Auto merge of rust-lang#103754 - SUPERCILEX:filled-mut, r=m-ou-se

Add back BorrowedBuf::filled_mut

This is useful if you want to do some processing on the bytes while still using the BorrowedBuf.

The API was removed in rust-lang#97015 with no explanation. The RFC also has it as part of its API, so this just seems like a mistake: [RFC](https://rust-lang.github.io/rfcs/2930-read-buf.html#:~:text=inline%5D%0A%20%20%20%20pub%20fn-,filled_mut,-(%26mut%20self))

ACP: rust-lang/libs-team#139

library/std/src/io/readbuf.rs

@@ -99,6 +99,13 @@ impl<'data> BorrowedBuf<'data> {
         unsafe { MaybeUninit::slice_assume_init_ref(&self.buf[0..self.filled]) }
     }
 
+    /// Returns a mutable reference to the filled portion of the buffer.
+    #[inline]
+    pub fn filled_mut(&mut self) -> &mut [u8] {
+        // SAFETY: We only slice the filled part of the buffer, which is always valid
+        unsafe { MaybeUninit::slice_assume_init_mut(&mut self.buf[0..self.filled]) }
+    }
+
     /// Returns a cursor over the unfilled part of the buffer.
     #[inline]
     pub fn unfilled<'this>(&'this mut self) -> BorrowedCursor<'this> {