Rollup merge of #109722 - hermitcore:read, r=Mark-Simulacrum

Implement read_buf for RustHermit

In principle, this PR extends rust-lang/rust#108326 for RustyHermit.

library/std/src/sys/hermit/net.rs

@@ -1,7 +1,7 @@
 #![allow(dead_code)]
 
 use crate::cmp;
-use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{self, BorrowedBuf, BorrowedCursor, IoSlice, IoSliceMut};
 use crate::mem;
 use crate::net::{Shutdown, SocketAddr};
 use crate::os::hermit::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, RawFd};
@@ -146,18 +146,35 @@ impl Socket {
         Ok(Socket(unsafe { FileDesc::from_raw_fd(fd) }))
     }
 
-    fn recv_with_flags(&self, buf: &mut [u8], flags: i32) -> io::Result<usize> {
-        let ret =
-            cvt(unsafe { netc::recv(self.0.as_raw_fd(), buf.as_mut_ptr(), buf.len(), flags) })?;
-        Ok(ret as usize)
+    fn recv_with_flags(&self, mut buf: BorrowedCursor<'_>, flags: i32) -> io::Result<()> {
+        let ret = cvt(unsafe {
+            netc::recv(
+                self.0.as_raw_fd(),
+                buf.as_mut().as_mut_ptr() as *mut u8,
+                buf.capacity(),
+                flags,
+            )
+        })?;
+        unsafe {
+            buf.advance(ret as usize);
+        }
+        Ok(())
     }
 
     pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
-        self.recv_with_flags(buf, 0)
+        let mut buf = BorrowedBuf::from(buf);
+        self.recv_with_flags(buf.unfilled(), 0)?;
+        Ok(buf.len())
     }
 
     pub fn peek(&self, buf: &mut [u8]) -> io::Result<usize> {
-        self.recv_with_flags(buf, netc::MSG_PEEK)
+        let mut buf = BorrowedBuf::from(buf);
+        self.recv_with_flags(buf.unfilled(), netc::MSG_PEEK)?;
+        Ok(buf.len())
+    }
+
+    pub fn read_buf(&self, buf: BorrowedCursor<'_>) -> io::Result<()> {
+        self.recv_with_flags(buf, 0)
     }
 
     pub fn read_vectored(&self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {