refactor(fuzz): add #[cfg(fuzzing)]

[mxinden]

quinn-proto exposes some hooks for fuzzing only. These are behind a #[cfg(fuzzing)].

The fuzz crate imports these hooks, but without the #[cfg(fuzzing)].

Running cargo check --workspace thus fails importing each of these hooks. For example:

error[E0432]: unresolved import `proto::fuzzing`
  --> fuzz/fuzz_targets/packet.rs:7:5
   |
7  |     fuzzing::{PacketParams, PartialDecode},
   |     ^^^^^^^ could not find `fuzzing` in `proto`

This commit adds the various #[cfg(fuzzing)] to fuzz/, preventing the above errors.

---

Needed for #1934.

[mxinden]

The cfg statements here are noisy. Would you prefer a single mod behind a cfg?

[Ralith]

I'm not sure this is the right approach. #[cfg(...)] disables code, so you're really just skipping the crate entirely by verbose means. What do other projects using this fuzzer do? Maybe we should exclude the crate from the workspace by default?

[djc]

I'm not sure this is the right approach. #[cfg(...)] disables code, so you're really just skipping the crate entirely by verbose means. What do other projects using this fuzzer do? Maybe we should exclude the crate from the workspace by default?

Yeah, I think using the workspace default-members config value might be a better approach.

[mxinden]

I wasn't aware of default-members. Thank you. Agreed, the status quo is much cleaner.

quinn/Cargo.toml

Lines 1 to 3 in 2d06eef

	[workspace]
	members = ["quinn", "quinn-proto", "quinn-udp", "bench", "perf", "fuzz"]
	default-members = ["quinn", "quinn-proto", "quinn-udp", "bench", "perf"]