add Infer function

Author: matthewmueller

mod.go

@@ -17,7 +17,7 @@ var ErrFileNotFound = fmt.Errorf(`mod: unable to find "go.mod": %w`, fs.ErrNotEx
 
 // New module
 func New(dir string) *Module {
-	modulePath := modulePathFromGoPath(dir)
+	modulePath := Infer(dir)
 	if modulePath == "" {
 		modulePath = "change.me"
 	}
@@ -128,13 +128,17 @@ func Abs(dir string) (absDir string, err error) {
 	return filepath.Dir(absPath), nil
 }
 
-// modulePathFromGoPath tries inferring the module path of directory. This only
-// works if you're in working within the $GOPATH
-func modulePathFromGoPath(path string) string {
-	src := filepath.Join(build.Default.GOPATH, "src") + "/"
-	if !strings.HasPrefix(path, src) {
+// GOPATH is the default GOPATH for the current build environment. This is
+// exposed for testing purposes, but should not be changed in production code.
+var GOPATH = build.Default.GOPATH
+
+// Infer the module path from the $GOPATH. This only works if you work inside
+// $GOPATH otherwise returns an empty string.
+func Infer(dir string) string {
+	src := filepath.Join(GOPATH, "src") + "/"
+	if !strings.HasPrefix(dir, src) {
 		return ""
 	}
-	modulePath := strings.TrimPrefix(path, src)
+	modulePath := strings.TrimPrefix(dir, src)
 	return modulePath
 }

mod_test.go

@@ -2,6 +2,7 @@ package mod_test
 
 import (
 	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/livebud/mod"
@@ -34,3 +35,19 @@ func TestNew(t *testing.T) {
 	is.Equal(dir, module.Dir())
 	is.Equal(module.Import(), "change.me")
 }
+
+func TestInferOutsideGoPath(t *testing.T) {
+	dir := t.TempDir()
+	is := is.New(t)
+	is.Equal(mod.Infer(dir), "")
+}
+
+func TestInferInsideGoPath(t *testing.T) {
+	gopath := mod.GOPATH
+	mod.GOPATH = t.TempDir()
+	t.Cleanup(func() {
+		mod.GOPATH = gopath
+	})
+	is := is.New(t)
+	is.Equal(mod.Infer(filepath.Join(mod.GOPATH, "src", "app.com")), "app.com")
+}

modcache.go

@@ -2,7 +2,6 @@ package mod
 
 import (
 	"fmt"
-	"go/build"
 	"os"
 	"path/filepath"
 
@@ -23,7 +22,7 @@ func getModCacheDir() string {
 		modCacheDir = env
 		return env
 	}
-	modCacheDir = filepath.Join(build.Default.GOPATH, "pkg", "mod")
+	modCacheDir = filepath.Join(GOPATH, "pkg", "mod")
 	return modCacheDir
 }