1 files changed, 97 insertions, 0 deletions
diff --git a/library/alloc/src/vec/spec_from_iter.rs b/library/alloc/src/vec/spec_from_iter.rs
new file mode 100644
index 00000000000..bbfcc68daef
--- /dev/null
+++ b/library/alloc/src/vec/spec_from_iter.rs
@@ -0,0 +1,97 @@
+use core::mem::ManuallyDrop;
+use core::ptr::{self};
+use core::slice::{self};
+
+use super::{IntoIter, SpecExtend, SpecFromIterNested, Vec};
+
+/// Specialization trait used for Vec::from_iter
+///
+/// ## The delegation graph:
+///
+/// ```text
+/// +-------------+
+/// |FromIterator |
+/// +-+-----------+
+///   |
+///   v
+/// +-+-------------------------------+  +---------------------+
+/// |SpecFromIter                  +---->+SpecFromIterNested   |
+/// |where I:                      |  |  |where I:             |
+/// |  Iterator (default)----------+  |  |  Iterator (default) |
+/// |  vec::IntoIter               |  |  |  TrustedLen         |
+/// |  SourceIterMarker---fallback-+  |  |                     |
+/// |  slice::Iter                    |  |                     |
+/// |  Iterator<Item = &Clone>        |  +---------------------+
+/// +---------------------------------+
+/// ```
+pub(super) trait SpecFromIter<T, I> {
+    fn from_iter(iter: I) -> Self;
+}
+
+impl<T, I> SpecFromIter<T, I> for Vec<T>
+where
+    I: Iterator<Item = T>,
+{
+    default fn from_iter(iterator: I) -> Self {
+        SpecFromIterNested::from_iter(iterator)
+    }
+}
+
+impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> {
+    fn from_iter(iterator: IntoIter<T>) -> Self {
+        // A common case is passing a vector into a function which immediately
+        // re-collects into a vector. We can short circuit this if the IntoIter
+        // has not been advanced at all.
+        // When it has been advanced We can also reuse the memory and move the data to the front.
+        // But we only do so when the resulting Vec wouldn't have more unused capacity
+        // than creating it through the generic FromIterator implementation would. That limitation
+        // is not strictly necessary as Vec's allocation behavior is intentionally unspecified.
+        // But it is a conservative choice.
+        let has_advanced = iterator.buf.as_ptr() as *const _ != iterator.ptr;
+        if !has_advanced || iterator.len() >= iterator.cap / 2 {
+            unsafe {
+                let it = ManuallyDrop::new(iterator);
+                if has_advanced {
+                    ptr::copy(it.ptr, it.buf.as_ptr(), it.len());
+                }
+                return Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap);
+            }
+        }
+
+        let mut vec = Vec::new();
+        // must delegate to spec_extend() since extend() itself delegates
+        // to spec_from for empty Vecs
+        vec.spec_extend(iterator);
+        vec
+    }
+}
+
+impl<'a, T: 'a, I> SpecFromIter<&'a T, I> for Vec<T>
+where
+    I: Iterator<Item = &'a T>,
+    T: Clone,
+{
+    default fn from_iter(iterator: I) -> Self {
+        SpecFromIter::from_iter(iterator.cloned())
+    }
+}
+
+// This utilizes `iterator.as_slice().to_vec()` since spec_extend
+// must take more steps to reason about the final capacity + length
+// and thus do more work. `to_vec()` directly allocates the correct amount
+// and fills it exactly.
+impl<'a, T: 'a + Clone> SpecFromIter<&'a T, slice::Iter<'a, T>> for Vec<T> {
+    #[cfg(not(test))]
+    fn from_iter(iterator: slice::Iter<'a, T>) -> Self {
+        iterator.as_slice().to_vec()
+    }
+
+    // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
+    // required for this method definition, is not available. Instead use the
+    // `slice::to_vec`  function which is only available with cfg(test)
+    // NB see the slice::hack module in slice.rs for more information
+    #[cfg(test)]
+    fn from_iter(iterator: slice::Iter<'a, T>) -> Self {
+        crate::slice::to_vec(iterator.as_slice(), crate::alloc::Global)
+    }
+}