unreachable code error prevents idiomatic usage of defer

[andrewrk]

const std = @import("std");
const assert = std.debug.assert;

pub fn main() void {
    const ptr = std.debug.global_allocator.create(i32);
    defer std.debug.global_allocator.destroy(ptr);

    std.os.exit(0);
}

Expected: this compiles.

Actual:

/home/andy/dev/zig/build/test.zig:6:11: error: unreachable code
    defer std.debug.global_allocator.destroy(ptr);
          ^

[tiehuis]

Existing example of this in the self-hosted compiler.

https://github.com/ziglang/zig/blob/master/src-self-hosted/main.zig#L65-L68

[andrewrk]

This might be solved with #283

[daurnimator]

oooo, this is really interesting, and infact a "good thing"!
If a longjmp-ing function is defined as noreturn then we correctly catch a skipped defer at compile time.

[Rocknest]

So defer should run before std.os.exit()?

[daurnimator]

So defer should run before std.os.exit()?

I'm not sure that's useful in the general case. If I have a function annotated noreturn like exit() do you even want defer statements to run?
What about if it's a sometimes-noreturn function like execve() (which can return an error if e.g. the executable is invalid)? => in that case I would probably want defer to run on failure but other times not at all.

[andrewrk]

This is now a duplicate of accepted proposal #2654. See the explanation there. Preferred solution to the above code is:

defer unreachable;