Function resident_set_bytes 

pub fn resident_set_bytes() -> Option<u64>

Returns the current process resident set size (RSS) in bytes, best-effort.

• Linux: 2nd field (resident pages) of /proc/self/statm × page size.
• macOS: mach task_info(MACH_TASK_BASIC_INFO) .resident_size.
• Other targets: None.

Returns None on any error; never panics.