Asking for a 'static bound when not needed

[carllerche]

Repro:

use std::io::{Reader,MemReader};

enum Wrapper<'a> {
  WrapReader(&'a Reader)
}

trait Wrap<'a> {
  fn wrap(self) -> Wrapper<'a>;
}

impl<'a, R: Reader> Wrap<'a> for &'a mut R {
  fn wrap(self) -> Wrapper<'a> {
    WrapReader(self as &'a mut Reader)
  }
}
pub fn main() {
  println!("Zomg!");
}

Error:

repro.rs:13:16: 13:20 error: value may contain references; add `'static` bound to `R`
repro.rs:13     WrapReader(self as &'a mut Reader)
                           ^~~~
error: aborting due to previous error

[alexcrichton]

cc @luqmana

[emberian]

I don't think it is unneeded, but I'm not sure.

[carllerche]

Converting everything to 100% trait objects compiles:

use std::io::{Reader,BufReader};

enum Wrapper<'a> {
  WrapReader(&'a Reader)
}

trait Wrap<'a> {
  fn wrap(self) -> Wrapper<'a>;
}

impl<'a> Wrap<'a> for &'a mut Reader {
  fn wrap(self) -> Wrapper<'a> {
    WrapReader(self)
  }
}

fn doit<'a>(b: &'a [u8]) {
  let mut r = BufReader::new(b);
  (&mut r as &mut Reader).wrap();
}

pub fn main() {
  let s = "zomg".to_string();
  doit(s.as_slice().as_bytes());
  println!("Zomg!");
}

So it seems that the compile error, at least in the given example, is not a safety problem.

[nikomatsakis]

cc me

[nikomatsakis]

I've been working on a fix for this (and general cleanup of this issue). Unfortunately, I've put it on hold for #5527, it seems like this kind of thing keeps coming up though.

[nikomatsakis]

I think the original error is legit, but there is no great way to fix it right now. I expect one to write something like R:'a+Reader -- basically, we can't cast R to an &'a mut Reader because we don't know whether R contains borrowed pointers with a lifetime shorter than 'a. Enabling this sort of syntax is what I was working on (among other related things).

[carllerche]

Looks like this is fixed by #16453

[sfackler]

Fixed by #18324