feat(eslint-plugin): add no-non-null-asserted-nullish-coalescing rule

[sonallux]

Adds a no-non-null-asserted-nullish-coalescing rule that disallows using the non-null assertion on the left operand of the nullish coalescing operator. The non-null assertion is not redundant here because the nullish coalescing operator is explicitly handling null and undefined.

Incorrect code for this rule:

foo! ?? bar

fixes #2853

[typescript-eslint]

Thanks for the PR, @sonallux!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #3349 (db9d5b9) into master (0bb96ca) will decrease coverage by 0.00%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #3349      +/-   ##
==========================================
- Coverage   93.53%   93.52%   -0.01%     
==========================================
  Files         149      150       +1     
  Lines        8043     8067      +24     
  Branches     2550     2556       +6     
==========================================
+ Hits         7523     7545      +22     
- Misses        164      165       +1     
- Partials      356      357       +1     

Flag	Coverage Δ
unittest	93.52% <91.66%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/eslint-plugin/src/configs/all.ts	100.00% <ø> (ø)
...c/rules/no-non-null-asserted-nullish-coalescing.ts	91.66% <91.66%> (ø)

[bradzacher]

Everything looks great so far! But there's one case we probably need to handle the case of a variable being used before it's assigned.

let x: string;
x! ?? '';

Reporting here is bad because there's no way for the user to satisfy it. Which is annoying and reduces user trust.

I think that we could handle this using scope analysis.

So by getting the variable declaration you can inspect its usages to see if it has any writes to it, and if it doesn't then it's a variable that is used before it's assigned.

You can also check the declaration for the weird "trust me it's assigned" syntax: const x!: number

Let me know if you want any pointers on how to use the scope analyser.
You can search this codebase for context.getScope() for a few examples.

[sonallux]

Everything looks great so far! But there's one case we probably need to handle the case of a variable being used before it's assigned.

let x: string;
x! ?? '';

@bradzacher I have added support for handling the use-before-assigned case. I hope I did not forget anything.

[bradzacher]

Tests look good. Implementation is almost correct.
Thanks for your work here!

[sonallux]

Tests look good. Implementation is almost correct.
Thanks for your work here!

@bradzacher Thanks for the feedback. I have already fixed all your comments.

[bradzacher]

LGTM - thanks for this!

[nx-cloud]

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	nx test @typescript-eslint/eslint-plugin-internal
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/eslint-plugin
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/eslint-plugin
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/eslint-plugin
	nx run-many --target=typecheck --all --parallel
	nx test @typescript-eslint/parser
	nx test @typescript-eslint/experimental-utils
	nx test @typescript-eslint/scope-manager
	nx test @typescript-eslint/visitor-keys
	nx test @typescript-eslint/typescript-estree
	nx run-many --target=build --all --parallel
	nx run-many --target=build --all --parallel
	nx run-many --target=build --all --parallel
	nx test @typescript-eslint/parser
	nx run-many --target=build --all --parallel

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