类型声明中的匿名字段？

I came across this type declaration:

type Handler func(*Conn)

type Server struct {
  Handshake func(*Config, *http.Request) error
  Handler
}

(this is a simplified version of https://github.com/golang/net/blob/38c17adf51120973d1735785a7c02f8ce8297c5e/websocket/server.go#L55-L66

The second field in the Server structure is anonymous. There is just type and no name.

Here is the grammar for type declarations (https://golang.org/ref/spec#Type_declarations):

TypeDecl     = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec     = identifier Type .

and it clearly requires an identifier name. But yet the section that I referenced that contains the grammar, also mentions anonymous fields.

I do not understand why this syntax is correct and how anonymous fields are used.

You want to look at the part of the grammar that has to do with structures, not just types. See: Struct types and the use of AnonymousField. Looking just at the production for TypeSpec is focusing attention on the wrong place. Instead, look at FieldDecl; the grammar shows that we have two possibilities: named fields (IdentifierList Type), or anonymous fields (AnonymousField).

Anonymous fields are typically used for embedding. In your example, a Server will act like a Handler because it has embedded that field.