improve the performance of the vec![] macro

Closes #17865

src/libstd/lib.rs

@@ -272,7 +272,9 @@ mod std {
     // The test runner calls ::std::os::args() but really wants realstd
     #[cfg(test)] pub use realstd::os as os;
     // The test runner requires std::slice::Vector, so re-export std::slice just for it.
-    #[cfg(test)] pub use slice;
+    //
+    // It is also used in vec![]
+    pub use slice;
 
-    pub use collections; // vec!() uses MutableSeq
+    pub use boxed; // used for vec![]
 }