Auto merge of #23206 - nagisa:print-io, r=alexcrichton

r? @alexcrichton or @aturon 

This still needs to somehow figure out how to avoid unstable warnings arising from the use of unstable functions. I tried to use `#[allow_internal_unstable]` but it still spits out warnings as far as I can see. @huonw (I think you implemented it) does `#[allow_internal_unstable]` not work for some reason or am I using it incorrectly?

4 files changed, 58 insertions, 27 deletions

diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index 821a0a0b06e..35ef375174a 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -33,10 +33,10 @@ pub use self::buffered::IntoInnerError;
 pub use self::cursor::Cursor;
 pub use self::error::{Result, Error, ErrorKind};
 pub use self::util::{copy, sink, Sink, empty, Empty, repeat, Repeat};
-pub use self::stdio::{stdin, stdout, stderr, Stdin, Stdout, Stderr};
+pub use self::stdio::{stdin, stdout, stderr, _print, Stdin, Stdout, Stderr};
 pub use self::stdio::{StdoutLock, StderrLock, StdinLock};
 #[doc(no_inline, hidden)]
-pub use self::stdio::set_panic;
+pub use self::stdio::{set_panic, set_print};
 
 #[macro_use] mod lazy;
 
diff --git a/src/libstd/io/stdio.rs b/src/libstd/io/stdio.rs
index 0e68be8d9e2..75d047d5c9c 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -11,6 +11,7 @@
 use prelude::v1::*;
 use io::prelude::*;
 
+use cell::RefCell;
 use cmp;
 use fmt;
 use io::lazy::Lazy;
@@ -18,6 +19,13 @@ use io::{self, BufReader, LineWriter};
 use sync::{Arc, Mutex, MutexGuard};
 use sys::stdio;
 
+/// Stdout used by print! and println! macroses
+thread_local! {
+    static LOCAL_STDOUT: RefCell<Option<Box<Write + Send>>> = {
+        RefCell::new(None)
+    }
+}
+
 /// A handle to a raw instance of the standard input stream of this process.
 ///
 /// This handle is not synchronized or buffered in any fashion. Constructed via
@@ -338,15 +346,15 @@ impl<'a> Write for StderrLock<'a> {
     fn flush(&mut self) -> io::Result<()> { self.inner.flush() }
 }
 
-/// Resets the task-local stdout handle to the specified writer
+/// Resets the task-local stderr handle to the specified writer
 ///
-/// This will replace the current task's stdout handle, returning the old
-/// handle. All future calls to `print` and friends will emit their output to
+/// This will replace the current task's stderr handle, returning the old
+/// handle. All future calls to `panic!` and friends will emit their output to
 /// this specified handle.
 ///
 /// Note that this does not need to be called for all new tasks; the default
-/// output handle is to the process's stdout stream.
-#[unstable(feature = "set_panic",
+/// output handle is to the process's stderr stream.
+#[unstable(feature = "set_stdio",
            reason = "this function may disappear completely or be replaced \
                      with a more general mechanism")]
 #[doc(hidden)]
@@ -360,3 +368,37 @@ pub fn set_panic(sink: Box<Write + Send>) -> Option<Box<Write + Send>> {
         Some(s)
     })
 }
+
+/// Resets the task-local stdout handle to the specified writer
+///
+/// This will replace the current task's stdout handle, returning the old
+/// handle. All future calls to `print!` and friends will emit their output to
+/// this specified handle.
+///
+/// Note that this does not need to be called for all new tasks; the default
+/// output handle is to the process's stdout stream.
+#[unstable(feature = "set_stdio",
+           reason = "this function may disappear completely or be replaced \
+                     with a more general mechanism")]
+#[doc(hidden)]
+pub fn set_print(sink: Box<Write + Send>) -> Option<Box<Write + Send>> {
+    use mem;
+    LOCAL_STDOUT.with(move |slot| {
+        mem::replace(&mut *slot.borrow_mut(), Some(sink))
+    }).and_then(|mut s| {
+        let _ = s.flush();
+        Some(s)
+    })
+}
+
+#[unstable(feature = "print",
+           reason = "implementation detail which may disappear or be replaced at any time")]
+#[doc(hidden)]
+pub fn _print(args: fmt::Arguments) {
+    if let Err(e) = LOCAL_STDOUT.with(|s| match s.borrow_mut().as_mut() {
+        Some(w) => w.write_fmt(args),
+        None => stdout().write_fmt(args)
+    }) {
+        panic!("failed printing to stdout: {}", e);
+    }
+}
diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index a5022943267..7378f43f757 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -60,19 +60,21 @@ macro_rules! panic {
     });
 }
 
+/// Macro for printing to the standard output.
+///
 /// Equivalent to the `println!` macro except that a newline is not printed at
 /// the end of the message.
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[allow_internal_unstable]
 macro_rules! print {
-    ($($arg:tt)*) => ($crate::old_io::stdio::print_args(format_args!($($arg)*)))
+    ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
 }
 
-/// Macro for printing to a task's stdout handle.
+/// Macro for printing to the standard output.
 ///
-/// Each task can override its stdout handle via `std::old_io::stdio::set_stdout`.
-/// The syntax of this macro is the same as that used for `format!`. For more
-/// information, see `std::fmt` and `std::old_io::stdio`.
+/// Use the `format!` syntax to write data to the standard output.
+/// See `std::fmt` for more information.
 ///
 /// # Examples
 ///
@@ -83,7 +85,8 @@ macro_rules! print {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! println {
-    ($($arg:tt)*) => ($crate::old_io::stdio::println_args(format_args!($($arg)*)))
+    ($fmt:expr) => (print!(concat!($fmt, "\n")));
+    ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
 }
 
 /// Helper macro for unwrapping `Result` values while returning early with an
diff --git a/src/libstd/old_io/stdio.rs b/src/libstd/old_io/stdio.rs
index 34b4ec94a44..dcc34505730 100644
--- a/src/libstd/old_io/stdio.rs
+++ b/src/libstd/old_io/stdio.rs
@@ -535,18 +535,4 @@ mod tests {
         stdout();
         stderr();
     }
-
-    #[test]
-    fn capture_stdout() {
-        use old_io::{ChanReader, ChanWriter};
-
-        let (tx, rx) = channel();
-        let (mut r, w) = (ChanReader::new(rx), ChanWriter::new(tx));
-        // FIXME (#22405): Replace `Box::new` with `box` here when/if possible.
-        let _t = thread::spawn(move|| {
-            set_stdout(Box::new(w));
-            println!("hello!");
-        });
-        assert_eq!(r.read_to_string().unwrap(), "hello!\n");
-    }
 }