Auto merge of #96490 - dtolnay:writetmpbackport, r=Mark-Simulacrum

Make [e]println macros eagerly drop temporaries (for backport)

This PR extracts the subset of #96455 which is only the parts necessary for fixing the 1.61-beta regressions in #96434.

My larger PR #96455 contains a few other changes relative to the pre-#94868 behavior; those are not necessary to backport into 1.61.

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

Author: bors

std/src/macros.rs

@@ -99,9 +99,9 @@ macro_rules! println {
     () => {
         $crate::print!("\n")
     };
-    ($($arg:tt)*) => {
-        $crate::io::_print($crate::format_args_nl!($($arg)*))
-    };
+    ($($arg:tt)*) => {{
+        $crate::io::_print($crate::format_args_nl!($($arg)*));
+    }};
 }
 
 /// Prints to the standard error.
@@ -164,9 +164,9 @@ macro_rules! eprintln {
     () => {
         $crate::eprint!("\n")
     };
-    ($($arg:tt)*) => {
-        $crate::io::_eprint($crate::format_args_nl!($($arg)*))
-    };
+    ($($arg:tt)*) => {{
+        $crate::io::_eprint($crate::format_args_nl!($($arg)*));
+    }};
 }
 
 /// Prints and returns the value of a given expression for quick and dirty