Unnecessary use of IpcSender/IpcReceiver in FontCacheThread::exit

[jdm]

While investigating servo/ipc-channel#29 I realized that the existing code is inefficient. We can use a regular std::sync::mpsc channel instead of the equivalent from ipc-channel, since both the Constellation and FontCacheThread code run in the same process.

Code: components/gfx/font_cache_thread.rs, in the exit function

[kishorbhat]

Can I try my hand at this?

Disclaimer: I might need to ping you if I don't get something at first; I'm new to Rust/Servo. :)

[KiChjang]

Sure thing! Come to #servo on irc.mozilla.org and ask your questions there, someone would definitely be in touch with you soon!

[Ms2ger]

Not quite clear this is possible:

   Compiling gfx v0.0.1 (file:///home/kishor/projects/servo/components/servo)
   Compiling script v0.0.1 (file:///home/kishor/projects/servo/components/servo)
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:89:10: 89:21 error: the trait `core::fmt::Debug` is not implemented for the type `std::sync::mpsc::Sender<()>` [E0277]
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:89     Exit(Sender<()>),
                                                                            ^~~~~~~~~~~
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:34: 83:39 note: in this expansion of #[derive_Debug] (defined in /home/kishor/projects/servo/components/gfx/font_cache_thread.rs)
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:89:10: 89:21 help: run `rustc --explain E0277` to see a detailed explanation
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:89:10: 89:21 note: `std::sync::mpsc::Sender<()>` cannot be formatted using `:?`; if it is defined in your crate, add `#[derive(Debug)]` or manually implement it
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:89:10: 89:21 note: required for the cast to the object type `core::fmt::Debug`
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:23: 83:32 error: the trait `serde::ser::Serialize` is not implemented for the type `std::sync::mpsc::Sender<()>` [E0277]
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83 #[derive(Deserialize, Serialize, Debug)]
                                                                                         ^~~~~~~~~
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:23: 83:32 note: in this expansion of #[derive_Serialize] (defined in /home/kishor/projects/servo/components/gfx/font_cache_thread.rs)
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:23: 83:32 help: run `rustc --explain E0277` to see a detailed explanation
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:23: 83:32 note: required by `serde::ser::Serializer::visit_newtype_variant`
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:21: 83:21 error: the trait `serde::de::Deserialize` is not implemented for the type `std::sync::mpsc::Sender<()>` [E0277]
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83 #[derive(Deserialize, Serialize, Debug)]
                                                                                       ^
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:10: 83:21 note: in this expansion of try! (defined in <std macros>)
/home/kishor/projects/servo/components/gfx/font_cache_thread.rs:83:21: 83:21 help: run `rustc --explain E0277` to see a detailed explanation
error: aborting due to 3 previous errors
Build failed, waiting for other jobs to finish...
^Cmach interrupted by signal or user action. Stopping.

[jdm]

I apologize for proposing an impossible change (at least without @ecoal95's experiment). The proper way to solve this would be to split the Command enum into two, and use mspc::Sender/Receiver<InProcessCommand> for the ones that do not need to be sent cross-process. This in turn requires changing the event loop for FontCacheThread to select over both in-process and out-of-process channels simultaneously, which makes it more complicated, etc. The end result is a bunch of work for something that isn't a significant burden right now. Sorry for nothing thinking it through earlier!