proposal: cmd/go: go build should build multiple main packages

[rasky]

go help build says:

When compiling multiple packages or a single non-main package,
build compiles the packages but discards the resulting object,
serving only as a check that the packages can be built.

The fact that go build doesn't build is counter-intuitive. It would be great to change it to generate binaries (just like it does when a single package is specified). Especially now with the build cache, it doesn't sound too much of a burden, and it would further reduce cases where an external build system is required.

[ianlancetaylor]

I guess we would have to prohibit the -o option in that case. Would it just put all the executables in the current directory?

[rasky]

-o is already prohibited for multiple packages:

The -o flag, only allowed when compiling a single package,
forces build to write the resulting executable or object
to the named output file, instead of the default behavior described
in the last two paragraphs.

Probably, putting all files into the current directory is the least surprising solution, and it's also compatible with how go build behaves with a single package.

[rasky]

I would attempt a patch, but I think it needs decision first... @rsc?

[rasky]

@rsc I would like to implement this, but can you please ack on the general idea first?

[rasky]

Ping @rsc @bradfitz @andybons ... should I just go and implement it?

[bradfitz]

IIRC (away from corp laptop), many people on the Go team are out this week, so the weekly NeedsDecision meeting will likely not happen today, and this needs @rsc's decision.

You could implement it now if you want, but @rsc might decide against it later.

[rasky]

Ping @rsc

[andybons]

Bumping to proposal so this will get looked at during our next meeting.

[rsc]

Duplicate of #14295, which I commented on last week. I think this should just work. #14295 (comment)