Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::significant_drop_in_scrutinee: Checks for temporaries returned from function calls in a match scrutinee that have the clippy::has_significant_drop attribute

What it meansChecks for temporaries returned from function calls in a match scrutinee that have the clippy::has_significant_drop attribute. It is in the nursery group and off by default.

Does it matter?

The clippy::has_significant_drop attribute can be added to types whose Drop impls have an important side-effect, such as unlocking a mutex, making it important for users to be able to accurately understand their lifetimes. When a temporary is returned in a function call in a match scrutinee, its lifetime lasts until the end of the match block, which may be surprising. For Mutexes this can lead to a deadlock. This happens when the match scrutinee uses a function call that returns a MutexGuard and then tries to lock again in one of the match arms. In that case the MutexGuard in the scrutinee will not be dropped until the end of the match block and thus will not unlock.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let mutex = Mutex::new(State {});

match mutex.lock().unwrap().foo() {
    true => {
        mutex.lock().unwrap().bar(); // Deadlock!
    }
    false => {}
};

println!("All done!");

Write this instead

let mutex = Mutex::new(State {});

let is_foo = mutex.lock().unwrap().foo();
match is_foo {
    true => {
        mutex.lock().unwrap().bar();
    }
    false => {}
};

println!("All done!");

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
significant_drop_in_scrutinee = "warn"

In code

#![warn(clippy::significant_drop_in_scrutinee)]

One run

cargo clippy -- -W clippy::significant-drop-in-scrutinee

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.60.0added in Rust

Clippy's own documentation

What it does

Checks for temporaries returned from function calls in a match scrutinee that have the clippy::has_significant_drop attribute.

Why is this bad?

The clippy::has_significant_drop attribute can be added to types whose Drop impls have an important side-effect, such as unlocking a mutex, making it important for users to be able to accurately understand their lifetimes. When a temporary is returned in a function call in a match scrutinee, its lifetime lasts until the end of the match block, which may be surprising.

For Mutexes this can lead to a deadlock. This happens when the match scrutinee uses a function call that returns a MutexGuard and then tries to lock again in one of the match arms. In that case the MutexGuard in the scrutinee will not be dropped until the end of the match block and thus will not unlock.

Example

let mutex = Mutex::new(State {});

match mutex.lock().unwrap().foo() {
    true => {
        mutex.lock().unwrap().bar(); // Deadlock!
    }
    false => {}
};

println!("All done!");

Use instead:

let mutex = Mutex::new(State {});

let is_foo = mutex.lock().unwrap().foo();
match is_foo {
    true => {
        mutex.lock().unwrap().bar();
    }
    false => {}
};

println!("All done!");

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

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