stage2: Copied union field aliases with source

[topolarity]

Zig Version

0.10.0-dev.4217+9d8cdb855

Steps to Reproduce

const std = @import("std");
const expect = std.testing.expect;

const U = union(enum) {
    array: [10]u32,
    other: u32,
};

test {
    var x = U{ .array = undefined };

    x.array[1] = 0;
    const a = x.array;
    x.array[1] = 15;

    try expect(a[1] == 0);
}

In -O Debug, the .load of the overall union object is not elided because of an intervening union tag check, so this bug does not manifest. However, that elision would actually be correct and is applied in .ReleaseFast mode.

The incorrect part is that .struct_field_val returns a pointer to the union field without doing a side effect check like what's done in airLoad

Expected Behavior

Test should pass (like in stage1).

Actual Behavior

$ zig test test.zig  -O ReleaseFast
Test [1/1] test_0... FAIL (TestUnexpectedResult)
0 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
/Users/toadman667/repos/zig/zig-cache/o/8907a365291a8522093c111f26d53494/test