[buro9]

Except for the build conditional flags: https://golang.org/pkg/go/build/#hdr-Build_Constraints

Which are pretty useful when you want to target tests to different versions of Go when std lib exhibits different behaviour (behaviour changed as std lib matured).

[vorg]

Also when you want to put package-main files in a library package directory, you need to put

  // +build ignore

near the top of it.