Rollup merge of #96455 - dtolnay:writetmp, r=m-ou-se

Make write/print macros eagerly drop temporaries

This PR fixes the 2 regressions in #96434 (`println` and `eprintln`) and changes all the other similar macros (`write`, `writeln`, `print`, `eprint`) to match the old pre-#94868 behavior of `println` and `eprintln`.

argument position | before #94868 | after #94868 | after this PR
--- |:---:|:---:|:---:
`write!($tmp, "…", …)` | :rage: | :rage: | :smiley_cat:
`write!(…, "…", $tmp)` | :rage: | :rage: | :smiley_cat:
`writeln!($tmp, "…", …)` | :rage: | :rage: | :smiley_cat:
`writeln!(…, "…", $tmp)` | :rage: | :rage: | :smiley_cat:
`print!("…", $tmp)` | :rage: | :rage: | :smiley_cat:
`println!("…", $tmp)` | :smiley_cat: | :rage: | :smiley_cat:
`eprint!("…", $tmp)` | :rage: | :rage: | :smiley_cat:
`eprintln!("…", $tmp)` | :smiley_cat: | :rage: | :smiley_cat:
`panic!("…", $tmp)` | :smiley_cat: | :smiley_cat: | :smiley_cat:

Example of code that is affected by this change:

```rust
use std::sync::Mutex;

fn main() {
    let mutex = Mutex::new(0);
    print!("{}", mutex.lock().unwrap()) /* no semicolon */
}
```

You can see several real-world examples like this in the Crater links at the top of #96434. This code failed to compile prior to this PR as follows, but works after this PR.

```console
error[E0597]: `mutex` does not live long enough
 --> src/main.rs:5:18
  |
5 |     print!("{}", mutex.lock().unwrap()) /* no semicolon */
  |                  ^^^^^^^^^^^^---------
  |                  |
  |                  borrowed value does not live long enough
  |                  a temporary with access to the borrow is created here ...
6 | }
  | -
  | |
  | `mutex` dropped here while still borrowed
  | ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `MutexGuard`
```

Author: GuillaumeGomez

library/core/src/macros/mod.rs

@@ -496,9 +496,10 @@ 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)*) => {
-        $dst.write_fmt($crate::format_args!($($arg)*))
-    };
+    ($dst:expr, $($arg:tt)*) => {{
+        let result = $dst.write_fmt($crate::format_args!($($arg)*));
+        result
+    }};
 }
 
 /// Write formatted data into a buffer, with a newline appended.
@@ -553,9 +554,10 @@ macro_rules! writeln {
     ($dst:expr $(,)?) => {
         $crate::write!($dst, "\n")
     };
-    ($dst:expr, $($arg:tt)*) => {
-        $dst.write_fmt($crate::format_args_nl!($($arg)*))
-    };
+    ($dst:expr, $($arg:tt)*) => {{
+        let result = $dst.write_fmt($crate::format_args_nl!($($arg)*));
+        result
+    }};
 }
 
 /// Indicates unreachable code.

library/std/src/macros.rs

@@ -62,9 +62,9 @@ macro_rules! panic {
 #[cfg_attr(not(test), rustc_diagnostic_item = "print_macro")]
 #[allow_internal_unstable(print_internals)]
 macro_rules! print {
-    ($($arg:tt)*) => {
-        $crate::io::_print($crate::format_args!($($arg)*))
-    };
+    ($($arg:tt)*) => {{
+        $crate::io::_print($crate::format_args!($($arg)*));
+    }};
 }
 
 /// Prints to the standard output, with a newline.
@@ -133,9 +133,9 @@ macro_rules! println {
 #[cfg_attr(not(test), rustc_diagnostic_item = "eprint_macro")]
 #[allow_internal_unstable(print_internals)]
 macro_rules! eprint {
-    ($($arg:tt)*) => {
-        $crate::io::_eprint($crate::format_args!($($arg)*))
-    };
+    ($($arg:tt)*) => {{
+        $crate::io::_eprint($crate::format_args!($($arg)*));
+    }};
 }
 
 /// Prints to the standard error, with a newline.

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

@@ -0,0 +1,70 @@
+// check-pass
+
+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.
+    }
+}
+
+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 */
+    };
+
+    let _println = {
+        let mutex = Mutex;
+        println!("{}", mutex.lock()) /* no semicolon */
+    };
+
+    let _eprint = {
+        let mutex = Mutex;
+        eprint!("{}", mutex.lock()) /* no semicolon */
+    };
+
+    let _eprintln = {
+        let mutex = Mutex;
+        eprintln!("{}", mutex.lock()) /* no semicolon */
+    };
+
+    let _panic = {
+        let mutex = Mutex;
+        panic!("{}", mutex.lock()) /* no semicolon */
+    };
+}