diff --git a/library/core/src/io/borrowed_buf.rs b/library/core/src/io/borrowed_buf.rs
index 088dea7812945..5df4f044b0b95 100644
--- a/library/core/src/io/borrowed_buf.rs
+++ b/library/core/src/io/borrowed_buf.rs
@@ -2,20 +2,18 @@
 
 use crate::fmt::{self, Debug, Formatter};
 use crate::mem::{self, MaybeUninit};
-use crate::{cmp, ptr};
+use crate::ptr;
 
-/// A borrowed byte buffer which is incrementally filled and initialized.
+/// A borrowed byte buffer which is incrementally filled.
 ///
-/// This type is a sort of "double cursor". It tracks three regions in the buffer: a region at the beginning of the
-/// buffer that has been logically filled with data, a region that has been initialized at some point but not yet
-/// logically filled, and a region at the end that is fully uninitialized. The filled region is guaranteed to be a
-/// subset of the initialized region.
+/// This type makes it safer to work with `MaybeUninit` buffers, such as to read into a buffer
+/// without having to initialize it first. It tracks the region of bytes that have been filled and
+/// whether the unfilled region was initialized.
 ///
 /// In summary, the contents of the buffer can be visualized as:
 /// ```not_rust
-/// [             capacity              ]
-/// [ filled |         unfilled         ]
-/// [    initialized    | uninitialized ]
+/// [                capacity                ]
+/// [ filled | unfilled (may be initialized) ]
 /// ```
 ///
 /// A `BorrowedBuf` is created around some existing data (or capacity for data) via a unique reference
@@ -30,8 +28,8 @@ pub struct BorrowedBuf<'data> {
     buf: &'data mut [MaybeUninit<u8>],
     /// The length of `self.buf` which is known to be filled.
     filled: usize,
-    /// The length of `self.buf` which is known to be initialized.
-    init: usize,
+    /// Whether the entire unfilled part of `self.buf` is known to be initialized.
+    init: bool,
 }
 
 impl Debug for BorrowedBuf<'_> {
@@ -48,24 +46,20 @@ impl Debug for BorrowedBuf<'_> {
 impl<'data> From<&'data mut [u8]> for BorrowedBuf<'data> {
     #[inline]
     fn from(slice: &'data mut [u8]) -> BorrowedBuf<'data> {
-        let len = slice.len();
-
         BorrowedBuf {
             // SAFETY: initialized data never becoming uninitialized is an invariant of BorrowedBuf
-            buf: unsafe { (slice as *mut [u8]).as_uninit_slice_mut().unwrap() },
+            buf: unsafe { &mut *(slice as *mut [u8] as *mut [MaybeUninit<u8>]) },
             filled: 0,
-            init: len,
+            init: true,
         }
     }
 }
 
 /// Creates a new `BorrowedBuf` from an uninitialized buffer.
-///
-/// Use `set_init` if part of the buffer is known to be already initialized.
 impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data> {
     #[inline]
     fn from(buf: &'data mut [MaybeUninit<u8>]) -> BorrowedBuf<'data> {
-        BorrowedBuf { buf, filled: 0, init: 0 }
+        BorrowedBuf { buf, filled: 0, init: false }
     }
 }
 
@@ -74,14 +68,13 @@ impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data> {
 /// Use `BorrowedCursor::with_unfilled_buf` instead for a safer alternative.
 impl<'data> From<BorrowedCursor<'data>> for BorrowedBuf<'data> {
     #[inline]
-    fn from(mut buf: BorrowedCursor<'data>) -> BorrowedBuf<'data> {
-        let init = buf.init_mut().len();
+    fn from(buf: BorrowedCursor<'data>) -> BorrowedBuf<'data> {
         BorrowedBuf {
             // SAFETY: no initialized byte is ever uninitialized as per
             // `BorrowedBuf`'s invariant
             buf: unsafe { buf.buf.buf.get_unchecked_mut(buf.buf.filled..) },
             filled: 0,
-            init,
+            init: buf.buf.init,
         }
     }
 }
@@ -100,8 +93,9 @@ impl<'data> BorrowedBuf<'data> {
     }
 
     /// Returns the length of the initialized part of the buffer.
+    #[unstable(feature = "borrowed_buf_init", issue = "78485")]
     #[inline]
-    pub fn init_len(&self) -> usize {
+    pub fn is_init(&self) -> bool {
         self.init
     }
 
@@ -159,32 +153,29 @@ impl<'data> BorrowedBuf<'data> {
 
     /// Clears the buffer, resetting the filled region to empty.
     ///
-    /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
+    /// The contents of the buffer are not modified.
     #[inline]
     pub fn clear(&mut self) -> &mut Self {
         self.filled = 0;
         self
     }
 
-    /// Asserts that the first `n` bytes of the buffer are initialized.
-    ///
-    /// `BorrowedBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
-    /// bytes than are already known to be initialized.
+    /// Asserts that the unfilled part of the buffer is initialized.
     ///
     /// # Safety
     ///
-    /// The caller must ensure that the first `n` unfilled bytes of the buffer have already been initialized.
+    /// All the bytes of the buffer must be initialized.
+    #[unstable(feature = "borrowed_buf_init", issue = "78485")]
     #[inline]
-    pub unsafe fn set_init(&mut self, n: usize) -> &mut Self {
-        self.init = cmp::max(self.init, n);
+    pub unsafe fn set_init(&mut self) -> &mut Self {
+        self.init = true;
         self
     }
 }
 
 /// A writeable view of the unfilled portion of a [`BorrowedBuf`].
 ///
-/// The unfilled portion consists of an initialized and an uninitialized part; see [`BorrowedBuf`]
-/// for details.
+/// The unfilled portion may be uninitialized; see [`BorrowedBuf`] for details.
 ///
 /// Data can be written directly to the cursor by using [`append`](BorrowedCursor::append) or
 /// indirectly by getting a slice of part or all of the cursor and writing into the slice. In the
@@ -238,21 +229,29 @@ impl<'a> BorrowedCursor<'a> {
         self.buf.filled
     }
 
-    /// Returns a mutable reference to the initialized portion of the cursor.
+    /// Returns `true` if the buffer is initialized.
+    #[unstable(feature = "borrowed_buf_init", issue = "78485")]
     #[inline]
-    pub fn init_mut(&mut self) -> &mut [u8] {
-        // SAFETY: We only slice the initialized part of the buffer, which is always valid
-        unsafe {
-            let buf = self.buf.buf.get_unchecked_mut(self.buf.filled..self.buf.init);
-            buf.assume_init_mut()
-        }
+    pub fn is_init(&self) -> bool {
+        self.buf.init
+    }
+
+    /// Set the buffer as fully initialized.
+    ///
+    /// # Safety
+    ///
+    /// All the bytes of the cursor must be initialized.
+    #[unstable(feature = "borrowed_buf_init", issue = "78485")]
+    #[inline]
+    pub unsafe fn set_init(&mut self) {
+        self.buf.init = true;
     }
 
     /// Returns a mutable reference to the whole cursor.
     ///
     /// # Safety
     ///
-    /// The caller must not uninitialize any bytes in the initialized portion of the cursor.
+    /// The caller must not uninitialize any bytes of the cursor if it is initialized.
     #[inline]
     pub unsafe fn as_mut(&mut self) -> &mut [MaybeUninit<u8>] {
         // SAFETY: always in bounds
@@ -271,10 +270,12 @@ impl<'a> BorrowedCursor<'a> {
     /// # Panics
     ///
     /// Panics if there are less than `n` bytes initialized.
+    #[unstable(feature = "borrowed_buf_init", issue = "78485")]
     #[inline]
-    pub fn advance(&mut self, n: usize) -> &mut Self {
+    pub fn advance_checked(&mut self, n: usize) -> &mut Self {
         // The subtraction cannot underflow by invariant of this type.
-        assert!(n <= self.buf.init - self.buf.filled);
+        let init_unfilled = if self.buf.init { self.buf.buf.len() - self.buf.filled } else { 0 };
+        assert!(n <= init_unfilled);
 
         self.buf.filled += n;
         self
@@ -291,40 +292,29 @@ impl<'a> BorrowedCursor<'a> {
     /// The caller must ensure that the first `n` bytes of the cursor have been properly
     /// initialised.
     #[inline]
-    pub unsafe fn advance_unchecked(&mut self, n: usize) -> &mut Self {
+    pub unsafe fn advance(&mut self, n: usize) -> &mut Self {
         self.buf.filled += n;
-        self.buf.init = cmp::max(self.buf.init, self.buf.filled);
         self
     }
 
-    /// Initializes all bytes in the cursor.
+    /// Initializes all bytes in the cursor and returns them.
+    #[unstable(feature = "borrowed_buf_init", issue = "78485")]
     #[inline]
-    pub fn ensure_init(&mut self) -> &mut Self {
+    pub fn ensure_init(&mut self) -> &mut [u8] {
         // 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 {
-            ptr::write_bytes(uninit.as_mut_ptr(), 0, uninit.len());
+        let unfilled = unsafe { self.buf.buf.get_unchecked_mut(self.buf.filled..) };
+
+        if !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 {
+                ptr::write_bytes(unfilled.as_mut_ptr(), 0, unfilled.len());
+            }
+            self.buf.init = true;
         }
-        self.buf.init = self.buf.capacity();
-
-        self
-    }
 
-    /// Asserts that the first `n` unfilled bytes of the cursor are initialized.
-    ///
-    /// `BorrowedBuf` assumes that bytes are never de-initialized, so this method does nothing when
-    /// called with fewer bytes than are already known to be initialized.
-    ///
-    /// # Safety
-    ///
-    /// The caller must ensure that the first `n` bytes of the buffer have already been initialized.
-    #[inline]
-    pub unsafe fn set_init(&mut self, n: usize) -> &mut Self {
-        self.buf.init = cmp::max(self.buf.init, self.buf.filled + n);
-        self
+        // SAFETY: these bytes have just been initialized if they weren't before
+        unsafe { unfilled.assume_init_mut() }
     }
 
     /// Appends data to the cursor, advancing position within its buffer.
@@ -341,10 +331,6 @@ impl<'a> BorrowedCursor<'a> {
             self.as_mut()[..buf.len()].write_copy_of_slice(buf);
         }
 
-        // SAFETY: We just added the entire contents of buf to the filled section.
-        unsafe {
-            self.set_init(buf.len());
-        }
         self.buf.filled += buf.len();
     }
 
@@ -365,7 +351,7 @@ impl<'a> BorrowedCursor<'a> {
         // Check that the caller didn't replace the `BorrowedBuf`.
         // This is necessary for the safety of the code below: if the check wasn't
         // there, one could mark some bytes as initialized even though there aren't.
-        assert!(core::ptr::addr_eq(prev_ptr, buf.buf));
+        assert!(core::ptr::eq(prev_ptr, buf.buf));
 
         let filled = buf.filled;
         let init = buf.init;
@@ -376,7 +362,7 @@ impl<'a> BorrowedCursor<'a> {
         // SAFETY: These amounts of bytes were initialized/filled in the `BorrowedBuf`,
         // and therefore they are initialized/filled in the cursor too, because the
         // buffer wasn't replaced.
-        self.buf.init = self.buf.filled + init;
+        self.buf.init = init;
         self.buf.filled += filled;
 
         res
diff --git a/library/coretests/tests/io/borrowed_buf.rs b/library/coretests/tests/io/borrowed_buf.rs
index aaa98d26ff8b9..de241e0cb85c9 100644
--- a/library/coretests/tests/io/borrowed_buf.rs
+++ b/library/coretests/tests/io/borrowed_buf.rs
@@ -8,7 +8,7 @@ fn new() {
     let mut rbuf: BorrowedBuf<'_> = buf.into();
 
     assert_eq!(rbuf.filled().len(), 0);
-    assert_eq!(rbuf.init_len(), 16);
+    assert!(rbuf.is_init());
     assert_eq!(rbuf.capacity(), 16);
     assert_eq!(rbuf.unfilled().capacity(), 16);
 }
@@ -20,7 +20,7 @@ fn uninit() {
     let mut rbuf: BorrowedBuf<'_> = buf.into();
 
     assert_eq!(rbuf.filled().len(), 0);
-    assert_eq!(rbuf.init_len(), 0);
+    assert!(!rbuf.is_init());
     assert_eq!(rbuf.capacity(), 16);
     assert_eq!(rbuf.unfilled().capacity(), 16);
 }
@@ -32,7 +32,7 @@ fn initialize_unfilled() {
 
     rbuf.unfilled().ensure_init();
 
-    assert_eq!(rbuf.init_len(), 16);
+    assert!(rbuf.is_init());
 }
 
 #[test]
@@ -40,7 +40,7 @@ fn advance_filled() {
     let buf: &mut [_] = &mut [0; 16];
     let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    rbuf.unfilled().advance(1);
+    rbuf.unfilled().advance_checked(1);
 
     assert_eq!(rbuf.filled().len(), 1);
     assert_eq!(rbuf.unfilled().capacity(), 15);
@@ -51,7 +51,7 @@ fn clear() {
     let buf: &mut [_] = &mut [255; 16];
     let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    rbuf.unfilled().advance(16);
+    rbuf.unfilled().advance_checked(16);
 
     assert_eq!(rbuf.filled().len(), 16);
     assert_eq!(rbuf.unfilled().capacity(), 0);
@@ -61,7 +61,7 @@ fn clear() {
     assert_eq!(rbuf.filled().len(), 0);
     assert_eq!(rbuf.unfilled().capacity(), 16);
 
-    assert_eq!(rbuf.unfilled().init_mut(), [255; 16]);
+    assert_eq!(rbuf.unfilled().ensure_init(), [255; 16]);
 }
 
 #[test]
@@ -70,24 +70,10 @@ fn set_init() {
     let mut rbuf: BorrowedBuf<'_> = buf.into();
 
     unsafe {
-        rbuf.set_init(8);
+        rbuf.set_init();
     }
 
-    assert_eq!(rbuf.init_len(), 8);
-
-    rbuf.unfilled().advance(4);
-
-    unsafe {
-        rbuf.set_init(2);
-    }
-
-    assert_eq!(rbuf.init_len(), 8);
-
-    unsafe {
-        rbuf.set_init(8);
-    }
-
-    assert_eq!(rbuf.init_len(), 8);
+    assert!(rbuf.is_init());
 }
 
 #[test]
@@ -97,7 +83,7 @@ fn append() {
 
     rbuf.unfilled().append(&[0; 8]);
 
-    assert_eq!(rbuf.init_len(), 8);
+    assert!(!rbuf.is_init());
     assert_eq!(rbuf.filled().len(), 8);
     assert_eq!(rbuf.filled(), [0; 8]);
 
@@ -105,7 +91,7 @@ fn append() {
 
     rbuf.unfilled().append(&[1; 16]);
 
-    assert_eq!(rbuf.init_len(), 16);
+    assert!(!rbuf.is_init());
     assert_eq!(rbuf.filled().len(), 16);
     assert_eq!(rbuf.filled(), [1; 16]);
 }
@@ -125,7 +111,7 @@ fn reborrow_written() {
     assert_eq!(cursor.written(), 32);
 
     assert_eq!(buf.unfilled().written(), 32);
-    assert_eq!(buf.init_len(), 32);
+    assert!(!buf.is_init());
     assert_eq!(buf.filled().len(), 32);
     let filled = buf.filled();
     assert_eq!(&filled[..16], [1; 16]);
@@ -136,30 +122,20 @@ fn reborrow_written() {
 fn cursor_set_init() {
     let buf: &mut [_] = &mut [MaybeUninit::zeroed(); 16];
     let mut rbuf: BorrowedBuf<'_> = buf.into();
+    let mut cursor = rbuf.unfilled();
 
     unsafe {
-        rbuf.unfilled().set_init(8);
+        cursor.set_init();
     }
 
-    assert_eq!(rbuf.init_len(), 8);
-    assert_eq!(rbuf.unfilled().init_mut().len(), 8);
-    assert_eq!(unsafe { rbuf.unfilled().as_mut().len() }, 16);
-
-    rbuf.unfilled().advance(4);
-
-    unsafe {
-        rbuf.unfilled().set_init(2);
-    }
+    assert!(cursor.is_init());
+    assert_eq!(unsafe { cursor.as_mut().len() }, 16);
 
-    assert_eq!(rbuf.init_len(), 8);
+    cursor.advance_checked(4);
 
-    unsafe {
-        rbuf.unfilled().set_init(8);
-    }
+    assert_eq!(unsafe { cursor.as_mut().len() }, 12);
 
-    assert_eq!(rbuf.init_len(), 12);
-    assert_eq!(rbuf.unfilled().init_mut().len(), 8);
-    assert_eq!(unsafe { rbuf.unfilled().as_mut().len() }, 12);
+    assert!(rbuf.is_init());
 }
 
 #[test]
@@ -173,26 +149,26 @@ fn cursor_with_unfilled_buf() {
         assert_eq!(buf.filled(), &[1, 2, 3]);
     });
 
-    assert_eq!(cursor.init_mut().len(), 0);
+    assert!(!cursor.is_init());
     assert_eq!(cursor.written(), 3);
 
     cursor.with_unfilled_buf(|buf| {
         assert_eq!(buf.capacity(), 13);
-        assert_eq!(buf.init_len(), 0);
+        assert!(!buf.is_init());
 
         buf.unfilled().ensure_init();
-        buf.unfilled().advance(4);
+        buf.unfilled().advance_checked(4);
     });
 
-    assert_eq!(cursor.init_mut().len(), 9);
+    assert!(cursor.is_init());
     assert_eq!(cursor.written(), 7);
 
     cursor.with_unfilled_buf(|buf| {
         assert_eq!(buf.capacity(), 9);
-        assert_eq!(buf.init_len(), 9);
+        assert!(buf.is_init());
     });
 
-    assert_eq!(cursor.init_mut().len(), 9);
+    assert!(cursor.is_init());
     assert_eq!(cursor.written(), 7);
 
     assert_eq!(rbuf.filled(), &[1, 2, 3, 0, 0, 0, 0]);
diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
index 0387b442562db..762baea57b881 100644
--- a/library/coretests/tests/lib.rs
+++ b/library/coretests/tests/lib.rs
@@ -11,6 +11,7 @@
 #![feature(async_iterator)]
 #![feature(bigint_helper_methods)]
 #![feature(bool_to_result)]
+#![feature(borrowed_buf_init)]
 #![feature(bstr)]
 #![feature(cfg_target_has_reliable_f16_f128)]
 #![feature(char_internals)]
diff --git a/library/std/src/fs/tests.rs b/library/std/src/fs/tests.rs
index 0a5d1153d860c..8838fe3be8f09 100644
--- a/library/std/src/fs/tests.rs
+++ b/library/std/src/fs/tests.rs
@@ -710,7 +710,7 @@ fn file_test_read_buf() {
     check!(file.read_buf(buf.unfilled()));
     assert_eq!(buf.filled(), &[1, 2, 3, 4]);
     // File::read_buf should omit buffer initialization.
-    assert_eq!(buf.init_len(), 4);
+    assert!(!buf.is_init());
 
     check!(fs::remove_file(filename));
 }
diff --git a/library/std/src/io/buffered/bufreader.rs b/library/std/src/io/buffered/bufreader.rs
index 40441dc057d0d..ac461f4f0307e 100644
--- a/library/std/src/io/buffered/bufreader.rs
+++ b/library/std/src/io/buffered/bufreader.rs
@@ -288,7 +288,7 @@ impl<R: ?Sized> BufReader<R> {
 #[cfg(test)]
 impl<R: ?Sized> BufReader<R> {
     #[allow(missing_docs)]
-    pub fn initialized(&self) -> usize {
+    pub fn initialized(&self) -> bool {
         self.buf.initialized()
     }
 }
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 9b600cd55758b..55fe33aaa800b 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -26,20 +26,20 @@ pub struct Buffer {
     // defensive initialization as possible. Note that while this often the same as `filled`, it
     // doesn't need to be. Calls to `fill_buf` are not required to actually fill the buffer, and
     // omitting this is a huge perf regression for `Read` impls that do not.
-    initialized: usize,
+    initialized: bool,
 }
 
 impl Buffer {
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
         let buf = Box::new_uninit_slice(capacity);
-        Self { buf, pos: 0, filled: 0, initialized: 0 }
+        Self { buf, pos: 0, filled: 0, initialized: false }
     }
 
     #[inline]
     pub fn try_with_capacity(capacity: usize) -> io::Result<Self> {
         match Box::try_new_uninit_slice(capacity) {
-            Ok(buf) => Ok(Self { buf, pos: 0, filled: 0, initialized: 0 }),
+            Ok(buf) => Ok(Self { buf, pos: 0, filled: 0, initialized: false }),
             Err(_) => {
                 Err(io::const_error!(ErrorKind::OutOfMemory, "failed to allocate read buffer"))
             }
@@ -70,7 +70,7 @@ impl Buffer {
 
     // This is only used by a test which asserts that the initialization-tracking is correct.
     #[cfg(test)]
-    pub fn initialized(&self) -> usize {
+    pub fn initialized(&self) -> bool {
         self.initialized
     }
 
@@ -110,13 +110,14 @@ impl Buffer {
     /// Read more bytes into the buffer without discarding any of its contents
     pub fn read_more(&mut self, mut reader: impl Read) -> io::Result<usize> {
         let mut buf = BorrowedBuf::from(&mut self.buf[self.filled..]);
-        let old_init = self.initialized - self.filled;
-        unsafe {
-            buf.set_init(old_init);
+
+        if self.initialized {
+            unsafe { buf.set_init() };
         }
+
         reader.read_buf(buf.unfilled())?;
         self.filled += buf.len();
-        self.initialized += buf.init_len() - old_init;
+        self.initialized = buf.is_init();
         Ok(buf.len())
     }
 
@@ -138,15 +139,16 @@ impl Buffer {
 
             let mut buf = BorrowedBuf::from(&mut *self.buf);
             // SAFETY: `self.filled` bytes will always have been initialized.
-            unsafe {
-                buf.set_init(self.initialized);
+
+            if self.initialized {
+                unsafe { buf.set_init() };
             }
 
             let result = reader.read_buf(buf.unfilled());
 
             self.pos = 0;
             self.filled = buf.len();
-            self.initialized = buf.init_len();
+            self.initialized = buf.is_init();
 
             result?;
         }
diff --git a/library/std/src/io/buffered/tests.rs b/library/std/src/io/buffered/tests.rs
index 6ad4158b92904..58be268573e66 100644
--- a/library/std/src/io/buffered/tests.rs
+++ b/library/std/src/io/buffered/tests.rs
@@ -1067,13 +1067,13 @@ fn bufreader_full_initialize() {
     }
     let mut reader = BufReader::new(OneByteReader);
     // Nothing is initialized yet.
-    assert_eq!(reader.initialized(), 0);
+    assert!(!reader.initialized());
 
     let buf = reader.fill_buf().unwrap();
     // We read one byte...
     assert_eq!(buf.len(), 1);
     // But we initialized the whole buffer!
-    assert_eq!(reader.initialized(), reader.capacity());
+    assert!(reader.initialized());
 }
 
 /// This is a regression test for https://github.com/rust-lang/rust/issues/127584.
diff --git a/library/std/src/io/copy.rs b/library/std/src/io/copy.rs
index 2b558efb8885e..0f3f890a964ad 100644
--- a/library/std/src/io/copy.rs
+++ b/library/std/src/io/copy.rs
@@ -214,15 +214,15 @@ impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
         }
 
         let mut len = 0;
-        let mut init = 0;
+        let mut init = false;
 
         loop {
             let buf = self.buffer_mut();
             let mut read_buf: BorrowedBuf<'_> = buf.spare_capacity_mut().into();
 
-            unsafe {
+            if init {
                 // SAFETY: init is either 0 or the init_len from the previous iteration.
-                read_buf.set_init(init);
+                unsafe { read_buf.set_init() };
             }
 
             if read_buf.capacity() >= DEFAULT_BUF_SIZE {
@@ -235,7 +235,7 @@ impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
                             return Ok(len);
                         }
 
-                        init = read_buf.init_len() - bytes_read;
+                        init = read_buf.is_init();
                         len += bytes_read as u64;
 
                         // SAFETY: BorrowedBuf guarantees all of its filled bytes are init
@@ -248,10 +248,6 @@ impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
                     Err(e) => return Err(e),
                 }
             } else {
-                // All the bytes that were already in the buffer are initialized,
-                // treat them as such when the buffer is flushed.
-                init += buf.len();
-
                 self.flush_buf()?;
             }
         }
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index b7756befa11e9..4442c4ccb0648 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -419,8 +419,6 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
         .and_then(|s| s.checked_add(1024)?.checked_next_multiple_of(DEFAULT_BUF_SIZE))
         .unwrap_or(DEFAULT_BUF_SIZE);
 
-    let mut initialized = 0; // Extra initialized bytes from previous loop iteration
-
     const PROBE_SIZE: usize = 32;
 
     fn small_probe_read<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
@@ -449,8 +447,6 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
         }
     }
 
-    let mut consecutive_short_reads = 0;
-
     loop {
         if buf.len() == buf.capacity() && buf.capacity() == start_cap {
             // The buffer might be an exact fit. Let's read into a probe buffer
@@ -474,11 +470,8 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
         spare = &mut spare[..buf_len];
         let mut read_buf: BorrowedBuf<'_> = spare.into();
 
-        // SAFETY: These bytes were initialized but not filled in the previous loop
-        unsafe {
-            read_buf.set_init(initialized);
-        }
-
+        // Note that we don't track already initialized bytes here, but this is fine
+        // because we explicitly limit the read size
         let mut cursor = read_buf.unfilled();
         let result = loop {
             match r.read_buf(cursor.reborrow()) {
@@ -489,9 +482,8 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
             }
         };
 
-        let unfilled_but_initialized = cursor.init_mut().len();
         let bytes_read = cursor.written();
-        let was_fully_initialized = read_buf.init_len() == buf_len;
+        let is_init = read_buf.is_init();
 
         // SAFETY: BorrowedBuf's invariants mean this much memory is initialized.
         unsafe {
@@ -506,15 +498,6 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
             return Ok(buf.len() - start_len);
         }
 
-        if bytes_read < buf_len {
-            consecutive_short_reads += 1;
-        } else {
-            consecutive_short_reads = 0;
-        }
-
-        // store how much was initialized but not filled
-        initialized = unfilled_but_initialized;
-
         // Use heuristics to determine the max read size if no initial size hint was provided
         if size_hint.is_none() {
             // The reader is returning short reads but it doesn't call ensure_init().
@@ -523,13 +506,12 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
             // When reading from disk we usually don't get any short reads except at EOF.
             // So we wait for at least 2 short reads before uncapping the read buffer;
             // this helps with the Windows issue.
-            if !was_fully_initialized && consecutive_short_reads > 1 {
+            if !is_init {
                 max_read_size = usize::MAX;
             }
-
             // we have passed a larger buffer than previously and the
             // reader still hasn't returned a short read
-            if buf_len >= max_read_size && bytes_read == buf_len {
+            else if buf_len >= max_read_size && bytes_read == buf_len {
                 max_read_size = max_read_size.saturating_mul(2);
             }
         }
@@ -587,8 +569,8 @@ pub(crate) fn default_read_buf<F>(read: F, mut cursor: BorrowedCursor<'_>) -> Re
 where
     F: FnOnce(&mut [u8]) -> Result<usize>,
 {
-    let n = read(cursor.ensure_init().init_mut())?;
-    cursor.advance(n);
+    let n = read(cursor.ensure_init())?;
+    cursor.advance_checked(n);
     Ok(())
 }
 
@@ -3098,7 +3080,7 @@ 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_mut().len());
+            let is_init = buf.is_init();
 
             // SAFETY: no uninit data is written to ibuf
             let ibuf = unsafe { &mut buf.as_mut()[..limit] };
@@ -3106,23 +3088,32 @@ impl<T: Read> Read for Take<T> {
             let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();
 
             // SAFETY: extra_init bytes of ibuf are known to be initialized
-            unsafe {
-                sliced_buf.set_init(extra_init);
+            if is_init {
+                unsafe { sliced_buf.set_init() };
             }
 
             let mut cursor = sliced_buf.unfilled();
             let result = self.inner.read_buf(cursor.reborrow());
 
-            let new_init = cursor.init_mut().len();
+            let should_init = cursor.is_init();
             let filled = sliced_buf.len();
 
             // cursor / sliced_buf / ibuf 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.
+                unsafe { buf.set_init() };
+            }
+
             unsafe {
-                // SAFETY: filled bytes have been filled and therefore initialized
-                buf.advance_unchecked(filled);
-                // SAFETY: new_init bytes of buf's unfilled buffer have been initialized
-                buf.set_init(new_init);
+                // SAFETY: filled bytes have been filled
+                buf.advance(filled);
             }
 
             self.limit -= filled as u64;
diff --git a/library/std/src/io/tests.rs b/library/std/src/io/tests.rs
index b22988d4a8a9d..00e52e6f33e62 100644
--- a/library/std/src/io/tests.rs
+++ b/library/std/src/io/tests.rs
@@ -214,8 +214,8 @@ fn read_buf_exact() {
 fn borrowed_cursor_advance_overflow() {
     let mut buf = [0; 512];
     let mut buf = BorrowedBuf::from(&mut buf[..]);
-    buf.unfilled().advance(1);
-    buf.unfilled().advance(usize::MAX);
+    buf.unfilled().advance_checked(1);
+    buf.unfilled().advance_checked(usize::MAX);
 }
 
 #[test]
diff --git a/library/std/src/io/util.rs b/library/std/src/io/util.rs
index 0410df3ef1a3e..a09c8bc069306 100644
--- a/library/std/src/io/util.rs
+++ b/library/std/src/io/util.rs
@@ -283,7 +283,7 @@ impl Read for Repeat {
         // SAFETY: No uninit bytes are being written.
         unsafe { buf.as_mut() }.write_filled(self.byte);
         // SAFETY: the entire unfilled portion of buf has been initialized.
-        unsafe { buf.advance_unchecked(buf.capacity()) };
+        unsafe { buf.advance(buf.capacity()) };
         Ok(())
     }
 
diff --git a/library/std/src/io/util/tests.rs b/library/std/src/io/util/tests.rs
index d0f106d7af416..fa1193f2b00fa 100644
--- a/library/std/src/io/util/tests.rs
+++ b/library/std/src/io/util/tests.rs
@@ -75,43 +75,43 @@ fn empty_reads() {
     let mut buf: BorrowedBuf<'_> = buf.into();
     e.read_buf(buf.unfilled()).unwrap();
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [_] = &mut [MaybeUninit::uninit()];
     let mut buf: BorrowedBuf<'_> = buf.into();
     e.read_buf(buf.unfilled()).unwrap();
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1024];
     let mut buf: BorrowedBuf<'_> = buf.into();
     e.read_buf(buf.unfilled()).unwrap();
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1024];
     let mut buf: BorrowedBuf<'_> = buf.into();
     Read::by_ref(&mut e).read_buf(buf.unfilled()).unwrap();
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [MaybeUninit<_>] = &mut [];
     let mut buf: BorrowedBuf<'_> = buf.into();
     e.read_buf_exact(buf.unfilled()).unwrap();
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [_] = &mut [MaybeUninit::uninit()];
     let mut buf: BorrowedBuf<'_> = buf.into();
     assert_eq!(e.read_buf_exact(buf.unfilled()).unwrap_err().kind(), ErrorKind::UnexpectedEof);
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1024];
     let mut buf: BorrowedBuf<'_> = buf.into();
     assert_eq!(e.read_buf_exact(buf.unfilled()).unwrap_err().kind(), ErrorKind::UnexpectedEof);
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1024];
     let mut buf: BorrowedBuf<'_> = buf.into();
@@ -120,7 +120,7 @@ fn empty_reads() {
         ErrorKind::UnexpectedEof,
     );
     assert_eq!(buf.len(), 0);
-    assert_eq!(buf.init_len(), 0);
+    assert!(!buf.is_init());
 
     let mut buf = Vec::new();
     assert_eq!(e.read_to_end(&mut buf).unwrap(), 0);
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 8fb1b1b05d20c..1aa79ebcccc62 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -316,6 +316,7 @@
 //
 // Library features (core):
 // tidy-alphabetical-start
+#![feature(borrowed_buf_init)]
 #![feature(bstr)]
 #![feature(bstr_internals)]
 #![feature(cast_maybe_uninit)]
diff --git a/library/std/src/net/tcp/tests.rs b/library/std/src/net/tcp/tests.rs
index 7c7ef7b2f7018..80d2aa6e58307 100644
--- a/library/std/src/net/tcp/tests.rs
+++ b/library/std/src/net/tcp/tests.rs
@@ -316,7 +316,7 @@ fn read_buf() {
         t!(s.read_buf(buf.unfilled()));
         assert_eq!(buf.filled(), &[1, 2, 3, 4]);
         // TcpStream::read_buf should omit buffer initialization.
-        assert_eq!(buf.init_len(), 4);
+        assert!(!buf.is_init());
 
         t.join().ok().expect("thread panicked");
     })
diff --git a/library/std/src/process/tests.rs b/library/std/src/process/tests.rs
index 12c5130defe5a..68c62a861075f 100644
--- a/library/std/src/process/tests.rs
+++ b/library/std/src/process/tests.rs
@@ -188,10 +188,10 @@ fn child_stdout_read_buf() {
     // ChildStdout::read_buf should omit buffer initialization.
     if cfg!(target_os = "windows") {
         assert_eq!(buf.filled(), b"abc\r\n");
-        assert_eq!(buf.init_len(), 5);
+        assert!(!buf.is_init());
     } else {
         assert_eq!(buf.filled(), b"abc\n");
-        assert_eq!(buf.init_len(), 4);
+        assert!(!buf.is_init());
     };
 }
 
diff --git a/library/std/src/sys/fd/hermit.rs b/library/std/src/sys/fd/hermit.rs
index 2666da16420c4..28fafdaf57d8a 100644
--- a/library/std/src/sys/fd/hermit.rs
+++ b/library/std/src/sys/fd/hermit.rs
@@ -33,7 +33,7 @@ impl FileDesc {
             )
         })?;
         // SAFETY: Exactly `result` bytes have been filled.
-        unsafe { buf.advance_unchecked(result as usize) };
+        unsafe { buf.advance(result as usize) };
         Ok(())
     }
 
diff --git a/library/std/src/sys/fd/unix.rs b/library/std/src/sys/fd/unix.rs
index bb6c0ac9e18e6..c5e8646dada1e 100644
--- a/library/std/src/sys/fd/unix.rs
+++ b/library/std/src/sys/fd/unix.rs
@@ -185,7 +185,7 @@ impl FileDesc {
 
         // SAFETY: `ret` bytes were written to the initialized portion of the buffer
         unsafe {
-            cursor.advance_unchecked(ret as usize);
+            cursor.advance(ret as usize);
         }
         Ok(())
     }
@@ -203,7 +203,7 @@ impl FileDesc {
 
         // SAFETY: `ret` bytes were written to the initialized portion of the buffer
         unsafe {
-            cursor.advance_unchecked(ret as usize);
+            cursor.advance(ret as usize);
         }
         Ok(())
     }
diff --git a/library/std/src/sys/fs/solid.rs b/library/std/src/sys/fs/solid.rs
index f6d5d3b784d3b..ec1db262855ad 100644
--- a/library/std/src/sys/fs/solid.rs
+++ b/library/std/src/sys/fs/solid.rs
@@ -401,7 +401,7 @@ impl File {
 
             // Safety: `num_bytes_read` bytes were written to the unfilled
             // portion of the buffer
-            cursor.advance_unchecked(num_bytes_read);
+            cursor.advance(num_bytes_read);
 
             Ok(())
         }
diff --git a/library/std/src/sys/net/connection/socket/hermit.rs b/library/std/src/sys/net/connection/socket/hermit.rs
index c32f8dcc8de86..8350d2b5fe4a0 100644
--- a/library/std/src/sys/net/connection/socket/hermit.rs
+++ b/library/std/src/sys/net/connection/socket/hermit.rs
@@ -143,7 +143,7 @@ impl Socket {
             )
         })?;
         unsafe {
-            buf.advance_unchecked(ret as usize);
+            buf.advance(ret as usize);
         }
         Ok(())
     }
diff --git a/library/std/src/sys/net/connection/socket/solid.rs b/library/std/src/sys/net/connection/socket/solid.rs
index 673d75046d3f2..ac06cdc00c8f0 100644
--- a/library/std/src/sys/net/connection/socket/solid.rs
+++ b/library/std/src/sys/net/connection/socket/solid.rs
@@ -190,7 +190,7 @@ impl Socket {
             netc::recv(self.as_raw_fd(), buf.as_mut().as_mut_ptr().cast(), buf.capacity(), flags)
         })?;
         unsafe {
-            buf.advance_unchecked(ret as usize);
+            buf.advance(ret as usize);
         }
         Ok(())
     }
diff --git a/library/std/src/sys/net/connection/socket/unix.rs b/library/std/src/sys/net/connection/socket/unix.rs
index 6d06a8d86bf16..323d6214347e7 100644
--- a/library/std/src/sys/net/connection/socket/unix.rs
+++ b/library/std/src/sys/net/connection/socket/unix.rs
@@ -294,7 +294,7 @@ impl Socket {
             )
         })?;
         unsafe {
-            buf.advance_unchecked(ret as usize);
+            buf.advance(ret as usize);
         }
         Ok(())
     }
diff --git a/library/std/src/sys/net/connection/socket/windows.rs b/library/std/src/sys/net/connection/socket/windows.rs
index 7355f0ce6bf5e..4da51d78ea69b 100644
--- a/library/std/src/sys/net/connection/socket/windows.rs
+++ b/library/std/src/sys/net/connection/socket/windows.rs
@@ -243,7 +243,7 @@ impl Socket {
                 }
             }
             _ => {
-                unsafe { buf.advance_unchecked(result as usize) };
+                unsafe { buf.advance(result as usize) };
                 Ok(())
             }
         }
diff --git a/library/std/src/sys/pal/sgx/abi/usercalls/mod.rs b/library/std/src/sys/pal/sgx/abi/usercalls/mod.rs
index 5041770faf661..f1e4a5a42577a 100644
--- a/library/std/src/sys/pal/sgx/abi/usercalls/mod.rs
+++ b/library/std/src/sys/pal/sgx/abi/usercalls/mod.rs
@@ -46,7 +46,7 @@ pub fn read_buf(fd: Fd, mut buf: BorrowedCursor<'_>) -> IoResult<()> {
         let mut userbuf = alloc::User::<[u8]>::uninitialized(buf.capacity());
         let len = raw::read(fd, userbuf.as_mut_ptr().cast(), userbuf.len()).from_sgx_result()?;
         userbuf[..len].copy_to_enclave(&mut buf.as_mut()[..len]);
-        buf.advance_unchecked(len);
+        buf.advance(len);
         Ok(())
     }
 }
diff --git a/library/std/src/sys/pal/windows/handle.rs b/library/std/src/sys/pal/windows/handle.rs
index 90e243e1aa038..ffa8507831acf 100644
--- a/library/std/src/sys/pal/windows/handle.rs
+++ b/library/std/src/sys/pal/windows/handle.rs
@@ -117,7 +117,7 @@ impl Handle {
             Ok(read) => {
                 // Safety: `read` bytes were written to the initialized portion of the buffer
                 unsafe {
-                    cursor.advance_unchecked(read);
+                    cursor.advance(read);
                 }
                 Ok(())
             }
@@ -140,7 +140,7 @@ impl Handle {
 
         // SAFETY: `read` bytes were written to the initialized portion of the buffer
         unsafe {
-            cursor.advance_unchecked(read);
+            cursor.advance(read);
         }
         Ok(())
     }
diff --git a/library/std/src/sys/process/windows/child_pipe.rs b/library/std/src/sys/process/windows/child_pipe.rs
index da7a86ca072e3..b848435ac275f 100644
--- a/library/std/src/sys/process/windows/child_pipe.rs
+++ b/library/std/src/sys/process/windows/child_pipe.rs
@@ -260,7 +260,7 @@ impl ChildPipe {
             Err(e) => Err(e),
             Ok(n) => {
                 unsafe {
-                    buf.advance_unchecked(n);
+                    buf.advance(n);
                 }
                 Ok(())
             }
diff --git a/library/std/src/sys/stdio/zkvm.rs b/library/std/src/sys/stdio/zkvm.rs
index f31c6c26e87cd..84496ac937363 100644
--- a/library/std/src/sys/stdio/zkvm.rs
+++ b/library/std/src/sys/stdio/zkvm.rs
@@ -19,7 +19,7 @@ impl io::Read for Stdin {
     fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> io::Result<()> {
         unsafe {
             let n = abi::sys_read(fileno::STDIN, buf.as_mut().as_mut_ptr().cast(), buf.capacity());
-            buf.advance_unchecked(n);
+            buf.advance(n);
         }
         Ok(())
     }