Support for go modules

[joshuarubin]

Beginning in go 1.11 go will have opt-in support for the vgo module system proposed by Russ Cox.

We've started testing support for this using Go tip and have run into an issue with mage.

We have a library that we use with our common mage operations. It is tagged and has a version greater than 1. As a result, its go.mod file lists the module as something like module github.com/foo/bar/v2. This means that imports of this module in the magefile would use import bar "github.com/foo/bar/v2" even though the module is downloaded (and would have been placed in the $GOPATH) at github.com/foo/bar.

The error we get when trying to compile the magefile is something like:

Error: failed to check types in directory: magefile.go:18:8: could not import github.com/foo/bar/v2 (cannot find package "github.com/foo/bar/v2" in any of:
        ~/go/src/github.com/foo/baz/vendor/github.com/foo/bar/v2 (vendor tree)
        ~/.gvm/gos/master/src/github.com/foo/bar/v2 (from $GOROOT)
        ~/go/src/github.com/foo/bar/v2 (from $GOPATH))

This stems from the fact that the stdlib go/* packages do not (and may never: golang/go#26433) support modules. The recommendation was to, instead, use the still in development golang.org/x/tools/go/packages package (which will only support Go 1.11 and later).

Until this issue is resolved, mage can't work seamlessly with go modules at all. Either we import with github.com/foo/bar in the magefile and vgo will use the latest v1 release (which is the wrong version), or we import github.com/foo/bar/v2 in the magefile and mage fails to build the magefile at all.

[natefinch]

Someone else saw this same error... I'll have to look into it. I think it may not be modules. It may be the fact that mage uses a customized version of go/build.

[rsc]

golang.org/x/tools/go/packages will support Go 1.10 as well, probably not Go 1.9.

[rsc]

I just tried the latest version of mage and reproduced the problem. It looks like it really is modules. But I have a fix for go/build for Go 1.11 that should make mage and many other programs keep working.

[natefinch]

Thank you @rsc, I really appreciate it.

[natefinch]

This is fixed by #131