rust: print: avoid evaluating arguments in pr_* macros in unsafe blocks

This is part of the effort to minimize the differences of the `rust`
branch with respect to mainline in order to eventually drop it.

Apply commit 6618d69 ("rust: print: avoid evaluating arguments in
`pr_*` macros in `unsafe` blocks") from mainline.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/print.rs

@@ -142,17 +142,24 @@ pub fn call_printk_cont(args: fmt::Arguments<'_>) {
 macro_rules! print_macro (
     // The non-continuation cases (most of them, e.g. `INFO`).
     ($format_string:path, false, $($arg:tt)+) => (
-        // SAFETY: This hidden macro should only be called by the documented
-        // printing macros which ensure the format string is one of the fixed
-        // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
-        // by the `module!` proc macro or fixed values defined in a kernel
-        // crate.
-        unsafe {
-            $crate::print::call_printk(
-                &$format_string,
-                crate::__LOG_PREFIX,
-                format_args!($($arg)+),
-            );
+        // To remain sound, `arg`s must be expanded outside the `unsafe` block.
+        // Typically one would use a `let` binding for that; however, `format_args!`
+        // takes borrows on the arguments, but does not extend the scope of temporaries.
+        // Therefore, a `match` expression is used to keep them around, since
+        // the scrutinee is kept until the end of the `match`.
+        match format_args!($($arg)+) {
+            // SAFETY: This hidden macro should only be called by the documented
+            // printing macros which ensure the format string is one of the fixed
+            // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
+            // by the `module!` proc macro or fixed values defined in a kernel
+            // crate.
+            args => unsafe {
+                $crate::print::call_printk(
+                    &$format_string,
+                    crate::__LOG_PREFIX,
+                    args,
+                );
+            }
         }
     );