cmd/go: list flag -json negates the -f flag

[mvdan]

$ go version
go version devel +2b50ab2aee Tue Mar 2 06:38:07 2021 +0000 linux/amd64

When writing scripts, my workflow is usually:

1. Do a go list -json query to find the fields/info I need
2. Narrow that down with -f, so I can then get the field I need directly
3. Write the script with the -f form

Sometimes, because I'm forgetful, I'll do step 2 as go list -json -f {{.Field}} without removing -json. That happened today again, and I was really confused for a solid five minutes:

$ go list -json unsafe
{
	"Dir": "/home/mvdan/tip/src/unsafe",
	"ImportPath": "unsafe",
	"Name": "unsafe",
	"Doc": "Package unsafe contains operations that step around the type safety of Go programs.",
	"Root": "/home/mvdan/tip",
	"Match": [
		"unsafe"
	],
	"Goroot": true,
	"Standard": true,
	"GoFiles": [
		"unsafe.go"
	]
}

$ go list -json -f {{.Dir}} unsafe
{
	"Dir": "/home/mvdan/tip/src/unsafe",
	"ImportPath": "unsafe",
	"Name": "unsafe",
	"Doc": "Package unsafe contains operations that step around the type safety of Go programs.",
	"Root": "/home/mvdan/tip",
	"Match": [
		"unsafe"
	],
	"Goroot": true,
	"Standard": true,
	"GoFiles": [
		"unsafe.go"
	]
}
$ go list -f {{.Dir}} unsafe
/home/mvdan/tip/src/unsafe

If both flags are present, I would expect one of the following:

1. An error, telling the user that the two flags don't make sense together
2. The -f flag would mean ignoring the -json flag, as it's more specific

I think the first option makes more sense. Silently ignoring the -f flag seems like the worst option to me.

[mvdan]

cc @bcmills @jayconrod @matloob per the owners doc

[bcmills]

I think the first option [an error telling the user that the two flags don't make sense together] makes more sense. Silently ignoring the -f flag seems like the worst option to me.

Agreed.

[mvdan]

Cool, I'll look into a small fix sometime this cycle if noone beats me to it in the next couple of weeks.

[gopherbot]

Change https://golang.org/cl/298029 mentions this issue: cmd/go: emit error when listing with -f and -json