Docs/examples/fixes to make rust-async, threadpool optional

[aran]

Is your feature request related to a problem? Please describe. From website/docs/guides/custom/rust/feature-flags.md it may be possible to remove unused features from the FRB import.

In my project at least, in disabling threadpool, the default generated executor seems to break compilation:

error[E0599]: the method `wrap_sync` exists for struct `FLUTTER_RUST_BRIDGE_HANDLER`, but its trait bounds were not satisfied
    --> .../frb_generated.rs:4454:33
     |
45   | flutter_rust_bridge::frb_generated_default_handler!();
     | ----------------------------------------------------- method `wrap_sync` not found for this struct
...
4454 |     FLUTTER_RUST_BRIDGE_HANDLER.wrap_sync::<flutter_rust_bridge::for_generated::SseCodec, _>(
     |     ----------------------------^^^^^^^^^ method cannot be called on `FLUTTER_RUST_BRIDGE_HANDLER` due to unsatisfied trait bounds
     |
    ::: /build/.cargo/registry/src/index.crates.io-6f17d22bba15001f/flutter_rust_bridge-2.3.0/src/handler/implementation/handler.rs:42:1
     |
42   | pub struct SimpleHandler<E: Executor, EL: ErrorListener> {
     | -------------------------------------------------------- doesn't satisfy `_: Handler`
     |
    ::: /build/.cargo/registry/src/index.crates.io-6f17d22bba15001f/flutter_rust_bridge-2.3.0/src/handler/implementation/executor.rs:23:1
     |
23   | pub struct SimpleExecutor<EL: ErrorListener, TP: BaseThreadPool, AR: BaseAsyncRuntime> {
     | -------------------------------------------------------------------------------------- doesn't satisfy `_: Executor`
     |
     = note: the following trait bounds were not satisfied:
             `SimpleExecutor<NoOpErrorListener, &LocalKey<SimpleThreadPool>, SimpleAsyncRuntime>: Executor`
             which is required by `SimpleHandler<SimpleExecutor<NoOpErrorListener, &LocalKey<SimpleThreadPool>, SimpleAsyncRuntime>, NoOpErrorListener>: Handler`
     = help: items from traits can only be used if the trait is implemented and in scope
     = note: the following trait defines an item `wrap_sync`, perhaps you need to implement it:
             candidate #1: `Handler`

In disabling rust-async—I am trying to use all methods as #[frb(sync)], but it's possible I've missed something—I see errors due to Lockable, RustAutoOpaqueInner, LockableOrderInfo and a few others. The RustAutoOpaqueInner errors are tied to types that I have pub use imported, frb(mirror(, and #[frb(external)]. E.g.:

error[E0412]: cannot find type `RustAutoOpaqueInner` in module `flutter_rust_bridge::for_generated`
   --> .../rust/src/frb_generated.rs:197:53
    |
197 |                 flutter_rust_bridge::for_generated::RustAutoOpaqueInner<MyType>,
    |                                                     ^^^^^^^^^^^^^^^^^^^ not found in `flutter_rust_bridge::for_generated`
    |
note: found an item that was configured out
   --> /build/.cargo/registry/src/index.crates.io-6f17d22bba15001f/flutter_rust_bridge-2.3.0/src/for_generated/mod.rs:62:42
    |
62  | pub use crate::rust_auto_opaque::{inner::RustAutoOpaqueInner, RustAutoOpaqueBase};
    |                                          ^^^^^^^^^^^^^^^^^^^
note: the item is gated behind the `rust-async` feature
   --> /build/.cargo/registry/src/index.crates.io-6f17d22bba15001f/flutter_rust_bridge-2.3.0/src/for_generated/mod.rs:61:7
    |
61  | #[cfg(feature = "rust-async")]
    |       ^^^^^^^^^^^^^^^^^^^^^^

Describe the solution you'd like Not sure. I am already using, or intending to use, all sync methods for thread safety on wasm platform. I don't think I am using the threadpools. I would be looking for either documentation, example code, or code-generator errors to lead me to what I've done wrong here.

Describe alternatives you've considered N/A

[fzyzcjy]

Firstly, I wonder why do you want to disable those features? IIRC Rust does a lot of optimizations and will eliminate dead code if you do not use a feature automatically.

Secondly, for thread pools: https://github.com/fzyzcjy/flutter_rust_bridge/issues/2215 may be related if you want to disable it because of the web things.

[aran]

My thinking was, I'm not (supposed to be) using them. If I were to use the threadpool accidentally, that would be a bug. I do see some extra activity in web console in setting up the service workers which I assume is due to the threadpool.

For async, just looking for opportunities to cut down surface area.

[fzyzcjy]

I see, then #2215 may be useful for thread pool thing. For async, looks like e.g. opaque types uses tokio rwlock currently, so it requires the tokio dependency, and thus the flag async.

[fzyzcjy]

Close since that issue is closed. But feel free to reopen this issue if you have any questions!

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