calling convention - Why does Rust have both call by value and call by reference?

Question

Some languages, like Haskell, make no distinction between pass-by-value and pass-by-reference. The compiler can then approximately choose the most efficient calling convention with a heuristic. One example heuristic would be for the Linux x64 ABI: if the size of parameter is greater than 16 bytes, pass a pointer to the stack otherwise pass the value in registers.

What is the advantage of keeping both notions of pass-by-value and pass-by-reference (non-mutable of course) in Rust and forcing the user to choose?

Could it be the case that pass-by-value is syntactic sugar for pass-by-reference + copy if the value is seen to be modified?

Answer 1

Two things:

1. Rust will transform certain pass-by-value calls into pass-by-reference, based on a similar heuristic.
2. Pass-by-value indicates ownership transfer, while pass-by-reference indicates borrowing. These are very different, and totally orthogonal from the asm-level concern you're asking about.

In other words, in Rust, these two forms have different semantics. That doesn't preclude also doing the optimization, though.