Dev builds Rust system monitor, uses unsafe FFI to fetch disk usage via statvfs

Developer Maneshwar is documenting a day-by-day build of a btop-style system monitor in Rust using the ratatui library. On day three, tackling disk usage metrics, he hit a wall: unlike CPU and memory stats, filesystem fullness is not exposed through any file in /proc or /sys. The only way to retrieve it is via the statvfs(2) syscall, which required using Rust's FFI and writing his first unsafe block. Rather than shell out to the df command and parse its output, he called the syscall directly through the libc crate, keeping the unsafe code isolated within a single function that returns a safe Option type. He also highlighted key considerations such as converting Rust strings to NUL-terminated CStrings and the importance of documenting safety assumptions inside every unsafe block.