with structs that dont implement debug rather than having a custom debug impl...

[cbeck88]

fwiw:

if this were coupled with a version bump to 0.8 it seems not unreasonable to tell users, there is a breaking change now deriving message does not also derive debug. it is very easy for users to adapt to the change, they just derive Debug as well as Message if they need it. that is the most intuitive behavior anyways and seems like the best place to end up. ofc that's just my 2 cents

[AlJohri]

related: #493 (this PR also makes debug configurable)

[caspermeijn]

I think this has been fixed in main branch in the mean time.

• If your code is generated by prost_build, then you can configure that to not generate a impl Debug with https://docs.rs/prost-build/latest/prost_build/struct.Config.html#method.skip_debug
• If your code uses prost_derive, then the skip_debug attribute exists: https://docs.rs/prost-derive/0.13.3/src/prost_derive/lib.rs.html#25
• Currently an impl Debug is required for each prost::Message, but that requirement is relaxed in this PR: feat: Relax Message Debug trait bound #1147

If this comment helps you, please consider updating the crate documentation to make these features more findable.