Add strictness flags

[nex3]

A number of users have requested the ability to opt in to stricter Sass compilation, in which code that would normally just causes a warning (especially a deprecation warning) instead produces a compile error. We should add these flags for all current deprecation warnings to the CLI and both APIs, and possibly add configuration file support so that the can be consistently configured for teams.

TypeScript's compiler options provide some prior art here. I suspect we don't want to add a general --strict flag that enables all strict checks, though, since this will mean some downstream users will break when we add new deprecations.

• CLI support
• Dart API support
• JS API support
• Documentation

[jathak]

As part of this, there should also be an option to error on @warn rules.

[Ariane-B]

So when can we hope to see this? I'm guessing not soon, since the proposal doesn't seem to be entirely decided upon?

[jathak]

We have many things we want to work on and while this is one of them, we don't currently have an ETA for this. We'll add an update here when we have news to share.

[jjonescz]

As part of this, there should also be an option to error on @warn rules.

Note: for failing on warnings in CI, I'm using the logger option like this (not ideal, but better than nothing):

logger: process.env.CI
  ? undefined
  : {
    warn: (message: string): void => {
      throw new Error(`Warning from SASS reported: ${message}`);
    },
  },

[jathak]

Forgot to close this issue, but this is now fully released. You can use the --fatal-deprecation flag on the CLI or the fatalDeprecations option in the JS API to make a particular deprecation type an error, or pass in a Dart Sass version to make all deprecations active in that version errors.

There's also --silence-deprecation/silenceDeprecations to silence a particular deprecation type.