Rollup merge of #78641 - the8472:buffered-copy, r=sfackler

Let io::copy reuse BufWriter buffers

This optimization will allow users to implicitly set the buffer size for io::copy by wrapping the writer into a `BufWriter` if the default block size is insufficient, which should fix #49921

Due to min_specialization limitations this approach only works with `BufWriter` but not for `BufReader<R>` since `R` is unconstrained and thus the necessary specialization on `R: Read` is not always applicable. Once specialization becomes more powerful this optimization could be extended to look at the reader and writer side and use whichever buffer is larger.

library/std/src/lib.rs

@@ -284,6 +284,7 @@
 #![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_ref)]
 #![feature(maybe_uninit_slice)]
+#![feature(maybe_uninit_uninit_array)]
 #![feature(min_specialization)]
 #![feature(needs_panic_runtime)]
 #![feature(negative_impls)]
@@ -327,6 +328,7 @@
 #![feature(unsafe_cell_raw_get)]
 #![feature(unwind_attributes)]
 #![feature(vec_into_raw_parts)]
+#![feature(vec_spare_capacity)]
 #![feature(wake_trait)]
 // NB: the above list is sorted to minimize merge conflicts.
 #![default_lib_allocator]