1 files changed, 56 insertions, 0 deletions
diff --git a/library/alloc/src/vec/spec_from_iter_nested.rs b/library/alloc/src/vec/spec_from_iter_nested.rs
new file mode 100644
index 00000000000..6abd4ff2a3f
--- /dev/null
+++ b/library/alloc/src/vec/spec_from_iter_nested.rs
@@ -0,0 +1,56 @@
+use core::iter::TrustedLen;
+use core::ptr::{self};
+
+use super::{SpecExtend, Vec};
+
+/// Another specialization trait for Vec::from_iter
+/// necessary to manually prioritize overlapping specializations
+/// see [`SpecFromIter`] for details.
+pub(super) trait SpecFromIterNested<T, I> {
+    fn from_iter(iter: I) -> Self;
+}
+
+impl<T, I> SpecFromIterNested<T, I> for Vec<T>
+where
+    I: Iterator<Item = T>,
+{
+    default fn from_iter(mut iterator: I) -> Self {
+        // Unroll the first iteration, as the vector is going to be
+        // expanded on this iteration in every case when the iterable is not
+        // empty, but the loop in extend_desugared() is not going to see the
+        // vector being full in the few subsequent loop iterations.
+        // So we get better branch prediction.
+        let mut vector = match iterator.next() {
+            None => return Vec::new(),
+            Some(element) => {
+                let (lower, _) = iterator.size_hint();
+                let mut vector = Vec::with_capacity(lower.saturating_add(1));
+                unsafe {
+                    ptr::write(vector.as_mut_ptr(), element);
+                    vector.set_len(1);
+                }
+                vector
+            }
+        };
+        // must delegate to spec_extend() since extend() itself delegates
+        // to spec_from for empty Vecs
+        <Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
+        vector
+    }
+}
+
+impl<T, I> SpecFromIterNested<T, I> for Vec<T>
+where
+    I: TrustedLen<Item = T>,
+{
+    fn from_iter(iterator: I) -> Self {
+        let mut vector = match iterator.size_hint() {
+            (_, Some(upper)) => Vec::with_capacity(upper),
+            _ => Vec::new(),
+        };
+        // must delegate to spec_extend() since extend() itself delegates
+        // to spec_from for empty Vecs
+        vector.spec_extend(iterator);
+        vector
+    }
+}