Allow specification of the output directory for cargo doc

[sfackler]

I'm hosting documentation for multiple crates in the same github.io repository. Rustdoc can only properly setup the crate list, search indices, etc, if it's writing directly into the same folder for each crate. Right now I'm symlinking target/doc to the right folder in each project, but that's pretty fragile. Ideally, I'd be able to specify cargo doc --out-dir=../sfackler.github.io/doc or something.

[alexcrichton]

Closing as a dupe of #482 as it's likely how this will be solved.

[zopsicle]

I don’t think #482 solves this properly (although it has proven very useful for other purposes), in the case where crates are built for different targets, for instance:

cargo doc --manifest-path foo/Cargo.toml --target x86_64-unknown-linux-gnu
cargo doc --manifest-path bar/Cargo.toml --target x86_64-pc-windows-msvc

The docs for foo and bar are written to target/x86_64-unknown-linux-gnu/doc and target/x86_64-pc-windows-msvc/doc, respectively. With separate crate lists and search indexes, etc.

Having an output directory option specifically for cargo doc (as suggested by OP) would therefore still be useful.