librustdoc: set cap-lints=allow when compiling documentation

[Ryman]

This sets the cap-lints setting to 'allow' for all doc compilations. There's precedent for this as rustdoc already whitelists unstable code when compiling documentation, with the expectation being that a regular compile will complain about any problems. I think the same justification applies here.

Problem case in the wild: laumann/compiletest-rs#28

r? @Manishearth

[Manishearth]

r? @alexcrichton

Not entirely familiar with the complete reasoning behind capping ar Allow, deferring to Alex.

[huonw]

@Manishearth FYI https://github.com/rust-lang/rfcs/blob/master/text/1193-cap-lints.md#motivation

[Manishearth]

I meant the unstable features stuff. I'm not sure if we should be capping at allow or warn here, and I'm not sure if this should even be a part of rustdoc (not cargo).

Happy with the code changes, though, r=me if nobody has an issue with allow over warn and it being in rustdoc.

[steveklabnik]

So, one thing is that when docs run without lints, we get into situations like #29523 . I would have loved to have seen a "hey, this feature isn't needed" warning, so that we wouldn't have had the bug.

[Manishearth]

So I guess we want to cap in docs but not doctests?

[Ryman]

@steveklabnik This change only applies to running rustdoc on rust files without the test flag, there's three more codepaths: testing rust files, compiling markdown files, testing markdown files. 😅

Just confirmed that make-check-docs still fires warnings from tests in rust files.

[steveklabnik]

@Ryman ah! ignore my objection then, sorry :)

[alexcrichton]

@steveklabnik oh we actually have support for that, it's just not turned on by default.

I also think that this is fine, seems kinda odd for a documentation generator to be warning you about various parts of your code! Thanks @Ryman!

@bors: r+ b1ef530

[bors]

⌛️ Testing commit b1ef530 with merge de11d2a...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt