Tracking issue for lints for the `html!` macro.

[teymour-aldridge]

Describe the feature you'd like
The html! macro should output warnings. The developer should be able to ignore these warnings as they see fit.

These warnings should integrate into the compiler and work just as any other lint/warning would.

Is your feature request related to a problem? Please describe. (Optional)
Often it's easy to make mistakes based on

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context (Optional)
This is blocked on the diagnostics API of rustc becoming available in a stable release of Rust.

Implemented lints include:

• Checking that all <a> tags have a href attribute (this is to enable screen readers to work on websites).
• Checking that all <img> tags have an alt attribute (this is to enable screen readers to work on websites).

Possible lints include:

• ...

Possible sources of inspiration for lints include:

• These two pages from the W3C: this page about getting started with web accessibility and this page about more detailed web accessibility concerns.

Please feel free to suggest lint ideas below, or further updates on the status of the diagnostics API.

[teymour-aldridge]

This crate might be of use in adding these errors (works on both stable and nightly).

[siku2]

Just keep in mind that warnings will still only be emitted on nightly but if we use it we don't have to wait for diagnostics to hit stable.
Do you know if the crate makes it possible to suppress warnings though?

[teymour-aldridge]

Do you know if the crate makes it possible to suppress warnings though?

The Diagnostics API probably has a way to do that built in. We can probably detect #[allow] attributes and filter warnings based on those.

Just keep in mind that warnings will still only be emitted on nightly

From my understanding the crate works on stable using a handcrafted API.

[siku2]

The Diagnostics API probably has a way to do that built in. We can probably detect #[allow] attributes and filter warnings based on those.

The Procedural Macro Diagnostics RFC proposes support for "Lint-Associated Warnings" which works with the #[allow()] attribute but AFAIK it hasn't been implemented yet.
Since the warnings are nightly only I don't think it's a problem for them to be unsilenceable. It's just something that will need to be addressed once lint-associated warnings are implemented.

The documentation says:

Beware: warnings are nightly only, they are completely ignored on stable.

There are still a few benefits we get from using the crate:

1. It gives us the ability to use non-abort errors. Currently an error aborts the macro compilation but there are places where it makes sense to keep going and emit all errors at the end.
2. We can start using diagnostics right away. Even though they're useless on stable they should start working when the features are stabilized without us having to do anything.
3. It provides a nice API for adding notes and other sections to the message.

[mc1098]

Unblocking this issue and marking the feature as accepted. I agree with @siku2 that being nightly provides an opt-in behaviour and allows us to start building up lints. Once the diagnostics API stabilises the switch, hopefully, will be relatively simply and we can hook into that API to provide ways to silence the warnings.

[ranile]

Now that #1748 is merged, we have main 2 areas to improve on:

1. Use console.warn in debug mode to display warnings in browser console. This can be a temporary solution until warnings from proc-macros are stabilized.
2. Use proper help/note messages for warnings emitted by the macro.