cmd/go: can only run "go env" in certain places

[bradfitz]

I've historically checked out my Go development trees to $HOME/go. (where it naturally falls if you do git clone https://go.googlesource.com/go without an additional argument)

That habit predates the new automatic GOPATH value of $HOME/go.

But that led to this surprising behavior on a new machine I was benchmarking:

In $HOME/go ...

[ec2-user@ip-172-31-7-89 go]$ pwd
/home/ec2-user/go
[ec2-user@ip-172-31-7-89 go]$ git remote -v
origin	https://go.googlesource.com/go (fetch)
origin	https://go.googlesource.com/go (push)

Here's $HOME/go/bin/go's version command showing that I just built Go...

[ec2-user@ip-172-31-7-89 go]$ ./bin/go version
go version devel +198f0452b0 Thu Nov 7 02:33:31 2019 +0000 linux/arm64

And can run go env from $HOME/go too....

[ec2-user@ip-172-31-7-89 go]$ ./bin/go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/home/ec2-user/.cache/go-build"
GOENV="/home/ec2-user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH=""
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/ec2-user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/ec2-user/go/pkg/tool/linux_arm64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
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 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build676408819=/tmp/go-build -gno-record-gcc-switches"

But if I cd down into src (which contains a go.mod file), I can't use the same go binary anymore to run go env (or run go tool dist test, etc):

[ec2-user@ip-172-31-7-89 go]$ cd src/
[ec2-user@ip-172-31-7-89 src]$ ../bin/go env
missing $GOPATH
[ec2-user@ip-172-31-7-89 src]$ ../bin/go tool dist test
2019/11/07 04:26:01 Error running go env CGO_ENABLED: exit status 1

Is this working as intended?

Can it be made more helpful if I'm holding it wrong?

/cc @bcmills

[bcmills]

Duplicate of #34628