go list in 1.21 gives an error import _ is a program, not an importable package

[alexbozhenko]

Per conversation here:
#25922 (comment)
it seems like managing tools dependencies with tools.go is still a best practice

The behavior of go list changed in 1.21, seems like in this commit by @matloob:
a5c7928
and it is now failing with "is a program, not an importable package", error. See below:

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

$ go version
go version devel go1.21-a5c79283f7 Mon Apr 10 20:27:52 2023 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes, reproducible on 1.21

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

go env Output

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/abozhenko/.cache/go-build'
GOENV='/home/abozhenko/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/abozhenko/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/abozhenko/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/abozhenko/code/tower/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2829969153=/tmp/go-build -gno-record-gcc-switches'

What did you do?

cat << EOF > tools.go
//go:build tools

package tools

// Manage tool dependencies via go.mod.
//
// https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module


// https://github.com/golang/go/issues/25922
import (
_ "github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway"
_ "github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-openapiv2"

_ "google.golang.org/grpc/cmd/protoc-gen-go-grpc"
_ "google.golang.org/protobuf/cmd/protoc-gen-go"

_ "github.com/envoyproxy/protoc-gen-validate"
_ "github.com/vektra/mockery/v2"
)
EOF

# Note that this step is needed to reproduce the bug. If go.mod/go.sum are absent, go list will not give a error.
go mod init test
go mod tidy

What did you expect to see?

Previous versions produced a list, that can be used by go install:

$/usr/local/go1.20.7/bin/go list -f '{{join .Imports " "}}' tools.go
github.com/envoyproxy/protoc-gen-validate github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-openapiv2 github.com/vektra/mockery/v2 google.golang.org/grpc/cmd/protoc-gen-go-grpc google.golang.org/protobuf/cmd/protoc-gen-go

What did you see instead?

Now it gives an error:

$ go list -f '{{join .Imports " "}}' tools.go
tools.go:17:2: import "github.com/envoyproxy/protoc-gen-validate" is a program, not an importable package

Running with -e flag

go list -e -f '{{join .Imports " "}}' tools.go

Does produce expected list of packages, but I am not sure if this is a right thing to do here.

[seankhliao]

Duplicate of #61857