io: add boxed module for boxed bytes like vec::zeroed...

- proxmox_io::boxed::uninitialized(len) -> Box<[u8]>
  same as vec::uninitialized, but as a box

- proxmox_io::boxed::zeroed(len) -> Box<[u8]>
  same as vec::zeroed, but as a box

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

proxmox-io/src/boxed.rs

@@ -0,0 +1,20 @@
+/// Uninitialized bytes, without `MaybeUninit`.
+///
+/// We're talking about bytes here, which we *allocate*. That is, we call a function, get a pointer
+/// to stuff, and can use it. It's not UB in that there's nothing undefined about what's going on
+/// here.
+pub fn uninitialized(len: usize) -> Box<[u8]> {
+    unsafe {
+        let data = std::alloc::alloc(std::alloc::Layout::array::<u8>(len).unwrap());
+        Box::from_raw(std::ptr::slice_from_raw_parts_mut(data, len))
+    }
+}
+
+/// Zero-initialized bytes, meant for large sizes to avoid busting the stack.
+pub fn zeroed(len: usize) -> Box<[u8]> {
+    let mut bytes = uninitialized(len);
+    unsafe {
+        std::ptr::write_bytes(bytes.as_mut_ptr(), 0, bytes.len());
+    }
+    bytes
+}

proxmox-io/src/lib.rs

@@ -23,4 +23,5 @@ pub use std_channel_writer::StdChannelWriter;
 mod byte_buffer;
 pub use byte_buffer::ByteBuffer;
 
+pub mod boxed;
 pub mod vec;