setExtra

fn setExtra(astgen: *AstGen, index: usize, extra: anytype) void {
    const fields = std.meta.fields(@TypeOf(extra));
    var i = index;
    inline for (fields) |field| {
        astgen.extra.items[i] = switch (field.type) {
            u32 => @field(extra, field.name),

            Zir.Inst.Ref,
            Zir.Inst.Index,
            Zir.Inst.Declaration.Name,
            std.zig.SimpleComptimeReason,
            Zir.NullTerminatedString,
            // Ast.TokenIndex is missing because it is a u32.
            Ast.OptionalTokenIndex,
            Ast.Node.Index,
            Ast.Node.OptionalIndex,
            => @intFromEnum(@field(extra, field.name)),

            Ast.TokenOffset,
            Ast.OptionalTokenOffset,
            Ast.Node.Offset,
            Ast.Node.OptionalOffset,
            => @bitCast(@intFromEnum(@field(extra, field.name))),

            i32,
            Zir.Inst.Call.Flags,
            Zir.Inst.BuiltinCall.Flags,
            Zir.Inst.SwitchBlock.Bits,
            Zir.Inst.SwitchBlockErrUnion.Bits,
            Zir.Inst.FuncFancy.Bits,
            Zir.Inst.Param.Type,
            Zir.Inst.Func.RetTy,
            => @bitCast(@field(extra, field.name)),

            else => @compileError("bad field type"),
        };
        i += 1;
    }
}