alt Hacker NewsI think it's a reference to certain optimizations possible due to aliasing rules in Rust that are not possible (or maybe only "not straight forward", I'm not sure) in C. So a transpiled program while keeping its semantics might not still compile to equally optimized assembly.
IIRC C can do the same things with correct usage of `restrict`, but that's extremely difficult by hand. So difficult that LLVM's `restrict` support was very buggy when Rust first started using the capabilities. Those bugs got fixed, but it's still impractical to use in handwritten C.