Change to non-line buffered output if output is not a TTY

This matches glibc behavior.

This is determined using the `isatty` function on Unixes, and not
attempted at all for other operating systems.

Fixes rust-lang#60673.

src/libstd/io/stdio.rs

@@ -84,6 +84,11 @@ impl Read for StdinRaw {
         Initializer::nop()
     }
 }
+impl StdoutRaw {
+    fn should_be_line_buffered(&self) -> bool {
+        self.0.should_be_line_buffered()
+    }
+}
 impl Write for StdoutRaw {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> { self.0.write(buf) }
 
@@ -496,9 +501,6 @@ impl fmt::Debug for StdinLock<'_> {
 /// [`io::stdout`]: fn.stdout.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stdout {
-    // FIXME: this should be LineWriter or BufWriter depending on the state of
-    //        stdout (tty or not). Note that if this is not line buffered it
-    //        should also flush-on-panic or some form of flush-on-abort.
     inner: Arc<ReentrantMutex<RefCell<StdioWriter<StdoutRaw>>>>,
 }
 
@@ -572,7 +574,14 @@ pub fn stdout() -> Stdout {
     fn stdout_init() -> Arc<ReentrantMutex<RefCell<StdioWriter<StdoutRaw>>>> {
         // This must not reentrantly access `INSTANCE`
         let stdout = match stdout_raw() {
-            Ok(stdout) => StdioWriter::new(stdout, StdioBufferKind::LineBuffered),
+            Ok(stdout) => {
+                let buffering = if stdout.should_be_line_buffered() {
+                    StdioBufferKind::LineBuffered
+                } else {
+                    StdioBufferKind::Buffered
+                };
+                StdioWriter::new(stdout, buffering)
+            },
             _ => StdioWriter::new_fake(),
         };
         Arc::new(ReentrantMutex::new(RefCell::new(stdout)))

src/libstd/sys/cloudabi/stdio.rs

@@ -21,6 +21,9 @@ impl Stdout {
     pub fn new() -> io::Result<Stdout> {
         Ok(Stdout(()))
     }
+    pub fn should_be_line_buffered(&self) -> bool {
+        true
+    }
 }
 
 impl io::Write for Stdout {

src/libstd/sys/sgx/stdio.rs

@@ -30,6 +30,10 @@ impl io::Read for Stdin {
 
 impl Stdout {
     pub fn new() -> io::Result<Stdout> { Ok(Stdout(())) }
+    pub fn should_be_line_buffered(&self) -> bool {
+        // FIXME: Implement me.
+        true
+    }
 }
 
 impl io::Write for Stdout {

src/libstd/sys/unix/stdio.rs

@@ -22,6 +22,11 @@ impl io::Read for Stdin {
 
 impl Stdout {
     pub fn new() -> io::Result<Stdout> { Ok(Stdout(())) }
+    pub fn should_be_line_buffered(&self) -> bool {
+        unsafe {
+            libc::isatty(libc::STDOUT_FILENO) != 0
+        }
+    }
 }
 
 impl io::Write for Stdout {

src/libstd/sys/vxworks/stdio.rs

@@ -20,6 +20,10 @@ impl io::Read for Stdin {
 
 impl Stdout {
     pub fn new() -> io::Result<Stdout> { Ok(Stdout(())) }
+    pub fn should_be_line_buffered(&self) -> bool {
+        // FIXME: Implement me.
+        true
+    }
 }
 
 impl io::Write for Stdout {

src/libstd/sys/wasi/stdio.rs

@@ -40,6 +40,12 @@ impl Stdout {
     pub fn flush(&self) -> io::Result<()> {
         Ok(())
     }
+
+    pub fn should_be_line_buffered(&self) -> bool {
+        // FIXME: Currently there seems to be no way to query whether stdout is
+        // a tty, `isatty` is not exposed by WASI.
+        true
+    }
 }
 
 impl Stderr {

src/libstd/sys/wasm/stdio.rs

@@ -30,6 +30,10 @@ impl io::Write for Stdout {
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }
+
+    fn should_be_line_buffered(&self) -> bool {
+        true
+    }
 }
 
 impl Stderr {

src/libstd/sys/windows/stdio.rs

@@ -246,6 +246,11 @@ impl Stdout {
     pub fn new() -> io::Result<Stdout> {
         Ok(Stdout)
     }
+    pub fn should_be_line_buffered(&self) -> bool {
+        // FIXME: Fill in. I don't know how to check whether output is
+        // redirected on Windows.
+        true
+    }
 }
 
 impl io::Write for Stdout {

src/libstd/sys/windows/stdio_uwp.rs

@@ -48,6 +48,11 @@ impl Stdout {
     pub fn new() -> io::Result<Stdout> {
         Ok(Stdout)
     }
+    pub fn should_be_line_buffered(&self) -> bool {
+        // FIXME: Fill in. I don't know how to check whether output is
+        // redirected on Windows.
+        true
+    }
 }
 
 impl io::Write for Stdout {