" change this to return FnMut instead of Fn" where the return type is a std::result::Result

[ambaxter]

I tried this code:
ambaxter/bbolt-rs@69eb00a#diff-4007187965c1fea4fd252a76f5f1007b2ea59f6d86e2f454edee547c5036be24R20

I expected to see this happen: Code compilation, or at least a sensible error.

Instead, this happened:

error[E0596]: cannot borrow `*f` as mutable, as it is a captured variable in a `Fn` closure
  --> src/util.rs:20:17
   |
14 | fn walk<F>(src: &Bolt, f: &mut F) -> crate::Result<()>
   |    ----                              ----------------- change this to return `FnMut` instead of `Fn`
...
18 |   src.view(|tx| {
   |            ---- in this closure
19 |     let tx_root = Breadcrumb::Root(b"tx".as_slice());
20 |     tx.for_each(|name, b| walk_bucket(&b, &tx_root, name, None, b.sequence(), f))
   |                 ^^^^^^^^^ cannot borrow as mutable                            - mutable borrow occurs due to use of `*f` in closure

For more information about this error, try `rustc --explain E0596`.

I tried to make a reproducer using the following code that expresses the same type and fn signatures on Rust Playground, but I'm unable to trigger it.

#![allow(dead_code)]
#![allow(unused_variables)]

use std::marker::PhantomData;

type Result<T, E = String> = std::result::Result<T, E>;

struct Bolt {}

trait BoltApi {
    fn view<'tx, F: FnMut(TxRef<'tx>) -> crate::Result<()>>(&'tx self, f: F) -> crate::Result<()>;
}

impl BoltApi for Bolt {
    fn view<'tx, F: FnMut(TxRef<'tx>) -> crate::Result<()>>(
        &'tx self,
        mut f: F,
    ) -> crate::Result<()> {
        Ok(())
    }
}

struct BucketImpl<'tx> {
    _tx: PhantomData<&'tx u64>,
}

trait BucketApi<'tx> {
    fn for_each<F: FnMut(&'tx [u8], Option<&'tx [u8]>) -> crate::Result<()>>(
        &self,
        f: F,
    ) -> crate::Result<()>;
}

impl<'tx> BucketApi<'tx> for BucketImpl<'tx> {
    fn for_each<F: FnMut(&'tx [u8], Option<&'tx [u8]>) -> crate::Result<()>>(
        &self,
        f: F,
    ) -> crate::Result<()> {
        Ok(())
    }
}

struct TxRef<'tx> {
    _tx: PhantomData<&'tx u64>,
}

trait TxApi<'tx> {
    fn for_each<F: FnMut(&[u8], BucketImpl<'tx>) -> crate::Result<()>>(
        &self,
        f: F,
    ) -> crate::Result<()>;
}

impl<'tx> TxApi<'tx> for TxRef<'tx> {
    fn for_each<F: FnMut(&[u8], BucketImpl<'tx>) -> crate::Result<()>>(
        &self,
        f: F,
    ) -> crate::Result<()>
where {
        Ok(())
    }
}

struct Tx {}

fn walk<F: FnMut(&[u8], Option<&[u8]>, u64) -> crate::Result<()>>(
    src: &Bolt,
    f: &mut F,
) -> crate::Result<()> {
    src.view(|tx| tx.for_each(|name, b| tx.for_each(|k, v| walk_bucket(&b, k, None, 0, f))))
}

fn walk_bucket<F: FnMut(&[u8], Option<&[u8]>, u64) -> crate::Result<()>>(
    b: &BucketImpl,
    key: &[u8],
    value: Option<&[u8]>,
    seq: u64,
    f: &mut F,
) -> crate::Result<()> {
    f(key, value, seq)?;
    b.for_each(|k, v| walk_bucket(b, k, v, 0, f))
}

fn main() {}

Meta

rustc --version --verbose:

% rustc --version --verbose
rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-apple-darwin
release: 1.78.0
LLVM version: 18.1.2

tested on Nightly as well.

Backtrace

 % RUST_BACKTRACE=1 cargo build
   Compiling bbolt-rs v1.3.9 (/Users/abaxter/RustroverProjects/bucket_test)
warning: unused import: `crate::common::cell::RefCell`
 --> src/common/pool.rs:1:5
  |
1 | use crate::common::cell::RefCell;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused import: `std::rc::Rc`
 --> src/common/pool.rs:6:5
  |
6 | use std::rc::Rc;
  |     ^^^^^^^^^^^

warning: unused import: `size::Size`
 --> src/util.rs:3:5
  |
3 | use size::Size;
  |     ^^^^^^^^^^

warning: unused import: `DerefMut`
  --> src/bucket.rs:24:34
   |
24 | use std::ops::{AddAssign, Deref, DerefMut};
   |                                  ^^^^^^^^

warning: unused import: `std::io::Write`
  --> src/tx.rs:28:5
   |
28 | use std::io::Write;
   |     ^^^^^^^^^^^^^^

warning: unused variable: `size`
 --> src/util.rs:9:11
  |
9 |   let mut size = 0u64;
  |           ^^^^ help: if this is intentional, prefix it with an underscore: `_size`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `dst_tx`
  --> src/util.rs:10:11
   |
10 |   let mut dst_tx = dst.begin_rw_tx()?;
   |           ^^^^^^ help: if this is intentional, prefix it with an underscore: `_dst_tx`

warning: unused variable: `max_size`
 --> src/util.rs:7:44
  |
7 | pub fn compact(src: &Bolt, dst: &mut Bolt, max_size: u64) -> crate::Result<()> {
  |                                            ^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_max_size`

warning: variable does not need to be mutable
 --> src/util.rs:9:7
  |
9 |   let mut size = 0u64;
  |       ----^^^^
  |       |
  |       help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

warning: variable does not need to be mutable
  --> src/util.rs:10:7
   |
10 |   let mut dst_tx = dst.begin_rw_tx()?;
   |       ----^^^^^^
   |       |
   |       help: remove this `mut`

error[E0596]: cannot borrow `*f` as mutable, as it is a captured variable in a `Fn` closure
  --> src/util.rs:20:17
   |
14 | fn walk<F>(src: &Bolt, f: &mut F) -> crate::Result<()>
   |    ----                              ----------------- change this to return `FnMut` instead of `Fn`
...
18 |   src.view(|tx| {
   |            ---- in this closure
19 |     let tx_root = Breadcrumb::Root(b"tx".as_slice());
20 |     tx.for_each(|name, b| walk_bucket(&b, &tx_root, name, None, b.sequence(), f))
   |                 ^^^^^^^^^ cannot borrow as mutable                            - mutable borrow occurs due to use of `*f` in closure

For more information about this error, try `rustc --explain E0596`.
warning: `bbolt-rs` (lib) generated 10 warnings
error: could not compile `bbolt-rs` (lib) due to 1 previous error; 10 warnings emitted

[pacak]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=35e078a9eb6f26e1e7ea82875b355e2d - and even smaller example

[ambaxter]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=ef5f474042707ec6372bbad0595b074c - a working reproduction with no external dependencies (updated to make smaller)

Thank you!!

[gurry]

@rustbot claim