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

Don't initialize `buf` if it was already initialized.

Move the `buf.advance()` call to make the initialization more like
calling `buf.ensure_init()`, then clarify how the code here is an
optimized variant of `ensure_init`.

Author: briansmith

library/std/src/io/mod.rs

@@ -3095,14 +3095,22 @@ impl<T: Read> Read for Take<T> {
             let mut cursor = sliced_buf.unfilled();
             let result = self.inner.read_buf(cursor.reborrow());
 
-            let should_init = cursor.is_init();
+            let is_init_up_to_limit = cursor.is_init();
             let filled = sliced_buf.len();
 
             // cursor / sliced_buf / ibuf must drop here
 
+            unsafe {
+                // SAFETY: filled bytes have been filled
+                buf.advance(filled);
+            }
+
             // Avoid accidentally quadratic behaviour by initializing the whole
             // cursor if only part of it was initialized.
-            if should_init {
+            if is_init_up_to_limit && !is_init {
+                // `buf.ensure_init()` would work, but it would additionally
+                // re-initialize `buf.as_mut()[buf.len()..limit]`.
+
                 // SAFETY: no uninit data is written
                 let uninit = unsafe { &mut buf.as_mut()[limit..] };
                 uninit.write_filled(0);
@@ -3111,11 +3119,6 @@ impl<T: Read> Read for Take<T> {
                 unsafe { buf.set_init() };
             }
 
-            unsafe {
-                // SAFETY: filled bytes have been filled
-                buf.advance(filled);
-            }
-
             self.limit -= filled as u64;
 
             result