cmd/go: improve diagnostics for mod subcommands within $GOPATH/src

[davecheney]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

% go version
go version devel +8a330454dc Fri Jul 13 03:53:00 2018 +0000 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

% go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/dfc/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/dfc"
GOPROXY=""
GORACE=""
GOROOT="/Users/dfc/go"
GOTMPDIR=""
GOTOOLDIR="/Users/dfc/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/by/3gf34_z95zg05cyj744_vhx40000gn/T/go-build891243944=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Daves-MacBook-Pro(~/src/github.com/pkg/errors) % go mod -init -module github.com/pkg/errors
go: creating new go.mod: module github.com/pkg/errors
Daves-MacBook-Pro(~/src/github.com/pkg/errors) % cat go.mod 
module github.com/pkg/errors
Daves-MacBook-Pro(~/src/github.com/pkg/errors) % go mod -verify
go mod: cannot use outside module

What did you expect to see?

Not a failure

What did you see instead?

All go mod commands report

go mod: cannot use outside module

[myitcv]

@davecheney looks like you're running these commands from within your GOPATH?

In which case, take a look through @rsc's answer in golang-dev:

https://groups.google.com/d/msg/golang-dev/a5PqQuBljF4/tYBmxfF4BgAJ

You'll need to set GO111MODULE=on to force module mode if you want to work with modules inside your GOPATH.

[davecheney]

Fair enough. Thanks for the answer.

…

On 13 Jul 2018, at 19:46, Paul Jolly ***@***.***> wrote: @davecheney looks like you're running these commands from within your GOPATH? In which case, take a look through @rsc's answer in golang-dev: https://groups.google.com/d/msg/golang-dev/a5PqQuBljF4/tYBmxfF4BgAJ You'll need to set GO111MODULE=on to force module mode if you want to work with modules inside your GOPATH. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or mute the thread.

[myitcv]

@rsc @bcmills is it worth adding GO111MODULE to go env to help in these sorts of situations?

Feels like that might be a very common first question we'll ask people.

[davecheney]

Maybe a better answer is to expand the error text to explain that when inside gopath you need to set an extra env var.

…

On 13 Jul 2018, at 23:36, Paul Jolly ***@***.***> wrote: @rsc @bcmills is it worth adding GO111MODULE to go env to help in these sorts of situations? Feels like that might be a very common first question we'll ask people. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or mute the thread.

[bcmills]

I suspect we should do both of those things. 🙂

[FiloSottile]

We should also show a helpful error for failed builds when a go.mod file was found and modules are off because of GOPATH.

[gopherbot]

Change https://golang.org/cl/124708 mentions this issue: cmd/go: diagnose 'go mod' in GOPATH/src better

[bcmills]

Should be much improved now. Please reopen if you still run into confusing behavior.