cmd/go: improve error message for package not found?

[thatreguy]

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

$ go version
1.12.4

Does this issue reproduce with the latest release?

Yes

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

Ubuntu 16.04.6/amd64

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/root/.go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/.go/src/github.com/firebase/firebase-admin-go/go.mod"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build012231754=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The following is my tree:
snippets/
├── constants
│   └── constants.go
├── libs
│   ├── auth.go
│   ├── db.go
│   ├── init.go
│   ├── messaging.go
│   └── storage.go
└── snippets.go
I was trying to import constants from snippets.go:

cat snippets.go

package main
import (
	"snippets/constants"
)
.
.
.

And did:
go build -x -v

What did you expect to see?

No error or verbose output from go build

What did you see instead?

Quite unhelpful output:
build firebase.google.com/go/snippets: cannot load snippets/constants: cannot find module providing package snippets/constants

Now for whatever reason my import might have failed, I can understand that. But I would at least like a detailed explanation from go build. (In fact, the same is true to go get, go run etc.)

[FiloSottile]

I am guessing your module here is called firebase.google.com/go/snippets, not snippets, so the import should have been firebase.google.com/go/snippets/constants. Not sure how go build could give a better error message here without guessing.

/cc @bcmills

[jayconrod]

@thatreguy You said your tree contains a directory snippets/constants. Where is this directory in relation to /root/.go/src/github.com/firebase/firebase-admin-go/go.mod? How was it created?

This is not something that would work in module mode: the Go command won't search local directories for imports outside of the main module. This worked for manually created GOPATH directories, but I'm not sure we can provide good diagnostics for that case.

[bcmills]

For the particular case where the import path doesn't start with a leading dot, there is no way such a module can exist without using a replace directive, so we can at least use that as a signal for a better error message.

[bcmills]

Duplicate of #30723