strict-macro checking should be enabled by default

[lukaslihotzki]

Describe the Bug

Unapplicable attributes are accepted inside wasm_bindgen impl and extern blocks.

Steps to Reproduce

#[wasm_bindgen]
pub struct Foo;

#[wasm_bindgen]
impl Foo {
    #[wasm_bindgen(inline_js = "hello world", raw_module = "something.js")]
    pub fn foo() {
    }
}

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(inline_js = "hello world", raw_module = "something.js")]
    pub fn bar();
}

Expected Behavior

Compile errors in Foo::foo and bar because the attributes inline_js and raw_module are not applicable for these items.

Actual Behavior

These attributes are silently ignored.

[Liamolucko]

wasm-bindgen already supports checking for this when the strict-macro feature is enabled:

error: unused #[wasm_bindgen] attribute
 --> src/lib.rs:8:20
  |
8 |     #[wasm_bindgen(inline_js = "hello world", raw_module = "something.js")]
  |                    ^^^^^^^^^

error: unused #[wasm_bindgen] attribute
 --> src/lib.rs:8:47
  |
8 |     #[wasm_bindgen(inline_js = "hello world", raw_module = "something.js")]
  |                                               ^^^^^^^^^^

error: unused #[wasm_bindgen] attribute
  --> src/lib.rs:14:20
   |
14 |     #[wasm_bindgen(inline_js = "hello world", raw_module = "something.js")]
   |                    ^^^^^^^^^

error: unused #[wasm_bindgen] attribute
  --> src/lib.rs:14:47
   |
14 |     #[wasm_bindgen(inline_js = "hello world", raw_module = "something.js")]
   |                                               ^^^^^^^^^^

error: could not compile `wasm-playground` due to 4 previous errors

So, perhaps that should be enabled by default? It would be a breaking change to do that, but it could also be argued that any code that does that is mistaken.

To do it non-breakingly, it could be a warning by default instead.

[lukaslihotzki]

Can proc_macros warn in stable Rust?

[lukaslihotzki]

Anyway, I think the strict-macro feature is poor design, because Cargo features should be additive, and this feature is not. Its sole purpose is to put additional restrictions on user code. This obviously breaks when some crate A uses unapplicable attributes, and crate B enables the strict-macro feature. Also, this feature is more useful for developers who do not have a lot of experience with wasm-bindgen, but these developers are unlikely to know this feature.

Therefore, I think strict checking should be always on, and the strict-macro feature should be removed (or made an no-op for compatibility). This is also allowed by the SemVer Compatibility chapter of the Cargo book:

Some changes are marked as "minor", even though they carry the potential risk of breaking a build. This is for situations where the potential is extremely low, and the potentially breaking code is unlikely to be written in idiomatic Rust, or is specifically discouraged from use.

[lukaslihotzki]

#2874 is another example of the confusion caused by the current approach.