bevy_utils: Export generate_composite_uuid utility function (#10496)

# Objective

The `generate_composite_uuid` utility function hidden in
`bevy_reflect::__macro_exports` could be generally useful to users.

For example, I previously relied on `Hash` to generate a `u64` to create
a deterministic `HandleId`. In v0.12, `HandleId` has been replaced by
`AssetId` which now requires a `Uuid`, which I could generate with this
function.

## Solution

Relocate `generate_composite_uuid` from `bevy_reflect::__macro_exports`
to `bevy_utils::uuid`.

It is still re-exported under `bevy_reflect::__macro_exports` so there
should not be any breaking changes (although, users should generally not
rely on pseudo-private/hidden modules like `__macro_exports`).

I chose to keep it in `bevy_reflect::__macro_exports` so as to not
clutter up our public API and to reduce the number of changes in this
PR. We could have also marked the export as `#[doc(hidden)]`, but
personally I like that we have a dedicated module for this (makes it
clear what is public and what isn't when just looking at the macro
code).

---

## Changelog

- Moved `generate_composite_uuid` to `bevy_utils::uuid` and made it
public
  - Note: it was technically already public, just hidden

crates/bevy_reflect/src/lib.rs

@@ -542,32 +542,7 @@ extern crate alloc;
 
 #[doc(hidden)]
 pub mod __macro_exports {
-    use crate::Uuid;
-
-    /// Generates a new UUID from the given UUIDs `a` and `b`,
-    /// where the bytes are generated by a bitwise `a ^ b.rotate_right(1)`.
-    /// The generated UUID will be a `UUIDv4` (meaning that the bytes should be random, not e.g. derived from the system time).
-    #[allow(clippy::unusual_byte_groupings)] // unusual byte grouping is meant to signal the relevant bits
-    pub const fn generate_composite_uuid(a: Uuid, b: Uuid) -> Uuid {
-        let mut new = [0; 16];
-        let mut i = 0;
-        while i < new.len() {
-            // rotating ensures different uuids for A<B<C>> and B<A<C>> because: A ^ (B ^ C) = B ^ (A ^ C)
-            // notice that you have to rotate the second parameter: A.rr ^ (B.rr ^ C) = B.rr ^ (A.rr ^ C)
-            // Solution: A ^ (B ^ C.rr).rr != B ^ (A ^ C.rr).rr
-            new[i] = a.as_bytes()[i] ^ b.as_bytes()[i].rotate_right(1);
-
-            i += 1;
-        }
-
-        // Version: the most significant 4 bits in the 6th byte: 11110000
-        new[6] = new[6] & 0b0000_1111 | 0b0100_0000; // set version to v4
-
-        // Variant: the most significant 3 bits in the 8th byte: 11100000
-        new[8] = new[8] & 0b000_11111 | 0b100_00000; // set variant to rfc4122
-
-        Uuid::from_bytes(new)
-    }
+    pub use bevy_utils::uuid::generate_composite_uuid;
 }
 
 #[cfg(test)]

crates/bevy_reflect/src/type_uuid.rs

@@ -1,5 +1,5 @@
 pub use bevy_reflect_derive::TypeUuid;
-pub use bevy_utils::Uuid;
+pub use bevy_utils::uuid::Uuid;
 
 /// A trait for types with a statically associated UUID.
 pub trait TypeUuid {

crates/bevy_utils/src/lib.rs

@@ -17,11 +17,14 @@ pub use short_names::get_short_name;
 pub mod synccell;
 pub mod syncunsafecell;
 
+pub mod uuid;
+
 mod cow_arc;
 mod default;
 mod float_ord;
 pub mod intern;
 
+pub use crate::uuid::Uuid;
 pub use ahash::{AHasher, RandomState};
 pub use bevy_utils_proc_macros::*;
 pub use cow_arc::*;
@@ -32,7 +35,6 @@ pub use instant::{Duration, Instant};
 pub use petgraph;
 pub use thiserror;
 pub use tracing;
-pub use uuid::Uuid;
 
 #[allow(missing_docs)]
 pub mod nonmax {

crates/bevy_utils/src/uuid.rs

@@ -0,0 +1,35 @@
+//! UUID utilities.
+//!
+//! This module re-exports the [`Uuid`] type from the [`uuid`] crate,
+//! and provides additional utility functions.
+//!
+//! [`uuid`]: ::uuid
+
+pub use ::uuid::Uuid;
+
+/// Generates a new UUID from the given UUIDs `a` and `b`,
+/// where the bytes are generated by a bitwise `a ^ b.rotate_right(1)`.
+///
+/// The generated UUID will be a `UUIDv4`
+/// (meaning that the bytes should be random, not e.g. derived from the system time).
+#[allow(clippy::unusual_byte_groupings)] // unusual byte grouping is meant to signal the relevant bits
+pub const fn generate_composite_uuid(a: Uuid, b: Uuid) -> Uuid {
+    let mut new = [0; 16];
+    let mut i = 0;
+    while i < new.len() {
+        // rotating ensures different uuids for A<B<C>> and B<A<C>> because: A ^ (B ^ C) = B ^ (A ^ C)
+        // notice that you have to rotate the second parameter: A.rr ^ (B.rr ^ C) = B.rr ^ (A.rr ^ C)
+        // Solution: A ^ (B ^ C.rr).rr != B ^ (A ^ C.rr).rr
+        new[i] = a.as_bytes()[i] ^ b.as_bytes()[i].rotate_right(1);
+
+        i += 1;
+    }
+
+    // Version: the most significant 4 bits in the 6th byte: 11110000
+    new[6] = new[6] & 0b0000_1111 | 0b0100_0000; // set version to v4
+
+    // Variant: the most significant 3 bits in the 8th byte: 11100000
+    new[8] = new[8] & 0b000_11111 | 0b100_00000; // set variant to rfc4122
+
+    Uuid::from_bytes(new)
+}