more precise RawDirEntry lifetime

[gmorer]

The lifetime returned by RawDir::next() is the RawDir's lifetime and not the buffer it uses ('buf). The file name lives inside the buffer provided to the kernel.

Example

fn test<'buf, 'fd>(fd: BorrowedFd<'fd>, buf: &'buf mut Vec<u8>) -> RawDirEntry<'buf> {
    RawDir::new(fd, buf.spare_capacity_mut())
        .next()
        .unwrap()
        .unwrap()
}

Without the change, the borrow checker will tell the following error:

error: lifetime may not live long enough
  --> brr-lib/src/fs/dir.rs:40:5
   |
   |   fn test<'buf, 'fd>(fd: BorrowedFd<'fd>, buf: &'buf mut Vec<u8>) -> RawDirEntry<'buf> {
   |           -----  --- lifetime `'fd` defined here
   |           |
   |           lifetime `'buf` defined here
   | /     RawDir::new(fd, buf.spare_capacity_mut())
   | |         .next()
   | |         .unwrap()
   | |         .unwrap()
   | |_________________^ function was supposed to return data with lifetime `'buf` but it is returning data with lifetime `'fd`
   |
   = help: consider adding the following bound: `'fd: 'buf`

error[E0515]: cannot return value referencing temporary value
  --> brr-lib/src/fs/dir.rs:40:5
   |
   |       RawDir::new(fd, buf.spare_capacity_mut())
   |       ^----------------------------------------
   |       |
   |  _____temporary value created here
   | |
   | |         .next()
   | |         .unwrap()
   | |         .unwrap()
   | |_________________^ returns a value referencing data owned by the current function

The borrow checker passes with the change