restore SpecFrom<T, TrustedLen<Item=T>> specialization by nesting

specializations

library/alloc/src/vec.rs

@@ -2108,7 +2108,13 @@ trait SpecFrom<T, I> {
     fn from_iter(iter: I) -> Self;
 }
 
-impl<T, I> SpecFrom<T, I> for Vec<T>
+// Another specialization trait for Vec::from_iter
+// necessary to manually prioritize overlapping specializations
+trait SpecFromNested<T, I> {
+    fn from_iter(iter: I) -> Self;
+}
+
+impl<T, I> SpecFromNested<T, I> for Vec<T>
 where
     I: Iterator<Item = T>,
 {
@@ -2137,6 +2143,28 @@ where
     }
 }
 
+impl<T, I> SpecFromNested<T, I> for Vec<T>
+where
+    I: TrustedLen<Item = T>,
+{
+    fn from_iter(iterator: I) -> Self {
+        let mut vector = Vec::new();
+        // must delegate to spec_extend() since extend() itself delegates
+        // to spec_from for empty Vecs
+        vector.spec_extend(iterator);
+        vector
+    }
+}
+
+impl<T, I> SpecFrom<T, I> for Vec<T>
+where
+    I: Iterator<Item = T>,
+{
+    default fn from_iter(iterator: I) -> Self {
+        SpecFromNested::from_iter(iterator)
+    }
+}
+
 struct InPlaceDrop<T> {
     inner: *mut T,
     dst: *mut T,