Add support for RUSTDOCFLAGS

[alexcrichton]

Like with RUSTFLAGS, parse this variable to pass along extra arguments to
invocations of rustdoc.

[rust-highfive]

r? @wycats

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[alexcrichton]

r? @brson

[brson]

Hm, rustdoc's interface is much less scrutinized than rustcs. When I look at it it's mostly a bunch of stuff I don't want people to touch (apparently rustdoc can load plugins...).

[alexcrichton]

Yeah it's true that most things I wouldn't want to expose, but the motivation here is twofold for me at least:

• Provide any means possible to pass options like --markdown-css and --html-in-header, alleviating pressure on PRs like Add "extension-css" option #2664
• Adding consistency with RUSTFLAGS and how to pass custom args to the underlying tool.

Ideally we'd provide configuration in Cargo.toml for various properties, but we unfortunately haven't figured out a story for passing through arguments to rustc/rustdoc like that yet.

[alexcrichton]

@brson, thoughts on how this is consistent with RUSTFLAGS?

[brson]

@bors r+

I suppose I don't care enough to argue against it.

[bors]

📌 Commit f639d38 has been approved by brson

[bors]

⌛ Testing commit f639d38 with merge 270200b...

[bors]

☀️ Test successful - cargo-cross-linux, cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64
Approved by: brson
Pushing 270200b to master...