confusing lifetime inference

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?

1. 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.

   Ariel Ben-Yehuda at 2016-05-10 05:27:09

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

   Jay at 2016-05-10 05:37:07

3. 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
   

   Esteban Kuber at 2018-02-10 01:37:04

4. 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`
   

   Esteban Kuber at 2019-07-20 02:31:59

5. Triage: no change since nll.

   Esteban Kuber at 2023-10-19 18:36:42