cmd/go: go mod tidy removes needed dependencies

[hyangah]

Go version

go1.22rc1

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='~/Library/Caches/go-build'
GOENV='~/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='~/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='~/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='~/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.22rc1.darwin-amd64'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='go1.22rc1'
GOTOOLDIR='~/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.22rc1.darwin-amd64/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.22rc1'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='~/projects/vscode-go/extension/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/5p/zn7ykc111kn3lm09h_47mz2w001py5/T/go-build2103652819=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

Reorganizing our project repo structure for golang/vscode-go#3121

The desired state is

github.com/golang/vscode-go (A) module hosts a small binary.
github.com/golang/vscode-go/extension (B) module requires github.com/golang/vscode-go, to use the binary in its testing. (B->A)

https://go-review.googlesource.com/c/vscode-go/+/554316 is the CL.

To enforce B->A dependency, I've tested blank import, go:generate, ... like:
https://go-review.googlesource.com/c/vscode-go/+/554316/9/extension/doc.go
https://go-review.googlesource.com/c/vscode-go/+/554316/9/extension/go.mod

In this state, go build works:

$ go run github.com/golang/vscode-go/vscgo version
vscgo: v0.0.0-00010101000000-000000000000
go: go1.22rc1

But if I run go mod tidy, this dependency is pruned.

$ go mod tidy
~/projects/vscode-go/extension$ git diff
diff --git a/extension/go.mod b/extension/go.mod
index 74acac558..cf12f6ee5 100644
--- a/extension/go.mod
+++ b/extension/go.mod
@@ -26,12 +26,10 @@ require (
        go4.org v0.0.0-20180809161055-417644f6feb5 // indirect
        golang.org/x/crypto v0.16.0 // indirect
        golang.org/x/exp v0.0.0-20230809094429-853ea248256d // indirect
-       golang.org/x/mod v0.14.0 // indirect
        golang.org/x/net v0.19.0 // indirect
        golang.org/x/oauth2 v0.15.0 // indirect
        golang.org/x/sync v0.5.0 // indirect
        golang.org/x/sys v0.16.0 // indirect
-       golang.org/x/telemetry v0.0.0-20240104150930-463b4b688e47 // indirect
        ...

Build fails.

$ cd extension
$ go run github.com/golang/vscode-go/vscgo version
../vscgo/main.go:20:2: missing go.sum entry for module providing package golang.org/x/telemetry/counter (imported by github.com/golang/vscode-go/vscgo); to add:
        go get github.com/golang/vscode-go/vscgo@v0.0.0-00010101000000-000000000000

Extra info:

• go mod why -m: same result before/after go mod tidy

$ go mod why -m github.com/golang/vscode-go
# github.com/golang/vscode-go
github.com/golang/vscode-go/extension
github.com/golang/vscode-go

• git status

$ git status --short --untracked=all | grep '\.go'
$

• This change is the very first one that makes A module include the vscgo package, so I used v0.0.0-00010101000000-000000000000.

What did you see happen?

Enforce my tool dependency requirements, and go mod tidy agrees :-)

What did you expect to see?

go mod tidy removes the dependency and makes the code unbuildable state.

[bcmills]

The dummy import in extension/doc.go is of package github.com/golang/vscode-go.

If you want to be able to build the package github.com/golang/vscode-go/vscgo specifically, then the dummy import needs to refer to that specific package so that its transitive dependencies will also be included.

~/src/github.com/golang/vscode-go/extension$ git diff
diff --git i/extension/doc.go w/extension/doc.go
index 53bc3ada..2fd96e3c 100644
--- i/extension/doc.go
+++ w/extension/doc.go
@@ -8,8 +8,10 @@ package extension

 // Dummy command to add dependency on vscgo.

+//go:build tools
+
 import (
-       _ "github.com/golang/vscode-go"
+       _ "github.com/golang/vscode-go/vscgo"
 )

 //go:generate go run github.com/golang/vscode-go/vscgo version

~/src/github.com/golang/vscode-go/extension$ go mod tidy

~/src/github.com/golang/vscode-go/extension$ go run github.com/golang/vscode-go/vscgo version
vscgo: v0.0.0-00010101000000-000000000000
go: devel go1.22-e9b3ff15f4 Wed Jan 10 17:35:49 2024 +0000