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

Implement `read_buf` for Hermit

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

cc ```@mkroening```

Author: matthiaskrgr

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

@@ -396,7 +396,7 @@ impl File {
     }
 
     pub fn read_buf(&self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
-        crate::io::default_read_buf(|buf| self.read(buf), cursor)
+        self.0.read_buf(cursor)
     }
 
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {

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(

library/std/src/sys/stdio/unix.rs

@@ -3,9 +3,7 @@ use hermit_abi::{EBADF, STDERR_FILENO, STDIN_FILENO, STDOUT_FILENO};
 #[cfg(target_family = "unix")]
 use libc::{EBADF, STDERR_FILENO, STDIN_FILENO, STDOUT_FILENO};
 
-#[cfg(target_family = "unix")]
-use crate::io::BorrowedCursor;
-use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut};
 use crate::mem::ManuallyDrop;
 #[cfg(target_os = "hermit")]
 use crate::os::hermit::io::FromRawFd;
@@ -28,7 +26,6 @@ impl io::Read for Stdin {
         unsafe { ManuallyDrop::new(FileDesc::from_raw_fd(STDIN_FILENO)).read(buf) }
     }
 
-    #[cfg(not(target_os = "hermit"))]
     fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> io::Result<()> {
         unsafe { ManuallyDrop::new(FileDesc::from_raw_fd(STDIN_FILENO)).read_buf(buf) }
     }