GitHub - jkeys089/interfaceguard: interfaceguard finds subtle interface-related issues, including improper comparisons and type mismatches.

interfaceguard finds subtle interface-related issues, including improper comparisons and type mismatches.

Example:

package main

import (
	"fmt"
	"time"
)

type Greeter interface {
	SayHi() string
}

type CostcoGreeter struct {}

func (c *CostcoGreeter) SayHi() string {
	return "Welcome to Costco, I love you."
}

func main() {
	var greeter Greeter = getCachedGreeter()
	
	if greeter == nil {
		panic("greeter is nil") // we might expect this to panic, but it doesn't!
	}
	
	fmt.Println("ok")
}

// getCachedGreeter simulates cache miss at runtime and will always return nil *CostcoGreeter
func getCachedGreeter() Greeter {
	now := time.Now().Unix()

	switch {
	case now == 0:
		return new(CostcoGreeter)
	case now == 1:
		return nil
	default:
		var c *CostcoGreeter
		return c
	}
}

This is a safer way to check if an interface is nil:

package main

import (
	"fmt"
	"reflect"
	"time"
)

type Greeter interface {
	SayHi() string
}

type CostcoGreeter struct {}

func (c *CostcoGreeter) SayHi() string {
	return "Welcome to Costco, I love you."
}

func main() {
	var greeter Greeter = getCachedGreeter()

	if isNil(greeter) {
		panic("greeter is nil") // now we catch the nil greeter and panic as expected
	}

	fmt.Println("ok")
}

// getCachedGreeter simulates cache miss at runtime and will always return nil *CostcoGreeter
func getCachedGreeter() Greeter {
	now := time.Now().Unix()

	switch {
	case now == 0:
		return new(CostcoGreeter)
	case now == 1:
		return nil
	default:
		var c *CostcoGreeter
		return c
	}
}

func isNil(val any) bool {
	//nolint:interfaceguard
	if val == nil {
		return true
	}

	v := reflect.ValueOf(val)

	//nolint:exhaustive
	switch v.Kind() {
	case reflect.Chan, reflect.Func, reflect.Map, reflect.Pointer,
		reflect.UnsafePointer, reflect.Interface, reflect.Slice:
		return v.IsNil()
	default:
		return false
	}
}

What about comparing two interface values to each other:

package main

import (
	"fmt"
	"time"
)

type Greeter interface {
	SayHi() string
}

type WallyWorldGreeter struct {}

func (w *WallyWorldGreeter) SayHi() string {
	return "Sorry, folks! We're closed for two weeks to clean and repair America's favorite family fun park. Sorry, uh-huh, uh-huh, uh-huh!"
}

func main() {
	var greeterA Greeter
	var greeterB Greeter = getCachedGreeter()
	
	if greeterA == greeterB {
		panic("greeterA == greeterB") // we might expect this to panic since both are nil, but it doesn't!
	}
	
	fmt.Println("ok")
}

// getCachedGreeter simulates cache miss at runtime and will always return nil *WallyWorldGreeter
func getCachedGreeter() Greeter {
	now := time.Now().Unix()

	switch {
	case now == 0:
		return new(WallyWorldGreeter)
	case now == 1:
		return nil
	default:
		var w *WallyWorldGreeter
		return w
	}
}

Interface equality checks can be tricky! Here is a safer approach:

package main

import (
	"fmt"
	"reflect"
	"time"
)

type Greeter interface {
	SayHi() string
}

type WallyWorldGreeter struct {}

func (w *WallyWorldGreeter) SayHi() string {
	return "Sorry, folks! We're closed for two weeks to clean and repair America's favorite family fun park. Sorry, uh-huh, uh-huh, uh-huh!"
}

func main() {
	var greeterA Greeter
	var greeterB Greeter = getCachedGreeter()

	if isEqual(greeterA, greeterB) {
		panic("greeterA == greeterB") // now we see them as equal and panic as expected
	}
	
	fmt.Println("ok")
}

// getCachedGreeter simulates cache miss at runtime and will always return nil *WallyWorldGreeter
func getCachedGreeter() Greeter {
	now := time.Now().Unix()

	switch {
	case now == 0:
		return new(WallyWorldGreeter)
	case now == 1:
		return nil
	default:
		var w *WallyWorldGreeter
		return w
	}
}

func isEqual(greeterA, greeterB Greeter) bool {
	if isNil(greeterA) && isNil(greeterB) {
		return true
	}
	
	return reflect.DeepEqual(greeterA, greeterB)
}

func isNil(val any) bool {
	//nolint:interfaceguard
	if val == nil {
		return true
	}

	v := reflect.ValueOf(val)

	//nolint:exhaustive
	switch v.Kind() {
	case reflect.Chan, reflect.Func, reflect.Map, reflect.Pointer,
		reflect.UnsafePointer, reflect.Interface, reflect.Slice:
		return v.IsNil()
	default:
		return false
	}
}

How To Use

Use it directly via command-line:

go install github.com/jkeys089/interfaceguard/cmd/interfaceguard@latest

# default all checks enabled
interfaceguard ./...

# disable interface-to-interface comparison checks
interfaceguard -i ./...

# disable interface nil comparison checks
interfaceguard -n ./...

Or use the golangci-lint plugin (see: plugin/golangci)