extraDataTrail

    fn extraDataTrail(
        self: *const WipFunction,
        comptime T: type,
        index: Instruction.ExtraIndex,
    ) struct { data: T, trail: ExtraDataTrail } {
        var result: T = undefined;
        const info = @typeInfo(T).@"struct";
        inline for (
            info.field_names,
            info.field_types,
            self.extra.items[index..][0..info.field_names.len],
        ) |field_name, field_type, value|
            @field(result, field_name) = switch (field_type) {
                u32 => value,
                Alignment,
                AtomicOrdering,
                Block.Index,
                FunctionAttributes,
                Type,
                Value,
                Instruction.BrCond.Weights,
                => @fromBackingInt(value),
                MemoryAccessInfo,
                Instruction.Alloca.Info,
                Instruction.Call.Info,
                => @bitCast(value),
                else => @compileError("bad field type: " ++ field_name ++ ": " ++ @typeName(field_type)),
            };
        return .{
            .data = result,
            .trail = .{ .index = index + @as(Type.Item.ExtraIndex, @intCast(info.field_names.len)) },
        };
    }