Revert write! and writeln! to late drop temporaries

library/core/src/macros/mod.rs

@@ -496,10 +496,9 @@ macro_rules! r#try {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "write_macro")]
 macro_rules! write {
-    ($dst:expr, $($arg:tt)*) => {{
-        let result = $dst.write_fmt($crate::format_args!($($arg)*));
-        result
-    }};
+    ($dst:expr, $($arg:tt)*) => {
+        $dst.write_fmt($crate::format_args!($($arg)*))
+    };
 }
 
 /// Write formatted data into a buffer, with a newline appended.
@@ -554,10 +553,9 @@ macro_rules! writeln {
     ($dst:expr $(,)?) => {
         $crate::write!($dst, "\n")
     };
-    ($dst:expr, $($arg:tt)*) => {{
-        let result = $dst.write_fmt($crate::format_args_nl!($($arg)*));
-        result
-    }};
+    ($dst:expr, $($arg:tt)*) => {
+        $dst.write_fmt($crate::format_args_nl!($($arg)*))
+    };
 }
 
 /// Indicates unreachable code.

src/test/ui/macros/format-args-temporaries-async.rs

@@ -0,0 +1,37 @@
+// check-pass
+// edition:2021
+
+use std::fmt::{self, Display};
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+struct AsyncStdout;
+
+impl AsyncStdout {
+    fn write_fmt<'a>(&'a mut self, _args: fmt::Arguments) -> WriteFmtFuture<'a, Self>
+    where
+        Self: Unpin,
+    {
+        WriteFmtFuture(self)
+    }
+}
+
+struct WriteFmtFuture<'a, T>(&'a mut T);
+
+impl<'a, T> Future for WriteFmtFuture<'a, T> {
+    type Output = io::Result<()>;
+    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+        unimplemented!()
+    }
+}
+
+async fn async_main() {
+    let _write = write!(&mut AsyncStdout, "...").await;
+    let _writeln = writeln!(&mut AsyncStdout, "...").await;
+}
+
+fn main() {
+    let _ = async_main;
+}

src/test/ui/macros/format-args-temporaries-in-write.rs

@@ -0,0 +1,50 @@
+// check-fail
+
+use std::fmt::{self, Display};
+
+struct Mutex;
+
+impl Mutex {
+    fn lock(&self) -> MutexGuard {
+        MutexGuard(self)
+    }
+}
+
+struct MutexGuard<'a>(&'a Mutex);
+
+impl<'a> Drop for MutexGuard<'a> {
+    fn drop(&mut self) {
+        // Empty but this is a necessary part of the repro. Otherwise borrow
+        // checker is fine with 'a dangling at the time that MutexGuard goes out
+        // of scope.
+    }
+}
+
+struct Out;
+
+impl Out {
+    fn write_fmt(&self, _args: fmt::Arguments) {}
+}
+
+impl<'a> Display for MutexGuard<'a> {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        Ok(())
+    }
+}
+
+fn main() {
+    // FIXME(dtolnay): We actually want both of these to work. I think it's
+    // sadly unimplementable today though.
+
+    let _write = {
+        let mutex = Mutex;
+        write!(Out, "{}", mutex.lock()) /* no semicolon */
+        //~^ ERROR `mutex` does not live long enough
+    };
+
+    let _writeln = {
+        let mutex = Mutex;
+        writeln!(Out, "{}", mutex.lock()) /* no semicolon */
+        //~^ ERROR `mutex` does not live long enough
+    };
+}

src/test/ui/macros/format-args-temporaries-in-write.stderr

@@ -0,0 +1,43 @@
+error[E0597]: `mutex` does not live long enough
+  --> $DIR/format-args-temporaries-in-write.rs:41:27
+   |
+LL |         write!(Out, "{}", mutex.lock()) /* no semicolon */
+   |                           ^^^^^^^^^^^^
+   |                           |
+   |                           borrowed value does not live long enough
+   |                           a temporary with access to the borrow is created here ...
+LL |
+LL |     };
+   |     -- ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `MutexGuard`
+   |     |
+   |     `mutex` dropped here while still borrowed
+   |
+help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
+  --> $SRC_DIR/core/src/macros/mod.rs:LL:COL
+   |
+LL |         $dst.write_fmt($crate::format_args!($($arg)*));
+   |                                                       +
+
+error[E0597]: `mutex` does not live long enough
+  --> $DIR/format-args-temporaries-in-write.rs:47:29
+   |
+LL |         writeln!(Out, "{}", mutex.lock()) /* no semicolon */
+   |                             ^^^^^^^^^^^^
+   |                             |
+   |                             borrowed value does not live long enough
+   |                             a temporary with access to the borrow is created here ...
+LL |
+LL |     };
+   |     -- ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `MutexGuard`
+   |     |
+   |     `mutex` dropped here while still borrowed
+   |
+help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
+  --> $SRC_DIR/core/src/macros/mod.rs:LL:COL
+   |
+LL |         $dst.write_fmt($crate::format_args_nl!($($arg)*));
+   |                                                          +
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/macros/format-args-temporaries.rs

@@ -20,29 +20,13 @@ impl<'a> Drop for MutexGuard<'a> {
     }
 }
 
-impl<'a> MutexGuard<'a> {
-    fn write_fmt(&self, _args: fmt::Arguments) {}
-}
-
 impl<'a> Display for MutexGuard<'a> {
     fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
         Ok(())
     }
 }
 
 fn main() {
-    let _write = {
-        let out = Mutex;
-        let mutex = Mutex;
-        write!(out.lock(), "{}", mutex.lock()) /* no semicolon */
-    };
-
-    let _writeln = {
-        let out = Mutex;
-        let mutex = Mutex;
-        writeln!(out.lock(), "{}", mutex.lock()) /* no semicolon */
-    };
-
     let _print = {
         let mutex = Mutex;
         print!("{}", mutex.lock()) /* no semicolon */