Code
use rayon::prelude::*;
fn main() {}
struct BadCounter {
counter: i32,
}
impl BadCounter {
fn count(mut self) -> i32 {
(0..8).into_par_iter().map(|_| self.counter += 1);
self.counter
}
}
struct EvilCounter {
counter: i32,
}
impl EvilCounter {
fn count(mut self) -> i32 {
(0..8)
.into_par_iter()
.flat_map(|_| (0..8).into_par_iter().map(|_| self.counter += 1));
self.counter
}
}Current output
error[E0594]: cannot assign to `self.counter`, as it is a captured variable in a `Fn` closure
--> src/main.rs:11:40
|
11 | (0..8).into_par_iter().map(|_| self.counter += 1);
| --- ^^^^^^^^^^^^^^^^^ cannot assign
| |
| in this closure
error[E0594]: cannot assign to `self.counter`, as it is a captured variable in a `Fn` closure
--> src/main.rs:24:58
|
21 | fn count(mut self) -> i32 {
| ----- --- change this to return `FnMut` instead of `Fn`
...
24 | .flat_map(|_| (0..8).into_par_iter().map(|_| self.counter += 1));
| --- ^^^^^^^^^^^^^^^^^ cannot assign
| |
| in this closure
error: lifetime may not live long enough
--> src/main.rs:24:27
|
24 | .flat_map(|_| (0..8).into_par_iter().map(|_| self.counter += 1));
| --- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
| | |
| | return type of closure `rayon::iter::Map<rayon::range::Iter<i32>, {closure@src/main.rs:24:54: 24:57}>` contains a lifetime `'2`
| lifetime `'1` represents this closure's body
|
= note: closure implements `Fn`, so references to captured variables can't escape the closure
help: consider adding 'move' keyword before the nested closure
|
24 | .flat_map(|_| (0..8).into_par_iter().map(move |_| self.counter += 1));
| ++++
error[E0596]: cannot borrow `self.counter` as mutable, as it is a captured variable in a `Fn` closure
--> src/main.rs:24:54
|
24 | .flat_map(|_| (0..8).into_par_iter().map(|_| self.counter += 1));
| --- ^^^ ------------ mutable borrow occurs due to use of `self.counter` in closure
| | |
| in this closure cannot borrow as mutable
Some errors have detailed explanations: E0594, E0596.
For more information about an error, try `rustc --explain E0594`.
error: could not compile `playground` (bin "playground") due to 4 previous errorsDesired output
No suggestion to "change this to return `FnMut` instead of `Fn`" for `EvilCounter::count`
Explanation that the argument to `rayon::ParallelIterator::map` is a `Fn` closure (so assignment is impossible) for both E0594 cases
Rationale and extra context
Issue only occurs on EvilCounter where the error is within a nested closure.
Seems related to #119985 and #125325.
In the context where I noticed this, the function in question returned an unrelated Fn closure, making the message very confusing. Being relatively new to Rust, it took me a while to realise that the error was in fact due to the map function requiring Fn. Referring to this in the error message (for both cases) would be very helpful for new users.
Other cases
Rust Version
rustc 1.97.0-nightly (36ba2c771 2026-04-23)
binary: rustc
commit-hash: 36ba2c7712052d731a7082d0eba5ed3d9d56c133
commit-date: 2026-04-23
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.2
Anything else?
No response
Code
Current output
Desired output
Rationale and extra context
Issue only occurs on
EvilCounterwhere the error is within a nested closure.Seems related to #119985 and #125325.
In the context where I noticed this, the function in question returned an unrelated
Fnclosure, making the message very confusing. Being relatively new to Rust, it took me a while to realise that the error was in fact due to themapfunction requiringFn. Referring to this in the error message (for both cases) would be very helpful for new users.Other cases
Rust Version
Anything else?
No response