Function parameter is not immutable

[jean-dao]

Came across this behavior, looks like a bug to me:

const std = @import("std");

const Nested = struct {
    x: u32,
};

const Test = struct {
    nested: Nested,

    fn update(self: *Test, arg: Nested) void {
        std.debug.print("t={} arg={}\n", .{self, arg});
        self.nested = .{ .x = 0 };
        std.debug.print("t={} arg={}\n", .{self, arg});
        self.nested = arg;
        std.debug.print("t={} arg={}\n", .{self, arg});
    }
};

pub
fn main() void {
    var t = Test{ .nested = .{ .x = 42 }};
    t.update(t.nested);
}

will print:

t=Test{ .nested = Nested{ .x = 42 } } arg=Nested{ .x = 42 }
t=Test{ .nested = Nested{ .x = 0 } } arg=Nested{ .x = 0 }
t=Test{ .nested = Nested{ .x = 0 } } arg=Nested{ .x = 0 }

When passing both a pointer to a struct and a member of this struct, the member is passed by reference (it seems), so changing the struct will change the second argument.

Tested with zig-linux-x86_64-0.9.0-dev.946+6237dc0a.

[Vexu]

This has the same root cause as #3696 and #4021 which is that the compiler doesn't recognize that there will be aliasing issues and passes the argument via a hidden pointer. The workaround is to copy the parameter to stack first but the underlying issue has not yet been solved.