Rollup merge of rust-lang#137353 - thaliaarchi:io-optional-methods/wasi-stdin, r=alexcrichton

Implement `read_buf` for WASI stdin

`WasiFd::read_buf` already exists. Simply use it in `Stdin`.

cc `@alexcrichton`

Tracked in rust-lang#136756

Author: matthiaskrgr

std/src/sys/pal/wasi/stdio.rs

@@ -1,7 +1,7 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
 use super::fd::WasiFd;
-use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut};
 use crate::mem::ManuallyDrop;
 use crate::os::raw;
 use crate::os::wasi::io::{AsRawFd, FromRawFd};
@@ -28,6 +28,10 @@ impl io::Read for Stdin {
         self.read_vectored(&mut [IoSliceMut::new(data)])
     }
 
+    fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> io::Result<()> {
+        ManuallyDrop::new(unsafe { WasiFd::from_raw_fd(self.as_raw_fd()) }).read_buf(buf)
+    }
+
     fn read_vectored(&mut self, data: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         ManuallyDrop::new(unsafe { WasiFd::from_raw_fd(self.as_raw_fd()) }).read(data)
     }
@@ -64,6 +68,7 @@ impl io::Write for Stdout {
     fn is_write_vectored(&self) -> bool {
         true
     }
+
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }