Assert all attributes are used by default

Cargo.toml

@@ -30,6 +30,10 @@ std = []
 serde-serialize = ["serde", "serde_json", "std"]
 nightly = []
 
+# Whether or not the `#[wasm_bindgen]` macro is strict and generates an error on
+# all unused attributes
+strict-macro = ["wasm-bindgen-macro/strict-macro"]
+
 # This is only for debugging wasm-bindgen! No stability guarantees, so enable
 # this at your own peril!
 xxx_debug_only_print_generated_code = ["wasm-bindgen-macro/xxx_debug_only_print_generated_code"]

crates/backend/src/error.rs

@@ -5,7 +5,7 @@ use syn::parse::Error;
 #[macro_export]
 macro_rules! err_span {
     ($span:expr, $($msg:tt)*) => (
-        $crate::Diagnostic::span_error(&$span, format!($($msg)*))
+        $crate::Diagnostic::spanned_error(&$span, format!($($msg)*))
     )
 }
 
@@ -43,7 +43,16 @@ impl Diagnostic {
         }
     }
 
-    pub fn span_error<T: Into<String>>(node: &ToTokens, text: T) -> Diagnostic {
+    pub fn span_error<T: Into<String>>(span: Span, text: T) -> Diagnostic {
+        Diagnostic {
+            inner: Repr::Single {
+                text: text.into(),
+                span: Some((span, span)),
+            },
+        }
+    }
+
+    pub fn spanned_error<T: Into<String>>(node: &ToTokens, text: T) -> Diagnostic {
         Diagnostic {
             inner: Repr::Single {
                 text: text.into(),

crates/macro-support/Cargo.toml

@@ -13,6 +13,7 @@ The part of the implementation of the `#[wasm_bindgen]` attribute that is not in
 [features]
 spans = ["wasm-bindgen-backend/spans"]
 extra-traits = ["syn/extra-traits"]
+strict-macro = []
 
 [dependencies]
 syn = { version = '0.15.0', features = ['visit'] }

crates/macro-support/src/lib.rs

@@ -20,12 +20,19 @@ 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)?;
 
     let mut tokens = proc_macro2::TokenStream::new();
     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)
 }