Support counters in v7 UUIDs

Cargo.toml

@@ -41,6 +41,9 @@ features = ["serde", "arbitrary", "slog", "borsh", "v1", "v3", "v4", "v5", "v6",
 [package.metadata.playground]
 features = ["serde", "v1", "v3", "v4", "v5", "v6", "v7", "v8"]
 
+[lints.rust]
+unexpected_cfgs = { level = "allow", check-cfg = ['cfg(uuid_unstable)'] }
+
 [badges.is-it-maintained-issue-resolution]
 repository = "uuid-rs/uuid"
 

src/builder.rs

@@ -609,9 +609,11 @@ impl Builder {
         ))
     }
 
-    /// Creates a `Builder` for a version 7 UUID using the supplied Unix timestamp and random bytes.
+    /// Creates a `Builder` for a version 7 UUID using the supplied Unix timestamp and counter bytes.
     ///
-    /// This method assumes the bytes are already sufficiently random.
+    /// This method will set the variant field within the counter bytes without attempting to shift
+    /// the data around it. Callers using the counter as a monotonic value should be careful not to
+    /// store significant data in the 2 least significant bits of the 3rd byte.
     ///
     /// # Examples
     ///
@@ -636,10 +638,10 @@ impl Builder {
     /// # Ok(())
     /// # }
     /// ```
-    pub const fn from_unix_timestamp_millis(millis: u64, random_bytes: &[u8; 10]) -> Self {
+    pub const fn from_unix_timestamp_millis(millis: u64, counter_random_bytes: &[u8; 10]) -> Self {
         Builder(timestamp::encode_unix_timestamp_millis(
             millis,
-            random_bytes,
+            counter_random_bytes,
         ))
     }
 

src/lib.rs

@@ -237,6 +237,9 @@ pub use timestamp::{context::NoContext, ClockSequence, Timestamp};
 #[cfg(any(feature = "v1", feature = "v6"))]
 pub use timestamp::context::Context;
 
+#[cfg(feature = "v7")]
+pub use timestamp::context::ContextV7;
+
 #[cfg(feature = "v1")]
 #[doc(hidden)]
 // Soft-deprecated (Rust doesn't support deprecating re-exports)
@@ -901,12 +904,7 @@ impl Uuid {
                 let seconds = millis / 1000;
                 let nanos = ((millis % 1000) * 1_000_000) as u32;
 
-                Some(Timestamp {
-                    seconds,
-                    nanos,
-                    #[cfg(any(feature = "v1", feature = "v6"))]
-                    counter: 0,
-                })
+                Some(Timestamp::from_unix_time(seconds, nanos, 0))
             }
             _ => None,
         }

src/rng.rs

@@ -1,5 +1,5 @@
 #[cfg(any(feature = "v4", feature = "v7"))]
-pub(crate) fn bytes() -> [u8; 16] {
+pub(crate) fn u128() -> u128 {
     #[cfg(not(feature = "fast-rng"))]
     {
         let mut bytes = [0u8; 16];
@@ -9,7 +9,7 @@ pub(crate) fn bytes() -> [u8; 16] {
             panic!("could not retrieve random bytes for uuid: {}", err)
         });
 
-        bytes
+        u128::from_ne_bytes(bytes)
     }
 
     #[cfg(feature = "fast-rng")]
@@ -29,7 +29,27 @@ pub(crate) fn u16() -> u16 {
             panic!("could not retrieve random bytes for uuid: {}", err)
         });
 
-        ((bytes[0] as u16) << 8) | (bytes[1] as u16)
+        u16::from_ne_bytes(bytes)
+    }
+
+    #[cfg(feature = "fast-rng")]
+    {
+        rand::random()
+    }
+}
+
+#[cfg(feature = "v7")]
+pub(crate) fn u64() -> u64 {
+    #[cfg(not(feature = "fast-rng"))]
+    {
+        let mut bytes = [0u8; 8];
+
+        getrandom::getrandom(&mut bytes).unwrap_or_else(|err| {
+            // NB: getrandom::Error has no source; this is adequate display
+            panic!("could not retrieve random bytes for uuid: {}", err)
+        });
+
+        u64::from_ne_bytes(bytes)
     }
 
     #[cfg(feature = "fast-rng")]