env GO111MODULE=on
[short] skip

# This script tests commands in module mode outside of any module.
#
# First, ensure that we really are in module mode, and that we really don't have
# a go.mod file.
go env GOMOD
stdout 'NUL|/dev/null'


# 'go list' without arguments implicitly operates on the current directory,
# which is not in a module.
! go list

        Indirect   bool          // module is only indirectly needed by main module
        Dir        string        // directory holding local copy of files, if any
        GoMod      string        // path to go.mod file describing module, if any
        GoVersion  string        // go version used in module
        Retracted  []string      // retraction information, if any (with -retracted or -u)

			}
		}
		tokens = append(tokens, tok.text)
		end = tok.endPos
	}
}

var (
	slashSlash = []byte("//")
	moduleStr  = []byte("module")
)

// ModulePath returns the module path from the gomod file text.
// If it cannot find a module path, it returns an empty string.
// It is tolerant of unrelated problems in the go.mod file.
func ModulePath(mod []byte) string {
	for len(mod) > 0 {
		line := mod

//	GOHOSTOS
//		The operating system (GOOS) of the Go toolchain binaries.
//	GOMOD
//		The absolute path to the go.mod of the main module.
//		If module-aware mode is enabled, but there is no go.mod, GOMOD will be
//		os.DevNull ("/dev/null" on Unix-like systems, "NUL" on Windows).
//		If module-aware mode is disabled, GOMOD will be the empty string.
//	GOTOOLDIR

-- .mod --
module example.com/retract

go 1.15

-- .info --
{"Version":"v1.0.0-good"}

-- retract.go --

		return errNoModules
	}

	// Predict whether module aware mode is enabled by checking the value of
	// GO111MODULE and looking for a go.mod file in the source directory or
	// one of its parents. Running 'go env GOMOD' in the source directory would
	// give a canonical answer, but we'd prefer not to execute another command.
	go111Module := os.Getenv("GO111MODULE")
	switch go111Module {
	case "off":
		return errNoModules

Constructed by hand.
(derived from rsc.io/quote@e7a685a342, but without an explicit go.mod file.)

-- .mod --
module "not-rsc.io/quote"
-- .info --
{"Version":"v0.1.0-nomod","Time":"2018-02-14T00:51:33Z"}
-- quote.go --
// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package quote collects pithy sayings.

		return nil, fmt.Errorf("%s: %w", args[0], err)
	}
	if deprecation != "" {
		fmt.Fprintf(os.Stderr, "go: module %s is deprecated: %s\n", rootMod.Path, modload.ShortMessage(deprecation, ""))
	}
	data, err := modfetch.GoMod(ctx, rootMod.Path, rootMod.Version)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", args[0], err)
	}
	f, err := modfile.Parse("go.mod", data, nil)
	if err != nil {

{
	"Replace": {
		"nomod/go.mod": "../../../overlay/carve-nomod-go-mod"
	}
}
-- carve/overlay_uncarve_module.json --
{
	"Replace": {
		"hasmod/go.mod": ""
	}
}
-- carve/hasmod/a.go --
package hasmod
-- carve/hasmod/go.mod --
module carve/hasmod
-- carve/nomod/b.go --
package nomod
-- $WORK/overlay/carve-nomod-go-mod --
module carve/nomod
-- carve2/go.mod --
module carve2

	cases := []struct {
		s    string
		want []string
		err  bool
	}{
		{s: "[good]", want: []string{"good"}},
		{s: "[good][abc][xyz]", want: []string{"good", "abc", "xyz"}},
		{s: "[]", want: []string{""}},
		{s: "[[good]", want: []string{"[good"}},
		{s: "[good]]", want: []string{"good]"}},
		{s: "[[good]]", want: []string{"[good]"}},
		{s: "x[bad]", err: true},
		{s: "[bad", err: true},
		{s: "bad]", err: true},