support compile time checks for regen

[bogdanlytvynovskyi]

Requested feature A clear description of the desired feature and an example of
how it would be used.

For each generated interface mock include a line like:
var _ Interface = (*MockInterface)(nil) in mock file

Why the feature is needed A clear description of how this feature is not
served by existing functionality in gomock.

Often build step and generation step are separated. The workflow for developer should look like:

• go generate ./...
• git add .
• git commit

However when working with old interfaces, and updating those - more often than not people tend to forget to do the generation step. If there is no mock usage within this repository everything is going to be fine in terms of build/test for this repository, but consumers of this package in a different repository are going to break. Which requires going back and releasing new "fixed" version. The proposal is to enforce build failure in case generated files are not updated appropriately.

WDYT?

[bogdanlytvynovskyi]

Proposed patch below:

diff --git a/mockgen/mockgen.go b/mockgen/mockgen.go
index 79cb921..8edd3ec 100644
--- a/mockgen/mockgen.go
+++ b/mockgen/mockgen.go
@@ -362,7 +362,7 @@ func (g *generator) Generate(pkg *model.Package, outputPkgName string, outputPac
 	g.p(")")
 
 	for _, intf := range pkg.Interfaces {
-		if err := g.GenerateMockInterface(intf, outputPackagePath); err != nil {
+		if err := g.GenerateMockInterface(pkg.Name, intf, outputPackagePath); err != nil {
 			return err
 		}
 	}
@@ -404,7 +404,7 @@ func (g *generator) formattedTypeParams(it *model.Interface, pkgOverride string)
 	return long.String(), short.String()
 }
 
-func (g *generator) GenerateMockInterface(intf *model.Interface, outputPackagePath string) error {
+func (g *generator) GenerateMockInterface(pkgName string, intf *model.Interface, outputPackagePath string) error {
 	mockType := g.mockName(intf.Name)
 	longTp, shortTp := g.formattedTypeParams(intf, outputPackagePath)
 
@@ -417,6 +417,7 @@ func (g *generator) GenerateMockInterface(intf *model.Interface, outputPackagePa
 	g.out()
 	g.p("}")
 	g.p("")
+	g.p("var _ %v.%v = (*%v)(nil)", pkgName, intf.Name, mockType)
 
 	g.p("// %vMockRecorder is the mock recorder for %v.", mockType, mockType)
 	g.p("type %vMockRecorder%v struct {", mockType, longTp)

[manfredlift]

@bogdanlytvynovskyi have you considered opening a PR for this? This is something I am also interested in! :)