Implement Send + Sync for VarLenUnicode, VarLenAscii, VarLenArray

[watsaig]

I am implementing a utility to write data to an HDF5 file in a thread, which works fine for some types but not others. For example, any type that includes VarLenUnicode, VarLenAscii or VarLenArray cannot be sent between threads.

Here is a minimal example:

use hdf5::types::VarLenUnicode;

#[derive(hdf5::H5Type)]
#[repr(C)]
struct CustomType {
    text: VarLenUnicode,
}

fn write_in_thread() {
    let file = hdf5::File::create("/tmp/dset.h5").unwrap();

    let data = vec![
        CustomType {
            text: VarLenUnicode::from_str("foo").unwrap(),
        },
        CustomType {
            text: VarLenUnicode::from_str("bar").unwrap(),
        },
    ];

    std::thread::spawn(move || {
        file.group("/")
            .unwrap()
            .new_dataset_builder()
            .with_data(&data)
            .create("dataset")
            .unwrap();
    });
}

Output (truncated):

error[E0277]: `*mut u8` cannot be sent between threads safely
   --> src\io\src\writer.rs:136:28
    |
136 |           std::thread::spawn(move || {
    |  _________------------------_^
    | |         |
    | |         required by a bound introduced by this call
137 | |             file.group("/")
138 | |                 .unwrap()
139 | |                 .new_dataset_builder()
...   |
142 | |                 .unwrap();
143 | |         });
    | |_________^ `*mut u8` cannot be sent between threads safely
    |
    = help: within `CustomType`, the trait `std::marker::Send` is not implemented for `*mut u8`
note: required because it appears within the type `VarLenUnicode`
   --> C:\Users\watsaig\.cargo\git\checkouts\hdf5-rust-c1072d7a2617f7d8\2e44e90\hdf5-types\src\string.rs:288:12
    |
288 | pub struct VarLenUnicode {
    |            ^^^^^^^^^^^^^
note: required because it appears within the type `CustomType`
   --> src\io\src\writer.rs:149:8
    |
149 | struct CustomType {
    |        ^^^^^^^^^^
    = note: required for `Unique<CustomType>` to implement `std::marker::Send`
note: required because it appears within the type `RawVec<CustomType>`

Is it possible to make these types thread-safe? I'm happy to help with a PR but not sure where to start.

[mulimoen]

It should be safe to implement Send for these types. The nomicon contains information on how to implement this trait. These trait impls should be added to string.rs and array.rs.