feat: add Fn matcher

Fn matcher allows to use custom logic for complex matching scenarios

gomock/matchers.go

@@ -97,6 +97,18 @@ func (anyMatcher) String() string {
 	return "is anything"
 }
 
+type fnMatcher struct {
+	fn func(x any) bool
+}
+
+func (f fnMatcher) Matches(x any) bool {
+	return f.fn(x)
+}
+
+func (fnMatcher) String() string {
+	return "a customizable behaviour"
+}
+
 type eqMatcher struct {
 	x any
 }
@@ -280,6 +292,15 @@ func All(ms ...Matcher) Matcher { return allMatcher{ms} }
 // Any returns a matcher that always matches.
 func Any() Matcher { return anyMatcher{} }
 
+// Fn returns a specialized matchers customizable for complex matching behaviour.
+// This is particularly useful in case you want to match over a field of a custom struct, or dynamic logic.
+//
+// Example usage:
+//
+//	Fn(func(x any){return x.(int) == 1}).Matches(1) // returns true
+//	Fn(func(x any){return x.(int) == 2}).Matches(1) // returns false
+func Fn(fn func(x any) bool) Matcher { return fnMatcher{fn} }
+
 // Eq returns a matcher that matches on equality.
 //
 // Example usage:

gomock/matchers_test.go

@@ -27,6 +27,9 @@ import (
 )
 
 type A []string
+type B struct {
+	Name string
+}
 
 func TestMatchers(t *testing.T) {
 	type e any
@@ -50,6 +53,7 @@ func TestMatchers(t *testing.T) {
 			[]e{[]string{"a", "b"}, A{"a", "b"}},
 			[]e{[]string{"a"}, A{"b"}},
 		},
+		{"test Fn", gomock.Fn(func(x any) bool { return x.(B).Name == "Dam" }), []e{B{Name: "Dam"}}, []e{B{Name: "Dave"}}},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

sample/imp4/imp4.go

@@ -1,3 +1,5 @@
 package imp_four
 
-type Imp4 struct{}
+type Imp4 struct {
+	Field string
+}

sample/user_test.go

@@ -7,6 +7,7 @@ import (
 	"go.uber.org/mock/gomock"
 	user "go.uber.org/mock/sample"
 	"go.uber.org/mock/sample/imp1"
+	imp_four "go.uber.org/mock/sample/imp4"
 )
 
 func TestRemember(t *testing.T) {
@@ -193,3 +194,19 @@ func TestDoAndReturnSignature(t *testing.T) {
 		mockIndex.Slice([]int{0}, []byte("meow"))
 	})
 }
+
+func TestExpectFnForeignFour(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	mockIndex := NewMockIndex(ctrl)
+	mockIndex.EXPECT().ForeignFour(gomock.Fn(func(x any) bool {
+		four, ok := x.(imp_four.Imp4)
+		if !ok {
+			return false
+		}
+		return four.Field == "Cool"
+	}))
+
+	mockIndex.ForeignFour(imp_four.Imp4{Field: "Cool"})
+}