Rollup merge of rust-lang#81599 - sdroege:fuse-trusted-len, r=m-ou-se

Implement `TrustedLen` for `Fuse<I: TrustedLen>`

This looks like it was simply forgotten.

library/core/src/iter/adapters/fuse.rs

@@ -1,6 +1,8 @@
 use crate::intrinsics;
 use crate::iter::adapters::{zip::try_get_unchecked, InPlaceIterable, SourceIter};
-use crate::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedRandomAccess};
+use crate::iter::{
+    DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedLen, TrustedRandomAccess,
+};
 use crate::ops::Try;
 
 /// An iterator that yields `None` forever after the underlying iterator
@@ -182,8 +184,19 @@ where
     }
 }
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+// SAFETY: `TrustedLen` requires that an accurate length is reported via `size_hint()`. As `Fuse`
+// is just forwarding this to the wrapped iterator `I` this property is preserved and it is safe to
+// implement `TrustedLen` here.
+unsafe impl<I> TrustedLen for Fuse<I> where I: TrustedLen {}
+
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
+// SAFETY: `TrustedRandomAccess` requires that `size_hint()` must be exact and cheap to call, and
+// `Iterator::__iterator_get_unchecked()` must be implemented accordingly.
+//
+// This is safe to implement as `Fuse` is just forwarding these to the wrapped iterator `I`, which
+// preserves these properties.
 unsafe impl<I> TrustedRandomAccess for Fuse<I>
 where
     I: TrustedRandomAccess,