牛肉 Cow更快，更紧凑的实现。 --- use beef :: Cow; let borrowed: Cow = Cow :: borrowed ( "Hello" ); let owned: Cow = Cow :: owned ( String :: from ( "World" )); assert_eq! ( format! ( "{} {}!" , borrowed, owned), "Hello World!" , ); 此板条箱暴露了Cow两个版本： be