Allow crates to opt-in to building a single target

[jyn514]

Based off of #532, but backwards compatible.

Closes #627.

Motivation: If crates are only built on one target, I'd be much more comfortable raising limits on a case-by-case basis (e.g. #608, #616). I would also be more comfortable adding more targets (i.e. not tier 1: #563), since specifying those targets would require setting extra-targets and thereby opting out of all others.

Waiting on me to test that this actually works since we don't have unit tests for builds. Done.

• Can build from a clean cache (no .rustwide-docker directory)
• Can add essential files (build add-essential-files)
• Does not pass --target for proc-macros (and in general, for the default build unless default-target is explicitly set) (build crate rstest 0.4.0, testing for Missing docs for procedural macro crate #422)
• Builds for cross-compiled targets work
• Builds for host target work
• targets behaves as documented (I only tested targets = ["x86_64-pc-windows-msvc"] and unset default-target; others are tested with unit tests)
• Passing a duplicate default-target still only builds the default target once

[jyn514]

r? @pietroalbini

[pietroalbini]

You don't have to do it in this PR, but it would be nice to just use #[derive(Deserialize)].

[pietroalbini]

Oh, also, at least for now we need to filter the targets to make sure someone doesn't use an unsupported target, and fail the build in that case.

[jyn514]

Oh, also, at least for now we need to filter the targets to make sure someone doesn't use an unsupported target, and fail the build in that case.

This is our current behavior for an unsupported default-target:

2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr] error[E0463]: can't find crate for `std`
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr]   |
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr]   = note: the `x86_64-pc-windows-gnu` target may not be installed
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr] 
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr] error: aborting due to previous error
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr] 
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr] For more information about this error, try `rustc --explain E0463`.
2020/03/07 12:51:25 [INFO] rustwide::cmd: [stderr] error: Could not document `hexponent`.

Our behavior for an unsupported extra-target is the same, I think it doesn't need to be handled explicitly.

[jyn514]

Addressed review comments.

[pietroalbini]

Our behavior for an unsupported extra-target is the same, I think it doesn't need to be handled explicitly.

Keep in mind that we only show the build log of the default target, so users wouldn't be able to see those errors.

[jyn514]

Keep in mind that we only show the build log of the default target, so users wouldn't be able to see those errors.

Hmm, that does make it harder ... I don't want to fail the default build if one of the targets isn't installed, since that would require publishing a full point release. Maybe we could show a popup on the /crate page that says the target doesn't exist?

[jyn514]

Also I plan to make a follow-up PR very shortly which will install targets dynamically so I'm not sure it's worth worrying about.

[jyn514]

This is ready for review.

[pietroalbini]

Other than these two comments this looks good to be merged!

[jyn514]

Addressed review comments.