cmd/go: 'go list -m -json' in an empty directory returns cryptic output

[hyangah]

$ go version
go version devel +56c9f8e8cf linux/amd64

$ cd $(mktemp -d)
$ GO111MODULE=on go list -m -json 
{
	"Path": "command-line-arguments",
	"Main": true
}

[bcmills]

This output is correct, if a bit strange: if you run a go command outside of any module, the effective module name for the resulting artifacts is command-line-arguments, and because you passed the -m flag it doesn't need a module path to resolve the effective import path of the current directory (and therefore doesn't error on the fact that there is no such module path).

I suppose that we could add some sort of special case, but I can't think of one would simplify this case more than it complicates others. Any concrete suggestions?

(CC @jayconrod)

[hyangah]

Without -m, the error message is more comprehensive.
I don't know how complex it will be to handle this special case.

$ GO111MODULE=on go list -json
go: cannot find main module; see 'go help modules'

[jayconrod]

Is there anything go list -m can do without a module root? The arguments are all module paths or queries, and those are only meaningful if we can generate the build list, which is going to be empty if there's no go.mod and no source files.

There's a branch in go list to handle -m behavior. I think we can just add a call to modload.ModRoot in there. That would print the cannot find main module message.

[bcmills]

go list -m --versions is very useful without a module root.

[jayconrod]

True, so is -u. And positional query arguments work, too.

So if we added a special case for this, it would just be for go list -m. No positional arguments. No -u, -versions or other options that affect semantics (-json would be okay though). The logic would just be in internal/list/list.go though; module and package loading infrastructure would be unaffected.

@hyangah WDYT? Worth adding?

[bcmills]

Depending on the outcome of #32027, this issue may become moot.

[gopherbot]

Change https://go.dev/cl/585055 mentions this issue: cmd/go: return error when directory has no go.mod in module mode

[matloob]

@gabyhelp we need your help

[matloob]

Is this still confusing people?