What is the difference between embedding a struct and embedding an interface inside a struct?

Both look syntactically similar but have very different semantics and use cases:

// Embedding a STRUCT — real method reuse
type Base struct{ ID int }
func (b Base) Describe() string { return fmt.Sprintf("id=%d", b.ID) }

type Derived struct { Base; Extra string }
d := Derived{Base: Base{ID: 1}, Extra: "x"}
d.Describe() // calls Base.Describe() — real implementation

// Embedding an INTERFACE — partial mock pattern
type Storage interface {
    Read(key string) ([]byte, error)
    Write(key string, val []byte) error
    Delete(key string) error
    List() ([]string, error)
}

// Test stub — only override Read; others panic (acceptable in tests)
type ReadOnlyStub struct {
    Storage              // embeds interface — zero value = nil
    data map[string][]byte
}
// Override only Read
func (r ReadOnlyStub) Read(key string) ([]byte, error) {
    v, ok := r.data[key]
    if !ok { return nil, errors.New("not found") }
    return v, nil
}

// ReadOnlyStub satisfies Storage (via embedded interface promotion)
// Calling Write, Delete, or List will panic — acceptable for a test stub
var s Storage = ReadOnlyStub{data: map[string][]byte{"k": []byte("v")}}
data, _ := s.Read("k") // works
// s.Write("k", nil)   // panics at runtime

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