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/ui-tests/Cargo.toml

@@ -9,7 +9,7 @@ doctest = false
 harness = false
 
 [dependencies]
-wasm-bindgen = { path = "../../.." }
+wasm-bindgen = { path = "../../..", features = ["strict-macro"] }
 
 [dev-dependencies]
 compiletest_rs = "0.3"

crates/macro/ui-tests/unused-attributes.rs

@@ -0,0 +1,11 @@
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+impl A {
+    #[wasm_bindgen(method)]
+    #[wasm_bindgen(method)]
+    pub fn foo() {
+    }
+}

crates/macro/ui-tests/unused-attributes.stderr

@@ -0,0 +1,14 @@
+error: unused #[wasm_bindgen] attribute
+ --> $DIR/unused-attributes.rs:7:20
+  |
+7 |     #[wasm_bindgen(method)]
+  |                    ^^^^^^
+
+error: unused #[wasm_bindgen] attribute
+ --> $DIR/unused-attributes.rs:8:20
+  |
+8 |     #[wasm_bindgen(method)]
+  |                    ^^^^^^
+
+error: aborting due to 2 previous errors
+