Rollup merge of rust-lang#83990 - the8472:take-trusted-len, r=dtolnay

implement `TrustedRandomAccess` for `Take` iterator adapter

`TrustedRandomAccess` requires the iterator length to fit within `usize`. `take(n)` only constrains the upper bound of an iterator. So if the inner is `TrustedRandomAccess` (which already implies a finite length) then so can be `Take`.

``````@rustbot`````` label T-libs-impl

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

@@ -1,5 +1,8 @@
 use crate::cmp;
-use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
+use crate::iter::{
+    adapters::zip::try_get_unchecked, adapters::SourceIter, FusedIterator, InPlaceIterable,
+    TrustedLen, TrustedRandomAccess,
+};
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator that only iterates over the first `n` iterations of `iter`.
@@ -111,6 +114,15 @@ where
 
         self.try_fold(init, ok(fold)).unwrap()
     }
+
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> <I as Iterator>::Item
+    where
+        Self: TrustedRandomAccess,
+    {
+        // SAFETY: the caller must uphold the contract for
+        // `Iterator::__iterator_get_unchecked`.
+        unsafe { try_get_unchecked(&mut self.iter, idx) }
+    }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]
@@ -207,3 +219,12 @@ impl<I> FusedIterator for Take<I> where I: FusedIterator {}
 
 #[unstable(feature = "trusted_len", issue = "37572")]
 unsafe impl<I: TrustedLen> TrustedLen for Take<I> {}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<I> TrustedRandomAccess for Take<I>
+where
+    I: TrustedRandomAccess,
+{
+    const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
+}