confusing lifetime inference

[BusyJay]

Consider following snippet:

use std::error::Error;

use std::io::{self, ErrorKind};

fn caused_of<'a>(mut err: &'a (Error + 'static)) -> Option<&'a io::Error> {
    loop {
        match err.downcast_ref::<io::Error>() {
            None => match err.cause() {
                None => return None,
                Some(cause) => err = cause,
            },
            d => return d,
        }
    }
}

fn main() {
    let e = io::Error::new(ErrorKind::Other, "oh no!");
    println!("{:?}", caused_of(&e));
}

According to Book, cause's lifetime should be err's lifetime. which is 'a, so it should be safe to assign cause to err directly. But I got following warnnings:

test.rs:8:34: 8:39 error: cannot infer an appropriate lifetime for autoref due to conflicting requirements [E0495]
test.rs:8                       None => match err.cause() {
                                           ^~~~~
test.rs:8:30: 8:41 note: first, the lifetime cannot outlive the method call at 8:29...
test.rs:8                       None => match err.cause() {
                                       ^~~~~~~~~~~
test.rs:8:30: 8:33 note: ...so that method receiver is valid for the method call
test.rs:8                       None => match err.cause() {
                                       ^~~
test.rs:5:75: 15:25 note: but, the lifetime must be valid for the lifetime 'a as defined on the block at 5:74...
test.rs:5 fn caused_of<'a>(mut err: &'a (Error + 'static)) -> Option<&'a io::Error> {
                                                                                    ^
test.rs:10:48: 10:53 note: ...so that trait type parameters matches those specified on the impl (expected std::ops::CoerceUnsized<&'a std::error::Error + 'static>, found std::ops::CoerceUnsized<&std::error::Error + 'static>)
test.rs:10                                                      Some(cause) => err = cause,
                                                          ^~~~~
error: aborting due to previous error

Am I missing something here?

[arielb1]

The problem is that fn cause returns a &'a (Error+'a) rather than an &'a (Error+'static).

@nikomatsakis: that sounds like a good test case for the new regionck error messages.

[BusyJay]

Is there any good way to iterate all the cause of error to find if the specific error exists?

[estebank]

Current output:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
 --> src/main.rs:8:31
  |
8 |             None => match err.cause() {
  |                               ^^^^^
  |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the function body at 5:1...
 --> src/main.rs:5:1
  |
5 | fn caused_of<'a>(mut err: &'a (Error + 'static)) -> Option<&'a io::Error> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
 --> src/main.rs:8:27
  |
8 |             None => match err.cause() {
  |                           ^^^
  = note: but, the lifetime must be valid for the static lifetime...
  = note: ...so that the expression is assignable:
          expected &'a std::error::Error + 'static
             found &std::error::Error

---

Update, minimal changes:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
 --> src/main.rs:8:31
  |
8 |             None => match err.cause() {
  |                               ^^^^^
  |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the function body at 5:14...
 --> src/main.rs:5:14
  |
5 | fn caused_of<'a>(mut err: &'a (Error + 'static)) -> Option<&'a io::Error> {
  |              ^^
note: ...so that reference does not outlive borrowed content
 --> src/main.rs:8:27
  |
8 |             None => match err.cause() {
  |                           ^^^
  = note: but, the lifetime must be valid for the static lifetime...
  = note: ...so that the expression is assignable:
          expected &'a (dyn std::error::Error + 'static)
             found &dyn std::error::Error

[estebank]

Output with nll enabled:

error: lifetime may not live long enough
  --> src/main.rs:11:32
   |
6  | fn caused_of<'a>(mut err: &'a (Error + 'static)) -> Option<&'a io::Error> {
   |              -- lifetime `'a` defined here
...
11 |                 Some(cause) => err = cause,
   |                                ^^^^^^^^^^^ cast requires that `'a` must outlive `'static`

[estebank]

Triage: no change since nll.