rand_core: implement reborrow for UnwrapMut

Author: baloo

rand_core/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 ### API changes
 - Relax `Sized` bound on impls of `TryRngCore`, `TryCryptoRng` and `UnwrapMut` (#1593)
+- Add `UnwrapMut::re` to reborrow the inner rng with a tighter lifetime (#1595)
 
 ### Other
 - Fixup clippy warnings (#1601)

rand_core/src/lib.rs

@@ -321,6 +321,21 @@ impl<R: TryCryptoRng> CryptoRng for UnwrapErr<R> {}
 #[derive(Debug, Eq, PartialEq, Hash)]
 pub struct UnwrapMut<'r, R: TryRngCore + ?Sized>(pub &'r mut R);
 
+impl<'r, R: TryRngCore + ?Sized> UnwrapMut<'r, R> {
+    /// Reborrow with a new lifetime
+    ///
+    /// Rust allows references like `&T` or `&mut T` to be "reborrowed" through
+    /// coercion: essentially, the pointer is copied under a new, shorter, lifetime.
+    /// Until rfcs#1403 lands, reborrows on user types require a method call.
+    #[inline(always)]
+    pub fn re<'b>(&'b mut self) -> UnwrapMut<'b, R>
+    where
+        'r: 'b,
+    {
+        UnwrapMut(self.0)
+    }
+}
+
 impl<R: TryRngCore + ?Sized> RngCore for UnwrapMut<'_, R> {
     #[inline]
     fn next_u32(&mut self) -> u32 {