Immutable struct changed in function call

[miquille]

Zig Version

0.9.1

Steps to Reproduce

const std = @import("std");

const Pair = struct {
    x: i32,
    y: i32,

    pub fn flip(t: Pair) Pair {
        return Pair{.y = t.x, .x = t.y};
    }
};

test "it should swap the values" {
    var v = Pair{.x = 10, .y = 4};
    v = v.flip();
    try std.testing.expectEqual(v.y, 10);
    try std.testing.expectEqual(v.x, 4); //Fails, it's 10 now
}

test "what actually happens" {
    var v = Pair{.x = 10, .y = 4};
    v = v.flip();
    try std.testing.expectEqual(v.x, 10);
    try std.testing.expectEqual(v.y, 10);
}

OS: Mac OS 12.3.1 (21E258), arm64

Note: Adding a temporary value resolves the problem, eg:

    pub fn flip(t: Pair) Pair {
        const tmp = Pair{.y = t.x, .x = t.y};
        return tmp;
    }

Expected Behavior

Either the behavior indicated by the test (eg: a new struct to be created with the values swapped (eg {.x = 4, .y = 10} after v.flip() and for that value to be assigned to v replacing it.)

test "it should swap the values" {
    var v = Pair{.x = 10, .y = 4};
    v = v.flip();
    try std.testing.expectEqual(v.y, 10);
    try std.testing.expectEqual(v.x, 4); //Fails, it's 10 now
}

OR

A warning or compile error suggesting that assigning to a return value that is a modification of this can be optimized in undefined ways.

Actual Behavior

The value of x is set to 10 because it assigned first, then y is set to the new value of x (10) instead of the value that is being passed in via the struct (4).

test "what actually happens" {
    var v = Pair{.x = 10, .y = 4};
    v = v.flip();
    try std.testing.expectEqual(v.x, 10);
    try std.testing.expectEqual(v.y, 10);
}

[rohlem]

This is a manifestation of #3696 (-> duplicate for the purpose of bug tracking, though I totally agree that it's an issue).