Deafult::default() 宏實(shí)現(xiàn)不同數(shù)據(jù)類型默認(rèn)值

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macro_rules! default_impl {
    ($t:ty, $v:expr, $doc:tt) => {
        #[stable(feature = "rust1", since = "1.0.0")]
        impl Default for $t {
            #[inline]
            #[doc = $doc]
            fn default() -> $t {
                $v
            }
        }
    };
}

default_impl! { (), (), "Returns the default value of `()`" }
default_impl! { bool, false, "Returns the default value of `false`" }
default_impl! { char, '\x00', "Returns the default value of `\\x00`" }

default_impl! { usize, 0, "Returns the default value of `0`" }
default_impl! { u8, 0, "Returns the default value of `0`" }
default_impl! { u16, 0, "Returns the default value of `0`" }
default_impl! { u32, 0, "Returns the default value of `0`" }
default_impl! { u64, 0, "Returns the default value of `0`" }
default_impl! { u128, 0, "Returns the default value of `0`" }

default_impl! { isize, 0, "Returns the default value of `0`" }
default_impl! { i8, 0, "Returns the default value of `0`" }
default_impl! { i16, 0, "Returns the default value of `0`" }
default_impl! { i32, 0, "Returns the default value of `0`" }
default_impl! { i64, 0, "Returns the default value of `0`" }
default_impl! { i128, 0, "Returns the default value of `0`" }

default_impl! { f32, 0.0f32, "Returns the default value of `0.0`" }
default_impl! { f64, 0.0f64, "Returns the default value of `0.0`" }

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