For some keys, it is too expensive to hash them on every lookup. Historically in Bevy, we have regrettably done the "wrong" thing in these cases (pre-computing hashes, then re-hashing them) because Rust's built in hashed collections don't give us the tools we need to do otherwise. Doing this is "wrong" because two different values can result in the same hash. Hashed collections generally get around this by falling back to equality checks on hash collisions. You can't do that if the key _is_ the hash. Additionally, re-hashing a hash increase the odds of collision!
#3959 needs pre-hashing to be viable, so I decided to finally properly solve the problem. The solution involves two different changes:
1. A new generalized "pre-hashing" solution in bevy_utils: `Hashed<T>` types, which store a value alongside a pre-computed hash. And `PreHashMap<K, V>` (which uses `Hashed<T>` internally) . `PreHashMap` is just an alias for a normal HashMap that uses `Hashed<T>` as the key and a new `PassHash` implementation as the Hasher.
2. Replacing the `std::collections` re-exports in `bevy_utils` with equivalent `hashbrown` impls. Avoiding re-hashes requires the `raw_entry_mut` api, which isn't stabilized yet (and may never be ... `entry_ref` has favor now, but also isn't available yet). If std's HashMap ever provides the tools we need, we can move back to that. The latest version of `hashbrown` adds support for the `entity_ref` api, so we can move to that in preparation for an std migration, if thats the direction they seem to be going in. Note that adding hashbrown doesn't increase our dependency count because it was already in our tree.
In addition to providing these core tools, I also ported the "table identity hashing" in `bevy_ecs` to `raw_entry_mut`, which was a particularly egregious case.
The biggest outstanding case is `AssetPathId`, which stores a pre-hash. We need AssetPathId to be cheaply clone-able (and ideally Copy), but `Hashed<AssetPath>` requires ownership of the AssetPath, which makes cloning ids way more expensive. We could consider doing `Hashed<Arc<AssetPath>>`, but cloning an arc is still a non-trivial expensive that needs to be considered. I would like to handle this in a separate PR. And given that we will be re-evaluating the Bevy Assets implementation in the very near future, I'd prefer to hold off until after that conversation is concluded.
#3457 adds the `doc_markdown` clippy lint, which checks doc comments to make sure code identifiers are escaped with backticks. This causes a lot of lint errors, so this is one of a number of PR's that will fix those lint errors one crate at a time.
This PR fixes lints in the `bevy_utils` crate.
# Objective
Fixes#2823.
## Solution
This PR adds notes to the `HashMap` and `HashSet` docs explaining why `HashMap::new()` (resp. `HashSet::new()`) is not available, and guiding the user toward using the `Default` implementation for an empty collection.
This matches `ahash::RandomState`, which provides both `Debug` and `Clone`.
Notably, implementing `Clone` allows the `StableHashMap`/`Set` to also implement `Clone`.
# Objective
- Allow `bevy_utils::StableHashMap` to be cloned.
## Solution
- Derive `Clone` for `bevy_utils::FixedState`.
- Also derive `Debug`, since we're touching it anyway, and this aligns `FixedState` with `ahash::RandomState`.
This is a rather simple but wide change, and it involves adding a new `bevy_app_macros` crate. Let me know if there is a better way to do any of this!
---
# Objective
- Allow adding and accessing sub-apps by using a label instead of an index
## Solution
- Migrate the bevy label implementation and derive code to the `bevy_utils` and `bevy_macro_utils` crates and then add a new `SubAppLabel` trait to the `bevy_app` crate that is used when adding or getting a sub-app from an app.
# Objective
Re-introduce `AHashExt` and respective `with_capacity()` implementations to give a more ergonomic way to set a `HashMap` / `HashSet` capacity.
As a note, this has also been discussed and agreed on issue #2115, which this PR addresses (leaving `new()` out of the `AHashExt` trait).
Fixes#2115.
## Solution
PR #1235 had removed the `AHashExt` trait and respective `with_capacity()`s implementations, leaving only the less ergonomic `HashMap::with_capacity_and_hasher(size, Default::default())` option available.
This re-introduces `AHashExt` and respective `with_capacity()` implementations to give a more ergonomic way to set a `HashMap` / `HashSet` capacity.
There are cases where we want an enum variant name. Right now the only way to do that with rust's std is to derive Debug, but this will also print out the variant's fields. This creates the unfortunate situation where we need to manually write out each variant's string name (ex: in #1963), which is both boilerplate-ey and error-prone. Crates such as `strum` exist for this reason, but it includes a lot of code and complexity that we don't need.
This adds a dead-simple `EnumVariantMeta` derive that exposes `enum_variant_index` and `enum_variant_name` functions. This allows us to make cases like #1963 much cleaner (see the second commit). We might also be able to reuse this logic for `bevy_reflect` enum derives.
* Remove AHashExt
There is little benefit of Hash*::new() over Hash*::default(), but it
does require more code that needs to be duplicated for every Hash* in
bevy_utils. It may also slightly increase compile times.
* Add StableHash* to bevy_utils
* Use StableHashMap instead of HashMap + BTreeSet for diagnostics
This is a significant reduction in the release mode compile times of
bevy_diagnostics
```
Benchmark #1: touch crates/bevy_diagnostic/src/lib.rs && cargo build --release -p bevy_diagnostic -j1
Time (mean ± σ): 3.645 s ± 0.009 s [User: 3.551 s, System: 0.094 s]
Range (min … max): 3.632 s … 3.658 s 20 runs
```
```
Benchmark #1: touch crates/bevy_diagnostic/src/lib.rs && cargo build --release -p bevy_diagnostic -j1
Time (mean ± σ): 2.938 s ± 0.012 s [User: 2.850 s, System: 0.090 s]
Range (min … max): 2.919 s … 2.969 s 20 runs
```
