Rollup merge of #143829 - a1phyr:trim_borrowed_buf, r=ChrisDenton

Trim `BorrowedCursor` API

This PR removes some method from the unstable `BorrowedCursor` type. A rational for each change can be found in the message of each commit.

I don't think that an ACP is required for this, please tell me if it is not the case.

Cc rust-lang/rust#78485 rust-lang/rust#117693

library/core/src/io/borrowed_buf.rs

@@ -149,7 +149,6 @@ impl<'data> BorrowedBuf<'data> {
     #[inline]
     pub fn unfilled<'this>(&'this mut self) -> BorrowedCursor<'this> {
         BorrowedCursor {
-            start: self.filled,
             // SAFETY: we never assign into `BorrowedCursor::buf`, so treating its
             // lifetime covariantly is safe.
             buf: unsafe {
@@ -205,9 +204,6 @@ pub struct BorrowedCursor<'a> {
     // we create a `BorrowedCursor`. This is only safe if we never replace `buf` by assigning into
     // it, so don't do that!
     buf: &'a mut BorrowedBuf<'a>,
-    /// The length of the filled portion of the underlying buffer at the time of the cursor's
-    /// creation.
-    start: usize,
 }
 
 impl<'a> BorrowedCursor<'a> {
@@ -225,7 +221,6 @@ impl<'a> BorrowedCursor<'a> {
                     self.buf,
                 )
             },
-            start: self.start,
         }
     }
 
@@ -235,23 +230,12 @@ impl<'a> BorrowedCursor<'a> {
         self.buf.capacity() - self.buf.filled
     }
 
-    /// Returns the number of bytes written to this cursor since it was created from a `BorrowedBuf`.
+    /// Returns the number of bytes written to the `BorrowedBuf` this cursor was created from.
     ///
-    /// Note that if this cursor is a reborrowed clone of another, then the count returned is the
-    /// count written via either cursor, not the count since the cursor was reborrowed.
+    /// In particular, the count returned is shared by all reborrows of the cursor.
     #[inline]
     pub fn written(&self) -> usize {
-        self.buf.filled - self.start
-    }
-
-    /// Returns a shared reference to the initialized portion of the cursor.
-    #[inline]
-    pub fn init_ref(&self) -> &[u8] {
-        // SAFETY: We only slice the initialized part of the buffer, which is always valid
-        unsafe {
-            let buf = self.buf.buf.get_unchecked(self.buf.filled..self.buf.init);
-            buf.assume_init_ref()
-        }
+        self.buf.filled
     }
 
     /// Returns a mutable reference to the initialized portion of the cursor.
@@ -264,15 +248,6 @@ impl<'a> BorrowedCursor<'a> {
         }
     }
 
-    /// Returns a mutable reference to the uninitialized part of the cursor.
-    ///
-    /// It is safe to uninitialize any of these bytes.
-    #[inline]
-    pub fn uninit_mut(&mut self) -> &mut [MaybeUninit<u8>] {
-        // SAFETY: always in bounds
-        unsafe { self.buf.buf.get_unchecked_mut(self.buf.init..) }
-    }
-
     /// Returns a mutable reference to the whole cursor.
     ///
     /// # Safety
@@ -325,7 +300,9 @@ impl<'a> BorrowedCursor<'a> {
     /// Initializes all bytes in the cursor.
     #[inline]
     pub fn ensure_init(&mut self) -> &mut Self {
-        let uninit = self.uninit_mut();
+        // SAFETY: always in bounds and we never uninitialize these bytes.
+        let uninit = unsafe { self.buf.buf.get_unchecked_mut(self.buf.init..) };
+
         // SAFETY: 0 is a valid value for MaybeUninit<u8> and the length matches the allocation
         // since it is comes from a slice reference.
         unsafe {

library/coretests/tests/io/borrowed_buf.rs

@@ -61,7 +61,7 @@ fn clear() {
     assert_eq!(rbuf.filled().len(), 0);
     assert_eq!(rbuf.unfilled().capacity(), 16);
 
-    assert_eq!(rbuf.unfilled().init_ref(), [255; 16]);
+    assert_eq!(rbuf.unfilled().init_mut(), [255; 16]);
 }
 
 #[test]
@@ -124,7 +124,7 @@ fn reborrow_written() {
     assert_eq!(cursor2.written(), 32);
     assert_eq!(cursor.written(), 32);
 
-    assert_eq!(buf.unfilled().written(), 0);
+    assert_eq!(buf.unfilled().written(), 32);
     assert_eq!(buf.init_len(), 32);
     assert_eq!(buf.filled().len(), 32);
     let filled = buf.filled();
@@ -142,9 +142,7 @@ fn cursor_set_init() {
     }
 
     assert_eq!(rbuf.init_len(), 8);
-    assert_eq!(rbuf.unfilled().init_ref().len(), 8);
     assert_eq!(rbuf.unfilled().init_mut().len(), 8);
-    assert_eq!(rbuf.unfilled().uninit_mut().len(), 8);
     assert_eq!(unsafe { rbuf.unfilled().as_mut().len() }, 16);
 
     rbuf.unfilled().advance(4);
@@ -160,9 +158,7 @@ fn cursor_set_init() {
     }
 
     assert_eq!(rbuf.init_len(), 12);
-    assert_eq!(rbuf.unfilled().init_ref().len(), 8);
     assert_eq!(rbuf.unfilled().init_mut().len(), 8);
-    assert_eq!(rbuf.unfilled().uninit_mut().len(), 4);
     assert_eq!(unsafe { rbuf.unfilled().as_mut().len() }, 12);
 }
 

library/std/src/io/mod.rs

@@ -489,7 +489,7 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
             }
         };
 
-        let unfilled_but_initialized = cursor.init_ref().len();
+        let unfilled_but_initialized = cursor.init_mut().len();
         let bytes_read = cursor.written();
         let was_fully_initialized = read_buf.init_len() == buf_len;
 
@@ -3053,7 +3053,7 @@ impl<T: Read> Read for Take<T> {
             // The condition above guarantees that `self.limit` fits in `usize`.
             let limit = self.limit as usize;
 
-            let extra_init = cmp::min(limit, buf.init_ref().len());
+            let extra_init = cmp::min(limit, buf.init_mut().len());
 
             // SAFETY: no uninit data is written to ibuf
             let ibuf = unsafe { &mut buf.as_mut()[..limit] };
@@ -3068,7 +3068,7 @@ impl<T: Read> Read for Take<T> {
             let mut cursor = sliced_buf.unfilled();
             let result = self.inner.read_buf(cursor.reborrow());
 
-            let new_init = cursor.init_ref().len();
+            let new_init = cursor.init_mut().len();
             let filled = sliced_buf.len();
 
             // cursor / sliced_buf / ibuf must drop here