> Exceptions only cause overheard when they happen.

They only execute instructions when they happen... but they still inhibit the optimizer when they don't. Calls that may throw act like branches in the control flow graph. They also introduce more code bloat for their landing pads.

> Rust’s model for errors is tied to a specific implementation.

The Rust ABI is unspecified for a reason. We're currently seeing a lot of optimization around struct and enum layout; I'm not sure why this couldn't be applied to the calling convention without any extra syntax for "throw."