Add a missing fmt::Debug impl lint

[sfackler]

Closes #20855

r? @alexcrichton

[alexcrichton]

Thanks! I think it's a bit too strong to set the default level to warn for this lint though. We've seen real compile time hits as well as code blow up sizes in the past from just adding derive(Debug) to everything. Especially seeing how many crates in the standard distribution are opting out of this lint I think it should be default allow if we're going to have it.

From a library author's perspective it's unfortunate to accidentally leave this out, but it's a backwards compatible change to add one at any time.

[sfackler]

I was under the impression that Debug was supposed to be implemented for all public types: https://github.com/rust-lang/rfcs/blob/master/text/0565-show-string-guidelines.md#debugging-fmtdebug. Should we be revising that?

I disabled the lint for all of the main rustc crates because I didn't feel like adding hundreds of Debug implementations in this PR. I don't see any reason to add a ton of impls in the rustc crates, but we should definitely flesh out the downstream facing ones.

[alexcrichton]

To me that guideline states that all public types should have a Debug implementation, but forcing it to be so seems somewhat unnecessary. There are many use cases (such as the hundreds of types in this repo) which don't want to have to have this lint turned one (another being *-sys crates for example).

[sfackler]

Updated to be allow by default. We may want to set the missing_copy_implementations to default allow as well.

[alexcrichton]

I would love to do so for that! It may want to have a separate PR though to prevent entangling the two.

[sfackler]

Yeah, for sure.

[alexcrichton]

@bors: r+ a7ad08c

[sfackler]

@bors: r=alexcrichton 7021491

[bors]

⌛ Testing commit 7021491 with merge 59dcba5...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt