cmd/go: document the meaning of solitary "command-line-arguments"

[kortschak]

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

$ go version

Does this issue reproduce with the latest release?

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build773661967=/tmp/go-build -gno-record-gcc-switches"

What did you do?

~ $ go list -m
command-line-arguments

and look for "command-line-arguments" in documentation and the source tree.

What did you expect to see?

An explanation of the significance of the output.

What did you see instead?

Crickets (though also some bread crumbs in the source and commit messages).

It would be nice if the meaning were properly documented.

[AlexRouSg]

I can reproduce on tip da4d585 and 1.13.5 and 1.12.14. Does not repo on 1.11.13

GO111MODULE=on go list -m in a non module directory prints command-line-arguments instead of go list -m: not using modules. Does not happen with GO111MODULE=auto or GO111MODULE=off

cc @heschik

[heschi]

This is coming from the go command, not go/packages -- the code linked above is just trying to mimic that behavior.

@bcmills, @jayconrod will know better but I think this is supposed to fail and instead is outputting a dummy value.

[jayconrod]

Confirming that this is a dummy value. It's the name of the main module when the go command runs in module mode outside a module (no go.mod file and GO111MODULE=on).

Perhaps it should be an error instead? go list -m all and go list -m ... report errors when run outside a module. We have a special case that suppresses matches for command-line-arguments and patterns that would match it like c....

[bcmills]

command-line-arguments is the package name that you get when you invoke go build or go run with a list of files instead of a package name.

The reason I chose that for go list -m outside of a module is that the module name is still significant in that case: in particular, the name of the main module affects which internal imports the files are allowed to use. (A module name of command-line-arguments implies that the files are only allowed to access packages in modules with paths beginning with the name command-line-arguments, of which there are none.)

The module name command-line-arguments is also more consistent from the perspective of debug information: for modules other than std, the package path is nearly always an extension of the module path, and the package path in this case has been command-line-arguments since long before module support.