Disallow explicit casting to unknown

[ThomasdenH]

Rule Suggestion

Once #4164 is fixed, it is not necessary anymore to cast to unknown. Implicit downcasting is always possible and should be preferred. A new rule could lint usages of as unknown and suggest implicit downcasting. I'd also like a general like to forbid casting altogether, but I think that is a separate issue.

Examples:

const a = JSON.parse(x) as unknown; // Cast not needed
const a: unknown = JSON.parse(x); // Use this instead

function testFunc(a: unknown): void {
}

testFunc(JSON.parse(x) as unknown); // Remove cast here
testFunc(JSON.parse(x)); // Like this

There is one exception to this rule:

const test = { a: 'This is a string' };
const testOther = test as unknown as { b: 9 }; // Without casting to unknown first, this would throw an error because the types have insufficient overlap

[ThomasdenH]

Thinking about it, this could be an extension to no-unnecessary-type-assertion. Currently the rule lints when the type stays the same, but often downcasting is also unnecessary.

[JoshuaKGoldberg]

Note: per #4534, this issue will be closed in less than a month if no PR is sent to add the rule. If you really need the rule, custom rules are always an option and can be maintained outside this repo!

[JoshuaKGoldberg]

💀 It's time! 💀

TSLint is being deprecated and no longer accepting pull requests for new rules. See #4534. 😱

If you'd like to see this rule implemented, you have two choices:

• Recommended: Use typescript-eslint to lint your TypeScript code with ESLint ✅
• Legacy: Write it as a custom TSLint rule 🤷‍♂️

👋 It was a pleasure open sourcing with you!

If you believe this message was posted here in error, please comment so we can re-open the issue!

[JoshuaKGoldberg]

🤖 Beep boop! 👉 TSLint is deprecated 👈 (#4534) and you should switch to typescript-eslint! 🤖

🔒 This issue is being locked to prevent further unnecessary discussions. Thank you! 👋