fix: import and arg collision (#219)

This PR fixes an issue where if the name of an argument to a method that is being mocked collides with the name of a package that needs to be imported, mockgen could generate uncompilable code.

Resolves #218

mockgen/internal/tests/import_collision/internalpackage/foo.go

@@ -0,0 +1,3 @@
+package internalpackage
+
+type FooExported struct{}

mockgen/internal/tests/import_collision/p2/p2.go

@@ -0,0 +1,12 @@
+package p2
+
+//go:generate mockgen -destination=mocks/mocks.go -package=internalpackage . Mything
+
+import (
+	"go.uber.org/mock/mockgen/internal/tests/import_collision/internalpackage"
+)
+
+type Mything interface {
+	// issue here, is that the variable has the same name as an imported package.
+	DoThat(internalpackage int) internalpackage.FooExported
+}

mockgen/mockgen.go

@@ -758,6 +758,17 @@ func (g *generator) GenerateMockReturnCallMethod(intf *model.Interface, m *model
 	return nil
 }
 
+// nameExistsAsPackage returns true if the name exists as a package name.
+// This is used to avoid name collisions when generating mock method arguments.
+func (g *generator) nameExistsAsPackage(name string) bool {
+	for _, symbolName := range g.packageMap {
+		if symbolName == name {
+			return true
+		}
+	}
+	return false
+}
+
 func (g *generator) getArgNames(m *model.Method, in bool) []string {
 	var params []*model.Parameter
 	if in {
@@ -766,16 +777,19 @@ func (g *generator) getArgNames(m *model.Method, in bool) []string {
 		params = m.Out
 	}
 	argNames := make([]string, len(params))
+
 	for i, p := range params {
 		name := p.Name
-		if name == "" || name == "_" {
+
+		if name == "" || name == "_" || g.nameExistsAsPackage(name) {
 			name = fmt.Sprintf("arg%d", i)
 		}
 		argNames[i] = name
 	}
 	if m.Variadic != nil && in {
 		name := m.Variadic.Name
-		if name == "" {
+
+		if name == "" || g.nameExistsAsPackage(name) {
 			name = fmt.Sprintf("arg%d", len(params))
 		}
 		argNames = append(argNames, name)