Rollup merge of #138301 - thaliaarchi:io-optional-methods/hermit, r=tgross35

Implement `read_buf` for Hermit

Following https://github.com/hermit-os/kernel/pull/1606, it is now safe to implement `Read::read_buf` for file descriptors on Hermit.

cc ```@mkroening```

1 files changed, 16 insertions, 1 deletions

diff --git a/library/std/src/sys/pal/hermit/fd.rs b/library/std/src/sys/pal/hermit/fd.rs
index 3d6b99cd77b..edd984d920a 100644
--- a/library/std/src/sys/pal/hermit/fd.rs
+++ b/library/std/src/sys/pal/hermit/fd.rs
@@ -2,7 +2,7 @@
 
 use super::hermit_abi;
 use crate::cmp;
-use crate::io::{self, IoSlice, IoSliceMut, Read, SeekFrom};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, Read, SeekFrom};
 use crate::os::hermit::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
 use crate::sys::{cvt, unsupported};
 use crate::sys_common::{AsInner, FromInner, IntoInner};
@@ -23,6 +23,21 @@ impl FileDesc {
         Ok(result as usize)
     }
 
+    pub fn read_buf(&self, mut buf: BorrowedCursor<'_>) -> io::Result<()> {
+        // SAFETY: The `read` syscall does not read from the buffer, so it is
+        // safe to use `&mut [MaybeUninit<u8>]`.
+        let result = cvt(unsafe {
+            hermit_abi::read(
+                self.fd.as_raw_fd(),
+                buf.as_mut().as_mut_ptr() as *mut u8,
+                buf.capacity(),
+            )
+        })?;
+        // SAFETY: Exactly `result` bytes have been filled.
+        unsafe { buf.advance_unchecked(result as usize) };
+        Ok(())
+    }
+
     pub fn read_vectored(&self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         let ret = cvt(unsafe {
             hermit_abi::readv(