go/build: imports from cgo files included in Imports when CGO_ENABLED=0

[jayconrod]

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

$ go version
go version devel +2ac1ca9160 Tue Dec 3 15:26:50 2019 +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 Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jayconrod/Library/Caches/go-build"
GOENV="/Users/jayconrod/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/jayconrod/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/jayconrod/Code/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/jayconrod/Code/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/jayconrod/Code/go/src/cmd/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/rq/x0692kqj6ml8cvrhcqh5bswc008xj1/T/go-build021162515=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

go run main.go
! stdout '^fmt$'

-- main.go --
package main

import (
	"fmt"
	"go/build"
	"log"
	"os"
	"path/filepath"
	"strings"
)

func main() {
	ctxt := new(build.Context)
	*ctxt = build.Default
	ctxt.CgoEnabled = false

	wd, err := os.Getwd()
	if err != nil {
		log.Fatal(err)
	}
	aDir := filepath.Join(wd, "a")
	p, err := ctxt.ImportDir(aDir, 0)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(strings.Join(p.Imports, "\n"))
}

-- a/cgo_enabled.go --
package a

import "C"

import _ "fmt"

-- a/empty.go --
package a

What did you expect to see?

The test script fails.

fmt should not appear in the output. It's only imported by cgo_enabled.go, which contains import "C". This file is ignored because ctxt.CgoEnabled is false. Its imports should be ignored, too.

Note that if we add // +build cgo to cgo_enabled.go, the imports from that file are not included in the output.

What did you see instead?

The test script should pass.

This can be tested more concisely with go list -f {{.Imports}} ./a. The root cause of the issue is in go/build though.

[gopherbot]

Change https://golang.org/cl/209660 mentions this issue: go/build: don't include imports from cgo files when CGO_ENABLED=0