Properly reset error return trace index

[Vexu]

Currently depends on #12807 to free up two Zir instruction slots, only the last commit is relevant to this PR.

Example:

conststd=@import("std");
test {
returnerror.SkipZigTest;
}
fnfoo() !void {
returnerror.DontShow;
}
test {
foo() catch {
std.debug.dumpStackTrace(@errorReturnTrace().?.*);
// return error.Second;
};
returnerror.Wtf;
}

With the comment:

Test [1/2] test_0... SKIP
Test [2/2] test_1... ./a.zig:6:5: 0x20f128 in foo (test)
return error.DontShow;
^
Test [2/2] test_1... FAIL (Wtf)
./a.zig:13:5: 0x20f185 in test_1 (test)
return error.Wtf;
^
0 passed; 1 skipped; 1 failed.

Uncommented:

Test [1/2] test_0... SKIP
Test [2/2] test_1... ./a.zig:6:5: 0x20f138 in foo (test)
return error.DontShow;
^
Test [2/2] test_1... FAIL (Second)
./a.zig:6:5: 0x20f138 in foo (test)
return error.DontShow;
^
./a.zig:11:9: 0x20f18e in test_1 (test)
return error.Second;
^
0 passed; 1 skipped; 1 failed.

Closes#1923

[Vexu]

Hmm, this still doesn't handle this case properly:

conststd=@import("std");
test {
returnerror.SkipZigTest;
}
fnfoo() !void {
returnerror.DontShow;
}
test {
foo() catch {
foo() catch {};
std.debug.dumpStackTrace(@errorReturnTrace().?.*);
};
}

Maybe it'd be better to start off with a simplified version which always resets to zero since it handles all the common cases properly.

[topolarity]

Hmm, this still doesn't handle this case properly:

Not sure if this is good news or bad, but I'm just finishing up an overlapping change that should handle that case correctly. It intends to implement the rest of #1923 (comment) (esp. supporting break)

Let me know how we can best coordinate the follow-up change so that I don't step on your toes here 🙂

[Vexu]

If you have a more complete fix then let's go with that, I just want this annoyance fixed to get the full benefit of #12807

[Vexu]

Closing in favor of the more complete solution.