Update build instructions for rustdoc

[dns2utf8]

Hi all

I updated the build instructions with all the things I learned while building the docs. Thank you @GuillaumeGomez for helping me!

Cheers, Stefan

[GuillaumeGomez]

@jyn514 In his case, it's triggering errors about "libtest not found". Which is why I recommended him to try with --stage 2 and to open an issue.

[jyn514]

@GuillaumeGomez that's fine, but we should help @dns2utf8 get --stage 1 working instead of recommending a command that takes twice as long.

[dns2utf8]

In my opinion building from stage 2 and the CI artefacts should be possible in ~5min or less, but my current test gave me ~23min. So not that great either

[dns2utf8]

The build error I was getting:

rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustdoc: error while loading shared libraries: http://libtest-767351f2ae2fb418.so: cannot open shared object file: No such file or directory
error: could not document `core`

I fixed this by building the docs with the master branch at 855c2d130fb70da1643cf8f696c7aad7537aef34 and then switching to my branch that has mostly JavaScript/CSS differences

[jyn514]

In my opinion building from stage 2 and the CI artefacts should be possible in ~5min or less, but my current test gave me ~23min. So not that great either

It should be less than ~3 minutes if you set download-rustc = true. profile = tools does that by default.

[jyn514]

Hi @dns2utf8, are you planning to work on this?

[JohnTitor]

Triage: I'm going to close this as inactive, but feel free to open another PR if anyone is still interested in this change (I think https://github.com/rust-lang/rustc-dev-guide/pull/1389/files#diff-d9ae61aa6e37d7cc365c1ae1c66c801e2741026fbc752d4628dc34f3c5ce9290 updated some instructions but the "Fast builds (for tier 1 targets)" section might be still useful). Thanks for contributing!