cmd/go: go list without flags calculates deps/imports

[chlunde]

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

go version go1.11 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/chlunde/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/chlunde/go"
GOPROXY=""
GORACE=""
GOROOT="/home/chlunde/opt/go"
GOTMPDIR=""
GOTOOLDIR="/home/chlunde/opt/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/chlunde/sample/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-build180931290=/tmp/go-build -gno-record-gcc-switches"

What did you do?

cd go/src/k8s.io/client-go
 ~/.../k8s.io/client-go$ time (for a in $(seq 10); do go list  ./...; done) | md5sum -
a1c615c3eb59b2af6e60274312bb4319  -

real	0m3.271s
user	0m3.397s
sys	0m0.745s
 ~/.../k8s.io/client-go$ time (for a in $(seq 10); do go list -find ./...; done) | md5sum -
a1c615c3eb59b2af6e60274312bb4319  -

real	0m1.437s
user	0m1.209s
sys	0m0.452s

What did you expect to see?

go list with just package names should be pretty fast

What did you see instead?

go list parses much of the source code even when -json, -f or -deps is not set, and is often 2-5x faster with -find.

Should -find be set automatically when no other advanced flags are set?

diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index f3cb4e47ec..fd9c2d2980 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -311,6 +311,11 @@ func runList(cmd *base.Command, args []string) {
 	out := newTrackingWriter(os.Stdout)
 	defer out.w.Flush()
 
+	// Ignore imports if they do not affect the output
+	if *listFmt == "" && !*listDeps && !*listTest && !*listM && !*listJson {
+		*listFind = true
+	}
+
 	if *listFmt == "" {
 		if *listM {
 			*listFmt = "{{.String}}"

I also wonder if go list could take shortcuts when only parsing package names, perhaps avoid reading all .go files when the package name is found.

/cc @bcmills

[bcmills]

Based on previous discussions with @rsc, I believe that go list is supposed to report more-or-less the same errors as go build. Determining whether an error exists is a non-trivial amount of work, although it's less work than building the package.

[bcmills]

It seems like we could certainly do this if -e is also set: if we're explicitly ignoring errors and also not including them in the output, then there's no point in computing them.

[bcmills]

Consolidating into #29666.