Common Truncate trait

I'd like to be able to generalise File using io::Read, io::Write, and io::Seek instead, but there's no common interface for truncating such an object. For example, it's useful to test file reads/writes on a Vec, but I can't ever reduce the size of the object if I delete data. This function is called set_len for files and truncate for Vec.

My best option right now is to make a Trait like so:

trait Truncate {
  fn truncate_len(&mut self, len: usize) -> io::Result<()>;
}
impl Truncate for File {
  fn truncate_len(&mut self, len: usize) -> io::Result<()> { self.set_len(len) }
}
impl Truncate for Vec<u8> {
  fn truncate_len(&mut self, len: usize) -> io::Result<()> { Ok(self.resize(len)) }
}

It'd be great to have something similar be part of std::io with support for more structs.

Start by reviewing the issue's proposed Truncate trait and the existing File::set_len and Vec::truncate APIs. Research how a common std::io abstraction could cover File, Vec, and other structs, then define the API, supported implementations, and acceptance criteria for a completed design.