use std::convert::TryInto;

fn main() {
  let a: i32 = 10;
  let b: u16 = 100;

  let b_ = b.try_into()
            .unwrap();

  if a < b_ {
    println!("Ten is less than one hundred.");
  }
}

这个代码里面，是不是因为 a<b_ ，然后编译器去推导 b.try_into() 转成什么类型

但我看了一下 u16实现的try_into

// TryFrom implies TryInto
#[stable(feature = "try_from", since = "1.34.0")]
impl<T, U> TryInto<U> for T
where
    U: TryFrom<T>,
{
    type Error = U::Error;

    #[inline]
    fn try_into(self) -> Result<U, U::Error> {
        U::try_from(self)
    }
}
}

这里有懂的可以解释一下吗，上面泛型U在let b_ = b.try_into().unwrap();是啥