Merge #503

503: Un-breaking-change Uuid::new_v4 r=KodrAus a=CAD97

<!--
    If this PR is a breaking change, ensure that you are opening it against 
    the `breaking` branch.  If the pull request is incomplete, prepend the Title with WIP: 
-->

**I'm submitting a(n)** refactor

# Description

#447 changed from using `rand::thread_rng` to using `getrandom` in `Uuid::new_v4`. This also changed the return type from `Uuid` to `Result<Uuid, getrandom::Error>`. This PR reverts the signature to the simpler `new_v4() -> Uuid`.

# Motivation

This signature is much simpler to use, and avoids a breaking change. `getrandom` is _highly_ unlikely to fail, and previously we used `thread_rng` here, which also panics if it fails to initialize from the OS entropy source. If `getrandom` fails, it is highly unlikely that any program creating v4 UUIDs has any reasonable recovery other than to abort, as the system is in a broken state.

If users absolutely need to recover in this situation, they can call `getrandom` first themselves to make sure that their system is working, or generate the bytes themselves and create the UUID from those bytes.

Additionally, actually wrapping `getrandom::Error` in `uuid::Error` comes with its own fun set of problems, described in #502.

# Tests

N/A

# Related Issue(s)

#475: this undoes the breaking change to `Uuid::new_v4`, thus making the requested publish a patch update

Closes #502: alternate approach to the same TODO

Co-authored-by: CAD97 <cad97@cad97.com>

src/v4.rs

@@ -4,10 +4,10 @@ impl Uuid {
     /// Creates a random UUID.
     ///
     /// This uses the [`getrandom`] crate to utilise the operating system's RNG
-    /// as the source of random numbers. If you'd like to use a custom generator,
-    /// don't use this method: generate random bytes using your custom generator
-    /// and pass them to the [`uuid::Builder::from_bytes`][from_bytes] function
-    /// instead.
+    /// as the source of random numbers. If you'd like to use a custom
+    /// generator, don't use this method: generate random bytes using your
+    /// custom generator and pass them to the
+    /// [`uuid::Builder::from_bytes`][from_bytes] function instead.
     ///
     /// Note that usage of this method requires the `v4` feature of this crate
     /// to be enabled.
@@ -24,16 +24,17 @@ impl Uuid {
     ///
     /// [`getrandom`]: https://crates.io/crates/getrandom
     /// [from_bytes]: struct.Builder.html#method.from_bytes
-    // TODO: change signature to support uuid's Error.
-    pub fn new_v4() -> Result<Uuid, getrandom::Error> {
+    pub fn new_v4() -> Uuid {
         let mut bytes = [0u8; 16];
-        getrandom::getrandom(&mut bytes)?;
+        getrandom::getrandom(&mut bytes).unwrap_or_else(|err| {
+            // NB: getrandom::Error has no source; this is adequate display
+            panic!("could not retreive random bytes for uuid: {}", err)
+        });
 
-        let uuid = crate::Builder::from_bytes(bytes)
+        crate::Builder::from_bytes(bytes)
             .set_variant(Variant::RFC4122)
             .set_version(Version::Random)
-            .build();
-        Ok(uuid)
+            .build()
     }
 }
 
@@ -43,15 +44,15 @@ mod tests {
 
     #[test]
     fn test_new() {
-        let uuid = Uuid::new_v4().unwrap();
+        let uuid = Uuid::new_v4();
 
         assert_eq!(uuid.get_version(), Some(Version::Random));
         assert_eq!(uuid.get_variant(), Some(Variant::RFC4122));
     }
 
     #[test]
     fn test_get_version() {
-        let uuid = Uuid::new_v4().unwrap();
+        let uuid = Uuid::new_v4();
 
         assert_eq!(uuid.get_version(), Some(Version::Random));
         assert_eq!(uuid.get_version_num(), 4)