addExtraAssumeCapacity

    fn addExtraAssumeCapacity(self: *WipFunction, extra: anytype) Instruction.ExtraIndex {
        const result: Instruction.ExtraIndex = @intCast(self.extra.items.len);
        const info = @typeInfo(@TypeOf(extra)).@"struct";
        inline for (info.field_names, info.field_types) |field_name, field_type| {
            const value = @field(extra, field_name);
            self.extra.appendAssumeCapacity(switch (field_type) {
                u32 => value,
                Alignment,
                AtomicOrdering,
                Block.Index,
                FunctionAttributes,
                Type,
                Value,
                Instruction.BrCond.Weights,
                => @backingInt(value),
                MemoryAccessInfo,
                Instruction.Alloca.Info,
                Instruction.Call.Info,
                => @bitCast(value),
                else => @compileError("bad field type: " ++ field_name ++ ": " ++ @typeName(field_type)),
            });
        }
        return result;
    }