Optimize hashing by implementing is_hash_trivial

[ironcev]

Description

This PR extends the Hash trait with the is_hash_trivial() -> bool associated function. This function is supposed to return true if the hash byte representation of the type is the same as its runtime memory representation.

By using is_hash_trivial, std::hash::sha256 and keccak256 module functions can skip expensive Bytes creation in the Hasher and directly hash the value's memory content.

The performance gains on o2 benchmarks are significant. In average, 25.47% improvements on order-book and 7.93% improvements on trade-account. Max matches benchmark improved for 37.62%. (Details are given in the comment below.)

This optimization is similar to what is_encode/decode_trivial() associated functions are providing for ABI encoding/decoding.

Similar to ABI encoding/decoding, implementation of hashing for a type is in full control of the Sway package that owns the type. In practice, and in std in particular, non-dynamic types are in general trivially hashable if their packed memory representation is the same as their runtime memory representation.

Because of this, when implementing Hash for tuples, we use the __runtime_mem_id and the __encoding_mem_id intrinsics. While using the __encoding_mem_id might look like coupling encoding and hashing, it is not. What we are actually using is the notion of the "packed memory representation" that the __encoding_mem_id provides. Unfortunately, the intrinsic that provides this information was originally created for ABI encoding and not for a more generic usage, e.g., __packed__mem_id.

To make the distinction clear and remove what looks like coupling of unrelated concepts, we will provide a follow-up PR that introduces intrinsics for use case based memory layouts:

• __mem_repr_id_runtime
• __mem_repr_id_encoding
• __mem_repr_id_hashing

Currently, all enums in the std hash their descriminators as u8. This makes them all being non-trivially hashable. For many of them, hashing the descriminator as u64 would make them trivially hashable. This is however a breaking change that potentially affects the hashed data we already have deployed, e.g., StorageMap keys.

In the future, we can provide this breaking change behind a feature flag. Enums that can benefit from being trivially hashed are marked in code with TODO: (HASH-TRIVIAL-ENUMS).

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[cursor]

PR Summary

High Risk
Breaking Hash API requires all manual implementations to add is_hash_trivial, and a wrong true silently changes sha256/keccak256 output—risky for StorageMap keys and other deployed hash-dependent state.

Overview
Adds Hash::is_hash_trivial() as a required associated function on std::hash::Hash, plus is_hash_trivial<T>(), so types can declare when their in-memory layout matches the bytes written by hash. sha256 and keccak256 use a compile-time fast path for trivial types (direct s256/k256 over __addr_of(val)), skipping Hasher buffer work for lower gas.

sway-lib-std implements the hook across primitives, tuples (via __runtime_mem_id / __encoding_mem_id), collections, and blockchain/crypto types; several types hash whole structs as raw slices (CallParams, U128). Std enums still hash tags as u8, so most remain non-trivial (HASH-TRIVIAL-ENUMS TODOs). The hashing example switches Location to u64 discriminators and documents trivial vs non-trivial custom Hash impls in a new Sway book page.

Tests expand hash coverage (tuple arity, trivial classification, composed hashing) and every custom Hash in the repo gains is_hash_trivial (typically false where unsure).

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[ironcev]

👍

[ironcev]

Performance improvements

o2 order-book benchmarks

	Improvements	Regressions
Count	35	—
Average	25.47%	—
Median	24.53%	—
Max	45.46%	—
Min	12.58%	—

Max matches benchmark improved for 37.62%.

o2 trade-account benchmarks

	Improvements	Regressions
Count	21	—
Average	7.93%	—
Median	8.29%	—
Max	11.53%	—
Min	4.43%	—