clippy::await_holding_refcell_ref: Checks for calls to await while holding a RefCell, Ref, or RefMut

async fn foo(x: &RefCell<u32>) {
  let mut y = x.borrow_mut();
  *y += 1;
  baz().await;
}

async fn bar(x: &RefCell<u32>) {
  let mut y = x.borrow_mut();
  *y += 1;
  drop(y); // explicit drop
  baz().await;
}

async fn foo(x: &RefCell<u32>) {
  {
     let mut y = x.borrow_mut();
     *y += 1;
  }
  baz().await;
}

async fn bar(x: &RefCell<u32>) {
  {
    let mut y = x.borrow_mut();
    *y += 1;
  } // y dropped here at end of scope
  baz().await;
}

#[allow(clippy::await_holding_refcell_ref)]
fn my_function() { /* ... */ }

[lints.clippy]
await_holding_refcell_ref = "allow"

cargo clippy -- -A clippy::await-holding-refcell-ref

Clippy's own documentation

What it does

Checks for calls to await while holding a RefCell, Ref, or RefMut.

Why is this bad?

RefCell refs only check for exclusive mutable access at runtime. Holding a RefCell ref across an await suspension point risks panics from a mutable ref shared while other refs are outstanding.

Known problems

Will report false positive for explicitly dropped refs (#6353). A workaround for this is to wrap the .borrow[_mut]() call in a block instead of explicitly dropping the ref.

Example

async fn foo(x: &RefCell<u32>) {
  let mut y = x.borrow_mut();
  *y += 1;
  baz().await;
}

async fn bar(x: &RefCell<u32>) {
  let mut y = x.borrow_mut();
  *y += 1;
  drop(y); // explicit drop
  baz().await;
}

Use instead:

async fn foo(x: &RefCell<u32>) {
  {
     let mut y = x.borrow_mut();
     *y += 1;
  }
  baz().await;
}

async fn bar(x: &RefCell<u32>) {
  {
    let mut y = x.borrow_mut();
    *y += 1;
  } // y dropped here at end of scope
  baz().await;
}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

Related lints

• arc_with_non_send_syncarc with non send sync
• await_holding_invalid_typeAllows users to configure types which should not be held across await suspension points
• await_holding_lockChecks for calls to await while holding a non-async-aware MutexGuard
• blanket_clippy_restriction_lints`clippy::restriction` is not meant to be enabled as a group
• block_scrutineeWarns when a match, if let, or while let scrutinee is wrapped in a block. This lint only triggers on the 2021 edition and older
• by_ref_peekable_peekcalling `.by_ref().peekable().peek()` will advance the underlying iterator and consume its first output

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker