bevy_platform

Cross-platform shims. Wraps std-only types so other Bevy crates compile under no_std, including the Wasm-without-atomics target.

Purpose

Bevy supports no_std and Wasm. Several types are usually provided by std — Instant, HashMap, Mutex, Arc, atomics — and have to be replaced with crate-level alternatives on these platforms. bevy_platform is the place that picks the right alternative based on cargo features.

Directory layout

crates/bevy_platform/src/
├── lib.rs              # Re-exports
├── time.rs             # Instant, Duration shims (uses web-time on wasm)
├── collections/        # HashMap, HashSet, BTreeMap, BTreeSet aliases
├── hash.rs             # PassHasher, FixedHasher (deterministic hashers)
├── sync/               # Mutex, RwLock, Arc shims
├── thread.rs           # spawn / thread_local on no_std
├── cell/               # SyncCell, SyncUnsafeCell
├── prelude.rs
└── …

Key abstractions

Type	File	Description
Instant	crates/bevy_platform/src/time.rs	Monotonic timestamp; web-time on wasm, std::time::Instant on native.
HashMap, HashSet	crates/bevy_platform/src/collections/	hashbrown::HashMap with a fixed hasher.
Arc, Mutex, RwLock	crates/bevy_platform/src/sync/	Re-exports of std::sync or portable-atomic-backed analogues.
FixedHasher, PassHasher	crates/bevy_platform/src/hash.rs	Hashers without DoS protection (faster, deterministic).

Why deterministic hashers

Bevy uses FixedHasher for ECS internal maps because the standard RandomState introduces frame-to-frame randomness that defeats deterministic replays. Game state hashes — used by netcode and rollback — need to be stable across runs and platforms.

Integration points

• Depends on: hashbrown, web-time (wasm), portable-atomic (no_std).
• Depended on by: Almost every Bevy crate. bevy_ecs, bevy_app, bevy_reflect, bevy_render, etc. import Instant, HashMap, and friends from here.

Entry points for modification

• New platform target: identify which std:: types break and add a shim here.
• Hasher tweak: hash.rs. The FixedHasher is the workspace default; changing it has performance and determinism implications.
• Sync primitive bug: sync/. The portable-atomic integration handles wasm-without-atomics.

See also

• bevy_utils — sister crate of higher-level helpers.