Flush to stdout from FileDescriptor::write for Stdout

Also, remove unnecessary `-Zmiri-disable-isolation` in test

src/shims/posix/foreign_items.rs

@@ -1,5 +1,3 @@
-use std::io::{self, Write};
-
 use log::trace;
 
 use rustc_middle::mir;
@@ -76,9 +74,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let count = this.read_scalar(n)?.to_machine_usize(this)?;
                 trace!("Called write({:?}, {:?}, {:?})", fd, buf, count);
                 let result = this.write(fd, buf, count)?;
-                if fd == 1 {
-                    io::stdout().flush().unwrap();
-                }
                 // Now, `result` is the value we return back to the program.
                 this.write_scalar(Scalar::from_machine_isize(result, this), dest)?;
             }

src/shims/posix/fs.rs

@@ -76,7 +76,15 @@ impl<'tcx> FileDescriptor<'tcx> for io::Stdout {
     }
 
     fn write(&mut self, bytes: &[u8]) -> InterpResult<'tcx, io::Result<usize>> {
-        Ok(Write::write(self, bytes))
+        let result = Write::write(self, bytes);
+        // Stdout is buffered, flush to make sure it appears on the
+        // screen.  This is the write() syscall of the interpreted
+        // program, we want it to correspond to a write() syscall on
+        // the host -- there is no good in adding extra buffering
+        // here.
+        io::stdout().flush().unwrap();
+
+        Ok(result)
     }
 
     fn seek(&mut self, _offset: SeekFrom) -> InterpResult<'tcx, io::Result<u64>> {

tests/compile-fail/fs/write_to_stdin.rs

@@ -1,4 +1,3 @@
-// compile-flags: -Zmiri-disable-isolation
 // ignore-windows: No libc on Windows
 
 #![feature(rustc_private)]