Support concurrent swc_common via feature

[spence]

When using swc_common with feature = ["concurrent"], deno_ast fails to compile.

error[E0277]: `Rc<RefCell<Vec<swc_common::errors::Diagnostic>>>` cannot be sent between threads safely
   --> .../deno_ast-0.38.2/src/transpiling/mod.rs:324:46
    |
324 | impl crate::swc::common::errors::Emitter for DiagnosticCollector {
    |                                              ^^^^^^^^^^^^^^^^^^^ 
    |    `Rc<RefCell<Vec<swc_common::errors::Diagnostic>>>` cannot be sent between threads safely
    |
    = help: within `DiagnosticCollector`, the trait `Send` is not implemented for 
      `Rc<RefCell<Vec<swc_common::errors::Diagnostic>>>`, which is required by `DiagnosticCollector: Send`

error[E0308]: mismatched types
  --> .../deno_ast-0.38.2/src/emit.rs:79:7
   |
78 |     let mut writer = Box::new(JsWriter::new(
   |                               ------------- arguments to this function are incorrect
79 |       source_map.clone(),
   |       ^^^^^^^^^^^^^^^^^^ expected `Arc<SourceMap>`, found `Rc<SourceMap>`
   |
   = note: expected struct `Arc<swc_common::SourceMap>`
              found struct `Rc<swc_common::SourceMap>`

Resolves #219

cc @dsherret

[CLAassistant]

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[spence]

we can feature gate these, but without concurrent, Send and Sync are noops:

#[cfg(not(feature = "concurrent"))]
mod single {
    pub use once_cell::unsync::{Lazy, OnceCell};
    /// Dummy trait because swc_common is in single thread mode.
    pub trait Send {}
    /// Dummy trait because swc_common is in single thread mode.
    pub trait Sync {}

    impl<T> Send for T where T: ?Sized {}
    impl<T> Sync for T where T: ?Sized {}

    pub use std::{
        cell::{
            Ref as ReadGuard, RefMut as WriteGuard, RefMut as MappedWriteGuard,
            RefMut as LockGuard, RefMut as MappedLockGuard,
        },
        rc::{Rc as Lrc, Weak},
    };
}

[dsherret]

If someone is using concurrent then this not being thread safe will cause thread concurrency issues. Probably this will need a thread safe implementation when compiling with concurrent.

[dsherret]

As mentioned in #219 (comment) , I'm not sure this is a scenario we want to bother to support.

Do you really need this feature? How much faster have you found it? We've just started using separate threads per file. Maybe in bundling scenarios this is faster?

[spence]

unfortunately, a number of swc libraries bake-in swc_common's concurrent feature and including any of them prevents deno_ast from compiling:

• swc_error_reporters
• swc_bundler
• swc_ecma_lints
• swc_estree_compat
• swc_node_bundler
• swc_plugin_runner

i'm working from a fork of farm, which uses rayon for concurrent builds, and they benchmark against other bundlers using swc (e.g., rspack).

[dsherret]

Ah, I see. We can add a concurrent feature here, but no guarantee it's going to work in the future. It might be removed at any time due to the maintenance cost and you might have to contribute fixes to get it working again.