self pass by copy copies before arguments are evaluated

[L3nn0x]

Zig Version

0.13.0 but also trunk on godbolt

Steps to Reproduce and Observed Behavior

Hello,

The following code will return done 0 whereas I would have expected done 1:

const std = @import("std");

const Test = struct {
 value: u8,

 fn inc(self: *Test) bool {
  self.value += 1;
  return true;
 }

 fn get(self: Test, _: bool) u8 {
  return self.value;
 }
};

pub fn main() void {
 var a = Test{.value=0};
 const v = a.get(a.inc());
 std.debug.print("done {}", .{v});
}

I suspect that this is because the self parameter is passed by copy and the copy happens before the arguments are evaluated. Since there is no control over the fact that the struct is passed by copy or by reference, the behavior isn't consistent based on the size of the struct.

A workaround today is to always pass the self argument as a pointer or const pointer.

Godbolt link: https://godbolt.org/z/dTEaPe3d3

Expected Behavior

The struct copy should be done after the arguments are evaluated so the result is done 1 in the example provided.

[mlugg]

Working as intended; evaluation is always left-to-right. The inconsistency based on struct size is going to be fixed by the elimination of PRO's semantic implications (#5973).