Add -Zrustdoc-scrape-examples to default Cargo args

[willcrichton]

This PR adds -Zrustdoc-scrape-examples to the set of default arguments passed to cargo doc. This (finally!) enables the scrape-examples feature to be enabled by default for all readers of docs.rs.

After extensive work within Cargo (rust-lang/cargo#10343, rust-lang/cargo#10533, rust-lang/cargo#10549, rust-lang/cargo#11430, rust-lang/cargo#11450), this option is now robust enough to avoid breaking any existing build. Any crate that documented before will still document with -Zrustdoc-scrape-examples. The core issue that was addressed is to not require dev-dependencies if they weren't already needed.

I have verified that -Zrustdoc-scrape-examples does not introduce breakage in a recent Crater run: https://crater-reports.s3.amazonaws.com/scrape-examples/index.html
(Note there are regressions, but they appear to be spurious and not related to the scrape-examples feature.)

This PR is currently a draft because I am waiting on rust-lang/cargo#11450 to reach nightly in a few days. But I wanted to put it up before then to provide room for discussion if someone is not comfortable enabling this flag.

cc @jyn514 @GuillaumeGomez @jsha

[willcrichton]

Now that rust-lang/rust@fb0ca59 has landed in nightly, this PR is ready for review.

[jyn514]

@willcrichton can you clarify for me where we ended up with the defaults? Is this bit still true? rust-lang/cargo#10343 (comment)

Packages have fine-grained control at the per-target level what's used as candidates for examples and what isn't (with the default being only the lib targets and examples)

If so, that seems good to me :) thank you for all your hard work pushing this feature through!

[willcrichton]

@jyn514 yes that's right. The longer explanation currently lives on the Unstable Features page in the Cargo book: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#scrape-examples

And thank you! Really excited to see this feature make it out in the world.

[syphar]

@jyn514 so this can be approved & merged?

[jyn514]

Yup!