Rollup merge of #82728 - calebsander:refactor/bufreader-buf, r=m-ou-se

Avoid unnecessary Vec construction in BufReader

As mentioned in #80460, creating a `Vec` and calling `Vec::into_boxed_slice()` emits unnecessary calls to `realloc()` and `free()`. Updated the code to use `Box::new_uninit_slice()` to create a boxed slice directly. I think this also makes it more explicit that the initial contents of the buffer are uninitialized.

r? ``@m-ou-se``

Author: m-ou-se

library/std/src/io/buffered/bufreader.rs

@@ -92,10 +92,9 @@ impl<R: Read> BufReader<R> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
         unsafe {
-            let mut buffer = Vec::with_capacity(capacity);
-            buffer.set_len(capacity);
-            inner.initializer().initialize(&mut buffer);
-            BufReader { inner, buf: buffer.into_boxed_slice(), pos: 0, cap: 0 }
+            let mut buf = Box::new_uninit_slice(capacity).assume_init();
+            inner.initializer().initialize(&mut buf);
+            BufReader { inner, buf, pos: 0, cap: 0 }
         }
     }
 }

library/std/src/lib.rs

@@ -289,6 +289,7 @@
 #![feature(needs_panic_runtime)]
 #![feature(negative_impls)]
 #![feature(never_type)]
+#![feature(new_uninit)]
 #![feature(nll)]
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(once_cell)]