Cleanup On Unwinding

This feature adds a new control-flow construct on_unwind $expr { $block }.

This evaluates $expr and returns its value, unless $expr causes unwinding to occur. In that case, the statements in $block are executed before unwinding continues.

This is used to make cleanup code explicit. This could look like:

#![allow(unused)]
fn main() {
let x = String::new();
function_call()

// becomes:
let x = String::new();
on_unwind function_call() {
  scope_end!(x);
};
}

(Where scope_end! is defined here).

This is different from catch_unwind because one cannot go back to normal function execution; when the end of the on_unwind block is reached, unwinding continues into the parent stack frame. (Tho maybe we could allow returning from such a block, and catch_unwind could just be implemented in terms of that?).

Backlinks

• Control-flow Desugarings
• Explicit Unwind Cleanup
• The Final Language