Unhelpful error message using impl trait to return a closure.

[peterjoel]

The following code (Rust Playground link)

fn create_recorder<'a>(log: &'a mut Vec<String>) -> impl FnMut(&'a String) {
    let handler = |msg: &String| -> () {
        log.push(format!("The messages was: {}", msg));
    };
    handler
}

produces the error:

rustc 1.15.0-nightly (71c06a56a 2016-12-18)
error[E0564]: only named lifetimes are allowed in `impl Trait`, but `` was found in the type `[closure@<anon>:4:19: 6:6 log:&mut &'a mut std::vec::Vec<std::string::String>]`
 --> <anon>:3:53
  |
3 | fn create_recorder<'a>(log: &'a mut Vec<String>) -> impl FnMut(&'a String) {
  |                                                     ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

This error can be gotten rid of, by adding a lifetime constraint:

fn create_recorder<'a>(log: &'a mut Vec<String>) -> impl FnMut(&'a String) + 'a {

But that still leaves an error about borrowing the log reference:

error[E0373]: closure may outlive the current function, but it borrows `log`, which is owned by the current function

Fixing this (by making the closure a move, as is suggested) doesn't require the named lifetime, so really the error second error message should be the only one presented.

[Seeker14491]

I ran into this too, but returning an iterator. In my case adding the lifetime is required in addition to marking the closure move.

Playground

fn apply_permutation<'a, T>(
    objects: &'a [T],
    permutation: &'a [usize],
) -> impl Iterator<Item = T> + 'a
where
    T: Copy,
{
    permutation.iter().map(move |&i| objects[i])
}

[gnzlbg]

I ran into this too while trying to return a generator...

#![feature(generators, generator_trait, conservative_impl_trait)]
use std::ops::{Generator};

struct Stack { 
    x: i32
}

impl Stack {
    fn new(x: i32) -> Stack { Stack{x: x} }
    fn gen(& mut self) -> impl Generator<Yield=i32,Return=()> { 
 // fn gen<'a>(&'a mut self) -> impl Generator<Yield=i32,Return=()> + 'a { 
        move || { for _i in 0..10 { self.x = self.x + 1; yield self.x; } return (); } 
    }
}

(The commented out line is the correct signature)

The error message is not useful, at all:

   Compiling playground v0.0.1 (file:///playground)
error[E0564]: only named lifetimes are allowed in `impl Trait`, but `` was found in the type `[generator@src/main.rs:13:9: 13:86 self:&mut Stack ((), std::ops::Range<i32>, i32, fn(std::ops::Range<i32>) -> <std::ops::Range<i32> as std::iter::IntoIterator>::IntoIter {<std::ops::Range<i32> as std::iter::IntoIterator>::into_iter}, Stack)]`
  --> src/main.rs:11:27
   |
11 |     fn gen(& mut self) -> impl Generator<Yield=i32,Return=()> { 
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This error message is so bad that it should IMO block the stabilization of conservative impl trait (@withoutboats).

Also, when I click the E0564 on the playground it sends me to a page that doesn't contain an explanation for it. Maybe that is a playground bug, and the playground should be sending me to some nightly page... Anyhow, this doesn't make this any better.

[gnzlbg]

The issues #43719 and #39722 seem to be duplicates of this one.

I've just rechecked, and the playground does indeed have a bug (rust-lang/rust-playground#207) in that the playground always links to the stable error explanations and not to the nightly ones when compiling with nightly.

However, the error at hand here, E0564, doesn't have an explanation at all.

Trying to return closures/generators, and trying to return iterators, seem to be the most common cause of triggering this error in all the issues currently filled, so maybe an explanation should cover that.

[cramertj]

This has been fixed.

[estebank]

Current output:

error[E0373]: closure may outlive the current function, but it borrows `log`, which is owned by the current function
 --> src/main.rs:5:19
  |
5 |     let handler = |msg: &String| -> () {
  |                   ^^^^^^^^^^^^^^^^^^^^ may outlive borrowed value `log`
6 |         log.push(format!("The messages was: {}", msg));
  |         --- `log` is borrowed here
help: to force the closure to take ownership of `log` (and any other referenced variables), use the `move` keyword
  |
5 |     let handler = move |msg: &String| -> () {
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^

The suggestion is correct.

---

#38615 (comment) now compiles.

---

#38615 (comment) now suggests the correct fix:

error: cannot infer an appropriate lifetime
  --> src/main.rs:12:9
   |
10 |     fn gen(& mut self) -> impl Generator<Yield=i32,Return=()> { 
   |                           ----------------------------------- this return type evaluates to the `'static` lifetime...
11 |  // fn gen<'a>(&'a mut self) -> impl Generator<Yield=i32,Return=()> + 'a { 
12 |         move || { for _i in 0..10 { self.x = self.x + 1; yield self.x; } return (); } 
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...but this borrow...
   |
note: ...can't outlive the anonymous lifetime #1 defined on the method body at 10:5
  --> src/main.rs:10:5
   |
10 | /     fn gen(& mut self) -> impl Generator<Yield=i32,Return=()> { 
11 | |  // fn gen<'a>(&'a mut self) -> impl Generator<Yield=i32,Return=()> + 'a { 
12 | |         move || { for _i in 0..10 { self.x = self.x + 1; yield self.x; } return (); } 
13 | |     }
   | |_____^
help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the method body at 10:5
   |
10 |     fn gen(& mut self) -> impl Generator<Yield=i32,Return=()> + '_ { 
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^