Feature request: use proc-macro-error crate

[ratijas]

There is new shiny proc-macro-error crate for better error reporting from within proc macros. It definitely has some wow-factor around it, stable 1.* tracks, large amount of downloads and low number of extra dependencies. I would consider using it for procedural derives and other macros in this project.

Checklist:

• Is it worth it? Prove by examples, please?
• Implementation
• Tests which show compiler UI (messages) enhancement

[sztomi]

I have an example of not-so-helpful errors with the proc macros:

error[E0308]: `match` arms have incompatible types
  --> src/ui/qt.rs:25:19
   |
25 | #[derive(Default, QObject)]
   |                   ^^^^^^^
   |                   |
   |                   expected enum `Result`, found `()`
   |                   this is found to be of type `Result<(), anyhow::Error>`
   |                   `match` arms have incompatible types
   |                   in this macro invocation
   |
  ::: /home/tamas/.cargo/registry/src/github.com-1ecc6299db9ec823/qmetaobject_impl-0.2.2/src/lib.rs:58:1
   |
58 | pub fn qobject_impl(input: TokenStream) -> TokenStream {
   | ------------------------------------------------------ in this expansion of `#[derive(QObject)]`
   |
   = note:   expected type `Result<(), anyhow::Error>`
           found unit type `()`

error: aborting due to 2 previous errors; 1 warning emitted

I have no idea what's going on here, actually. It would be great to get a backtrace from the proc macro for sure.

[ratijas]

Thanks for your input, @sztomi! Prioritized, and hopefully it will be resolved after upcoming refactoring macros into parse & generate steps.

With proc-macro-error yet to come, meanwhile let's debug your example. I guess, your custom QObject sub-class uses qt_method! syntax with inline function definition, so you are not getting any meaningful errors — since they occur inside macro body. Try splitting your code into declaration and definition, i.e. rewriting with out-of-line qt_method!() form. Here is an example of how it could be done:

• qmetaobject-rs/examples/graph/src/main.rs

  Line 44 in b81438c

  appendSample: qt_method!(fn(&mut self, value: f64)),

  .

• qmetaobject-rs/examples/graph/src/main.rs

  Lines 87 to 93 in b81438c

  	fn appendSample(&mut self, value: f64) {
  	self.m_samples.push(value);
  	self.m_samplesChanged = true;
  	// FIXME! find a better way maybe
  	self.set_flag(QQuickItemFlag::ItemHasContents);
  	(self as &dyn QQuickItem).update();
  	}

I'd say, README should probably mention this feature, instead of only relying on users to read the docs.

[sztomi]

Hey, thanks a lot for that. I actually had an out-of-line definition, I think I found that in one of the examples. And the problem was that I changed the return type of the out-of-line definition while I left the fn type inside the qt_method! call unchanged.