Feature request: option to mark function as safe

[DanielEScherzer]

External functions are assumed to be unsafe (https://doc.rust-lang.org/reference/items/external-blocks.html#r-items.extern.fn.safety) but there should be an option to mark one as safe

For example, for FFI bindings for the PHP source, the function zend_html_putc (https://github.com/php/php-src/blob/d03163dc0af54f9c0687ab345f5db2584563b2a7/Zend/zend_highlight.c#L28) is always going to be safe - it just prints a character. Similarly, zend_gdb_present (https://github.com/php/php-src/blob/PHP-8.5.10/Zend/zend_gdb.c#L110) takes no parameters and returns a boolean reporting if GDB is present - this function is always going to be safe to call

There should be a way to indicate that a specific function should be marked as safe

[emilio]

Opting in seems fine. But I'd note that there's nothing necessarily safe about those functions per se, e.g. they could be making assumptions about which thread it's called from or what not.