Add rule: strict-string-expressions

[ColCh]

PR checklist

• Addresses an existing issue: fixes Rule suggestion: strict-string-expressions (no implicit function to string) #4512
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update ???

Overview of change:

adds implementation for rule discussed in #4512

Is there anything you'd like reviewers to focus on?

CHANGELOG.md entry:

[new-rule] strict-string-expressions reports errors on type coercions found in string expressions

[JoshuaKGoldberg]

@ColCh looks like you've got some lint failures from CI running this rule on TSLint itself. https://circleci.com/gh/palantir/tslint/16921?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

That looks like it's exposed an edge case or two. Should | null and | undefined be allowed in string expressions? I think so; it's common for error logging to have strings like:

function complain(error?: Error | null) {
    console.error(`Oh dear: ${error}`);
}

new Error().stack being string | undefined is also pretty annoying.

If you really wanted, you could make an issue to add an option to disallow this, or file a followup issue after this PR... but IMO it's fine to just always allow these.

[ColCh]

Yes, I agree with you. That undefined in union as well as other empty-in-runtime values like never or null creates so much problems :D

I think that better option is make those as configurable thing

[ColCh]

real case here 🎉

[ColCh]

@JoshuaKGoldberg seems to pass now

I don't understand why ci/circleci: testNext is failing but it's failing in master branch too

so... please review :)

[JoshuaKGoldberg]

LGTM, thanks for pushing this through @ColCh! (and the helpful PR comments!)

[ColCh]

Thank you!