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

Author: dtolnay

library/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

src/test/pretty/dollar-crate.pp

@@ -9,5 +9,5 @@
 // pp-exact:dollar-crate.pp
 
 fn main() {
-    ::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"], &[]));
+    { ::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"], &[])); };
 }

src/test/ui/macros/trace-macro.stderr

@@ -5,5 +5,5 @@ LL |     println!("Hello, World!");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: expanding `println! { "Hello, World!" }`
-   = note: to `$crate :: io :: _print($crate :: format_args_nl! ("Hello, World!"))`
+   = note: to `{ $crate :: io :: _print($crate :: format_args_nl! ("Hello, World!")) ; }`
 

src/test/ui/pattern/usefulness/tuple-struct-nonexhaustive.stderr

@@ -12,7 +12,7 @@ LL | struct Foo(isize, isize);
    = note: the matched value is of type `Foo`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
-LL ~         Foo(2, b) => println!("{}", b),
+LL ~         Foo(2, b) => println!("{}", b)
 LL +         Foo(_, _) => todo!()
    |