std::io::Take: Clarify & optimize BorrowedBuf::set_init usage.

library/std/src/io/mod.rs

@@ -3083,32 +3083,31 @@ impl<T: Read> Read for Take<T> {
             let is_init = buf.is_init();
 
             // SAFETY: no uninit data is written to ibuf
-            let ibuf = unsafe { &mut buf.as_mut()[..limit] };
-
-            let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();
+            let mut sliced_buf = BorrowedBuf::from(unsafe { &mut buf.as_mut()[..limit] });
 
             if is_init {
                 // SAFETY: `sliced_buf` is a subslice of `buf`, so if `buf` was initialized then
                 // `sliced_buf` is.
                 unsafe { sliced_buf.set_init() };
             }
 
-            let mut cursor = sliced_buf.unfilled();
-            let result = self.inner.read_buf(cursor.reborrow());
+            let result = self.inner.read_buf(sliced_buf.unfilled());
 
-            let should_init = cursor.is_init();
+            let did_init_up_to_limit = sliced_buf.is_init();
             let filled = sliced_buf.len();
 
-            // cursor / sliced_buf / ibuf must drop here
+            // sliced_buf must drop here
 
             // Avoid accidentally quadratic behaviour by initializing the whole
             // cursor if only part of it was initialized.
-            if should_init {
-                // SAFETY: no uninit data is written
-                let uninit = unsafe { &mut buf.as_mut()[limit..] };
-                uninit.write_filled(0);
-                // SAFETY: all bytes that were not initialized by `T::read_buf`
-                // have just been written to.
+            if did_init_up_to_limit && !is_init {
+                // SAFETY: No uninit data will be written.
+                let unfilled_before_advance = unsafe { buf.as_mut() };
+
+                unfilled_before_advance[limit..].write_filled(0);
+
+                // SAFETY: `unfilled_before_advance[..limit]` was initialized by `T::read_buf`, and
+                // `unfilled_before_advance[limit..]` was just initialized.
                 unsafe { buf.set_init() };
             }