Wrong pointer from captured field

[jonascloud]

Zig Version

0.14.0

Steps to Reproduce and Observed Behavior

Hey,

I believe I've encountered a bug in the following Zig code. The function below returns a pointer to a captured field (self.bar) from a recursive function. In debug mode (when running zig build test), the code works fine. However, when using the -Doptimize flag, the returned pointer is incorrect.

Here is the problematic code:

const Foo = struct {
    bar: ?u32 = null,
    pub fn get(self: *Foo) !*const u32 {
        if (self.bar) |b| {
            return &b;
        } else {
            self.bar = 0;
            return self.get();
        }
    }
};

pub fn main() !void {
    var foo: Foo = .{};
    const bar = try foo.get();
    std.debug.print("{}", .{bar.*});

    std.debug.assert(bar.* == 0);
}

The code compiles correctly if I return the field directly without specifying a const pointer and let the compiler handle it. Using |*b| also works fine.

Regards,
Jonas

Expected Behavior

Function returns the correct pointer or &b is disallowed.

[milogreg]

b is not an alias for bar's u32 component, it is a constant local variable equal to bar's u32 component. A pointer to a variable will not be valid outside of the variable's scope, so you are invoking undefined behavior by dereferencing the pointer returned by get.

[jonascloud]

Ah, yes. That's probably what's happening, thank you. b is a "copy" of the underlying memory and there are no guarantees, that this copy lives after the scope ends.
Referencing "introduce runtime safety for dangling stack pointers" and closing as duplicate.