defer, %defer, and ?defer

[andrewrk]

• defer expr, evaluates expr when control flow leaves scope
• %defer expr, evaluates expr when control flow leaves scope by returning an error from the function
• ?defer expr, evaluates expr when control flow leaves scope by returning null from the function
• defer valid at statement level only in parser
• error when there is a defer with 0 block exit paths
• make the returns respect defer
• error when there is a %return in a function with void return type
• optimization where the node remembers if it was error wrapped from a payload, if that is returned, then we know it is not an error. when %return returns it is for sure an error. otherwise we have to do a runtime check to find out if it's an error and then do the defer expression
• better defer implementation - instead of duplicating the code at every exit path, make a basic block that exit paths point to.
• error for control flow inside an expression exiting the defer expression scope? for example you probably shouldn't be able to return in a defer.

[ddevault]

Not sure how it would take shape, but it would be nice to be able to assign and defer in one go (sugar wise). Something to this effect:

const foo %defer deallocate_foo(foo) = %return try_to_foo();

Further sugared:

const foo %defer deallocate_foo = %return try_to_foo();

With the implication being that deallocate_foo just takes one parameter that is foo.

[andrewrk]

The point of this syntax is so that the deallocation becomes "attached" to the allocation and you can't accidentally separate them. Is that right?

[ddevault]

No, it's just sugar to make it so you can write less code.

[andrewrk]

Moved to individual issues.