Rollup merge of #42717 - ollie27:into_to_from2, r=sfackler

Convert `Into<Box<[T]>> for Vec<T>` into `From<Vec<T>> for Box<[T]>`

As the `collections` crate has been merged into `alloc` in #42648 this impl is now possible. This is the final part of #42129 missing from #42227.

src/liballoc/vec.rs

@@ -2124,10 +2124,12 @@ fn from(s: Box<[T]>) -> Vec<T> {
     }
 }
 
-#[stable(feature = "box_from_vec", since = "1.18.0")]
-impl<T> Into<Box<[T]>> for Vec<T> {
-    fn into(self) -> Box<[T]> {
-        self.into_boxed_slice()
+// note: test pulls in libstd, which causes errors here
+#[cfg(not(test))]
+#[stable(feature = "box_from_vec", since = "1.20.0")]
+impl<T> From<Vec<T>> for Box<[T]> {
+    fn from(v: Vec<T>) -> Box<[T]> {
+        v.into_boxed_slice()
     }
 }