Repository: rust
Issue: Tracking Issue for map_try_insert
Labels: A-collections,T-libs-api,C-tracking-issue

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Feature gate: `#![feature(map_try_insert)]`

This is a tracking issue for `BTreeMap::try_insert` and `HashMap::try_insert`.

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Unlike `.insert()`, `.try_insert()` does not overwrite existing values, and has a meaningful error type with more context. See #82764

### Public API

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```rust
// alloc::collections::btree_map

impl<K: Ord, V> BTreeMap<K, V> {
    pub fn try_insert(&mut self, key: K, value: V) -> Result<&mut V, OccupiedError<'_, K, V>>;
}

pub struct OccupiedError<'a, K: 'a, V: 'a> {
    pub entry: OccupiedEntry<'a, K, V>,
    pub value: V,
}

impl<K: Debug + Ord, V: Debug> Debug for OccupiedError<'_, K, V>;
impl<'a, K: Debug + Ord, V: Debug> Display for OccupiedError<'a, K, V>;
impl<'a, K: Debug + Ord, V: Debug> Error for OccupiedError<'a, K, V>;

// std::collections::hash_map

impl<K: Eq + Hash, V, S: BuildHasher> HashMap<K, V, S> {
    pub fn try_insert(&mut self, key: K, value: V) -> Result<&mut V, OccupiedError<'_, K, V>>;
}

pub struct OccupiedError<'a, K: 'a, V: 'a> {
    pub entry: OccupiedEntry<'a, K, V>,
    pub value: V,
}

impl<K: Debug, V: Debug> Debug for OccupiedError<'_, K, V>;
impl<'a, K: Debug, V: Debug> fmt::Display for OccupiedError<'a, K, V>;
impl<'a, K: Debug, V: Debug> Error for OccupiedError<'a, K, V>;
```

### Steps / History

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- Issue: https://github.com/rust-lang/rfcs/issues/3092
- [x] Implementation: #82764
- [ ] Final commenting period (FCP)
- [ ] Stabilization PR

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### Unresolved Questions

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- Should `OccupiedError` implement Error (and Display)? https://github.com/rust-lang/rust/pull/82764#discussion_r587557677
- Should `OccupiedError` have public fields, or accessors? https://github.com/rust-lang/rust/pull/82764#discussion_r587555473
- `try_` might imply it returns an `Err` when allocating fails, which is not the case. Maybe rename it to `insert_new` or something?
- Put the `key: K` argument into the `OccupiedError`?