add default FromIterator for types with Default and Extend trait

library/core/src/iter/traits/collect.rs

@@ -152,6 +152,18 @@ pub trait FromIterator<A>: Sized {
     fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self;
 }
 
+// 为同时实现 Default 和 Extend 的类型实现 FromIterator
+#[cfg(feature = "specialization")]
+#[stable(feature = "from_iter_default", since = "CURRENT_RUSTC_VERSION")]
+impl<A, T: Default + Extend<A>> FromIterator<A> for T {
+    #[rustc_diagnostic_item = "from_iter_default"]
+    default fn from_iter<I: IntoIterator<Item = A>>(iter: I) -> Self {
+        let mut collection = T::default();
+        collection.extend(iter);
+        collection
+    }
+}
+
 /// Conversion into an [`Iterator`].
 ///
 /// By implementing `IntoIterator` for a type, you define how it will be
@@ -635,40 +647,40 @@ macro_rules! spec_tuple_impl {
             }
         }
 
-        /// This implementation turns an iterator of tuples into a tuple of types which implement
-        /// [`Default`] and [`Extend`].
-        ///
-        /// This is similar to [`Iterator::unzip`], but is also composable with other [`FromIterator`]
-        /// implementations:
-        ///
-        /// ```rust
-        /// # fn main() -> Result<(), core::num::ParseIntError> {
-        /// let string = "1,2,123,4";
-        ///
-        /// // Example given for a 2-tuple, but 1- through 12-tuples are supported
-        /// let (numbers, lengths): (Vec<_>, Vec<_>) = string
-        ///     .split(',')
-        ///     .map(|s| s.parse().map(|n: u32| (n, s.len())))
-        ///     .collect::<Result<_, _>>()?;
-        ///
-        /// assert_eq!(numbers, [1, 2, 123, 4]);
-        /// assert_eq!(lengths, [1, 1, 3, 1]);
-        /// # Ok(()) }
-        /// ```
-        #[$meta]
-        $(#[$doctext])?
-        #[stable(feature = "from_iterator_for_tuple", since = "1.79.0")]
-        impl<$($ty_names,)* $($extend_ty_names,)*> FromIterator<($($extend_ty_names,)*)> for ($($ty_names,)*)
-        where
-            $($ty_names: Default + Extend<$extend_ty_names>,)*
-        {
-            fn from_iter<Iter: IntoIterator<Item = ($($extend_ty_names,)*)>>(iter: Iter) -> Self {
-                let mut res = <($($ty_names,)*)>::default();
-                res.extend(iter);
+        // /// This implementation turns an iterator of tuples into a tuple of types which implement
+        // /// [`Default`] and [`Extend`].
+        // ///
+        // /// This is similar to [`Iterator::unzip`], but is also composable with other [`FromIterator`]
+        // /// implementations:
+        // ///
+        // /// ```rust
+        // /// # fn main() -> Result<(), core::num::ParseIntError> {
+        // /// let string = "1,2,123,4";
+        // ///
+        // /// // Example given for a 2-tuple, but 1- through 12-tuples are supported
+        // /// let (numbers, lengths): (Vec<_>, Vec<_>) = string
+        // ///     .split(',')
+        // ///     .map(|s| s.parse().map(|n: u32| (n, s.len())))
+        // ///     .collect::<Result<_, _>>()?;
+        // ///
+        // /// assert_eq!(numbers, [1, 2, 123, 4]);
+        // /// assert_eq!(lengths, [1, 1, 3, 1]);
+        // /// # Ok(()) }
+        // /// ```
+        // #[$meta]
+        // $(#[$doctext])?
+        // #[stable(feature = "from_iterator_for_tuple", since = "1.79.0")]
+        // impl<$($ty_names,)* $($extend_ty_names,)*> FromIterator<($($extend_ty_names,)*)> for ($($ty_names,)*)
+        // where
+        //     $($ty_names: Default + Extend<$extend_ty_names>,)*
+        // {
+        //     fn from_iter<Iter: IntoIterator<Item = ($($extend_ty_names,)*)>>(iter: Iter) -> Self {
+        //         let mut res = <($($ty_names,)*)>::default();
+        //         res.extend(iter);
 
-                res
-            }
-        }
+        //         res
+        //     }
+        // }
 
     };
 }

library/core/src/lib.rs

@@ -174,6 +174,7 @@
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
 #![feature(simd_ffi)]
+#![feature(specialization)]
 #![feature(staged_api)]
 #![feature(stmt_expr_attributes)]
 #![feature(strict_provenance_lints)]