HashMap: panic in element destructor causes leaks of unrelated elements #132222
Description
When dropping a HashMap (or HashSet) and an element's destructor panics, then all elements that would be dropped after are leaked. This is inconsistent with other std collections (Vec, LinkedList, BTreeMap), where after panic in one destructor the remaining destructors are still called, potentially causing an abort if another one panics.
I tried this code: playground
use std::cell::Cell; use std::collections::{BTreeMap, HashMap, LinkedList}; use std::panic::{catch_unwind, AssertUnwindSafe}; struct Dropper<'a>(&'a Cell<u32>); impl Drop for Dropper<'_> { fn drop(&mut self) { let count = self.0.get(); self.0.set(count + 1); if count == 0 { panic!("oh no"); } } } fn main() { // Vec let count = Cell::new(0); catch_unwind(AssertUnwindSafe(|| { drop(vec![[Dropper(&count), Dropper(&count)]]); })) .unwrap_err(); println!("vec: {}", count.get()); // LinkedList let count = Cell::new(0); catch_unwind(AssertUnwindSafe(|| { drop(LinkedList::from([Dropper(&count), Dropper(&count)])); })) .unwrap_err(); println!("linked list: {}", count.get()); // BTreeMap let count = Cell::new(0); catch_unwind(AssertUnwindSafe(|| { drop(BTreeMap::from([(1, Dropper(&count)), (2, Dropper(&count))])); })) .unwrap_err(); println!("b-tree map: {}", count.get()); // HashMap let count = Cell::new(0); catch_unwind(AssertUnwindSafe(|| { drop(HashMap::from([(1, Dropper(&count)), (2, Dropper(&count))])); })) .unwrap_err(); println!("hash map: {}", count.get()); }I expected to see this happen: The drop behavior of Vec, LinkedList, BTreeMap and HashMap should be consistent.
Instead, this happened: HashMap drops only one element if the destructor unwinds, but the others drop both elements.
running Miri on the code shows a memory leak
error: memory leaked: alloc17016 (Rust heap, size: 76, align: 8), allocated here: --> /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/alloc.rs:15:15 | 15 | match alloc.allocate(layout) { | ^^^^^^^^^^^^^^^^^^^^^^ | = note: BACKTRACE: = note: inside `hashbrown::raw::alloc::inner::do_alloc::<std::alloc::Global>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/alloc.rs:15:15: 15:37 = note: inside `hashbrown::raw::RawTableInner::new_uninitialized::<std::alloc::Global>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:1534:38: 1534:61 = note: inside `hashbrown::raw::RawTableInner::fallible_with_capacity::<std::alloc::Global>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:1572:30: 1572:96 = note: inside `hashbrown::raw::RawTableInner::prepare_resize::<std::alloc::Global>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:2633:13: 2633:94 = note: inside `hashbrown::raw::RawTableInner::resize_inner::<std::alloc::Global>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:2829:29: 2829:86 = note: inside `hashbrown::raw::RawTableInner::reserve_rehash_inner::<std::alloc::Global>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:2719:13: 2725:14 = note: inside `hashbrown::raw::RawTable::<(i32, Dropper<'_>)>::reserve_rehash::<{closure@hashbrown::map::make_hasher<i32, Dropper<'_>, std::hash::RandomState>::{closure#0}}>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:1045:13: 1056:14 = note: inside `hashbrown::raw::RawTable::<(i32, Dropper<'_>)>::reserve::<{closure@hashbrown::map::make_hasher<i32, Dropper<'_>, std::hash::RandomState>::{closure#0}}>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/raw/mod.rs:993:20: 994:81 = note: inside `hashbrown::map::HashMap::<i32, Dropper<'_>, std::hash::RandomState>::reserve` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/map.rs:1102:9: 1103:77 = note: inside `<hashbrown::map::HashMap<i32, Dropper<'_>, std::hash::RandomState> as std::iter::Extend<(i32, Dropper<'_>)>>::extend::<[(i32, Dropper<'_>); 2]>` at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.0/src/map.rs:4489:9: 4489:30 = note: inside `<std::collections::HashMap<i32, Dropper<'_>> as std::iter::Extend<(i32, Dropper<'_>)>>::extend::<[(i32, Dropper<'_>); 2]>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:3185:9: 3185:31 = note: inside `<std::collections::HashMap<i32, Dropper<'_>> as std::iter::FromIterator<(i32, Dropper<'_>)>>::from_iter::<[(i32, Dropper<'_>); 2]>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:3170:9: 3170:25 = note: inside `<std::collections::HashMap<i32, Dropper<'_>> as std::convert::From<[(i32, Dropper<'_>); 2]>>::from` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:1405:9: 1405:29 note: inside closure --> src/main.rs:45:14 | 45 | drop(HashMap::from([(1, Dropper(&count)), (2, Dropper(&count))])); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ = note: inside `<{closure@src/main.rs:44:35: 44:37} as std::ops::FnOnce<()>>::call_once - shim` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5: 250:71 = note: inside `<std::panic::AssertUnwindSafe<{closure@src/main.rs:44:35: 44:37}> as std::ops::FnOnce<()>>::call_once` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panic/unwind_safe.rs:272:9: 272:19 = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<{closure@src/main.rs:44:35: 44:37}>, ()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:557:40: 557:43 = note: inside `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<{closure@src/main.rs:44:35: 44:37}>>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:520:19: 520:88 = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<{closure@src/main.rs:44:35: 44:37}>, ()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:358:14: 358:33 note: inside `main` --> src/main.rs:44:5 | 44 | / catch_unwind(AssertUnwindSafe(|| { 45 | | drop(HashMap::from([(1, Dropper(&count)), (2, Dropper(&count))])); 46 | | })) | |_______^ Meta
rustc --version --verbose:
rustc 1.84.0-nightly (c1db4dc24 2024-10-25) binary: rustc commit-hash: c1db4dc24267a707409c9bf2e67cf3c7323975c8 commit-date: 2024-10-25 host: x86_64-unknown-linux-gnu release: 1.84.0-nightly LLVM version: 19.1.1 @rustbot label T-libs A-collections A-destructors I-memleak