RWLock Wrapper - doesn't have a size known at compile-time

help
1

I have some code I found on this forum that I am hoping will help me diagnose a deadlock
by printing to the console on write() and drop

The code is at

I would like to ad an identifier to the RWLock struct.
Ideally this would be a static str of some kind but when I try to add that or even a u8
like

pub struct RwLock<T: ?Sized> {
    pub lock: sync::RwLock<T>,
    id: u8,
}

I start getting ```
pub lock: sync::RwLock,
| ^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time


isn't a u8 known at compile time ? 
Can someone explain how I would get around this and why the error occurs.

Thanks
2

There can only be one unsized field in a struct, and it must be the last one, even though Rust otherwise reserves the right to reorder fields. Move the declaration of id before the unsized lock and it will work.

1 Like
3

Thanks. Sounds like a place the compiler output could be better.

5

You're right. The compiler should make this explicit. The reason as why this rule is true is not hard to conceive, however, this feature would be good for beginners

6

The last note in full error message does state this restriction:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/lib.rs:2:5
  |
2 |     data: [u8],
  |     ^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `[u8]`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: only the last field of a struct may have a dynamically sized type

Maybe the error could be refined in this particular case, with only one unsized field but not the last.

7

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