为什么impl线和new_in线的where线都是A: allocator？在文档中对单个方法可见trait绑定是否只是为了文档，或者如果省略where绑定，含义会有所不同？
在这段摘自https://doc.rust-lang.org/src/alloc/boxed.rs.html#378-380
impl<T, A: Allocator> Box<T, A> {
/// Allocates memory in the given allocator then places `x` into it.
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let five = Box::new_in(5, System);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[must_use]
#[inline]
pub fn new_in(x: T, alloc: A) -> Self
where
A: Allocator,
{
let mut boxed = Self::new_uninit_in(alloc);
unsafe {
boxed.as_mut_ptr().write(x);
boxed.assume_init()
}
}