Assert all attributes are used by default

This commit implements a system that will assert that all
`#[wasm_bindgen]` attributes are actually used during compilation. This
should help ensure that we don't sneak in stray attributes that don't
actually end up having any meaning, and hopefully make it a bit easier
to learn `#[wasm_bindgen]`!

crates/macro-support/src/lib.rs

@@ -20,6 +20,7 @@ mod parser;
 
 /// Takes the parsed input from a `#[wasm_bindgen]` macro and returns the generated bindings
 pub fn expand(attr: TokenStream, input: TokenStream) -> Result<TokenStream, Diagnostic> {
+    parser::reset_attrs_used();
     let item = syn::parse2::<syn::Item>(input)?;
     let opts = syn::parse2(attr)?;
 
@@ -27,5 +28,11 @@ pub fn expand(attr: TokenStream, input: TokenStream) -> Result<TokenStream, Diag
     let mut program = backend::ast::Program::default();
     item.macro_parse(&mut program, (Some(opts), &mut tokens))?;
     program.try_to_tokens(&mut tokens)?;
+
+    // If we successfully got here then we should have used up all attributes
+    // and considered all of them to see if they were used. If one was forgotten
+    // that's a bug on our end, so sanity check here.
+    parser::assert_all_attrs_checked();
+
     Ok(tokens)
 }