Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

Since --enforceReadonly was never released (microsoft/TypeScript#58296 is still open), it may be a good idea to think about implementing this.

Fail Cases

type MutableA = { a: string };
type ReadonlyA = { readonly a: string };

const mutate = (mut: MutableA): void => {
    mut.a = "whoops";
};

const readonlyA: ReadonlyA = { a: "readonly?" };
mutate(readonlyA);

Pass Cases

type MutableA = { a: string };
type ReadonlyA = { readonly a: string };

const read = (mut: ReadonlyA): void => {
    // No longer allowed:
    // mut.a = "whoops";
};

const readonlyA: ReadonlyA = { a: "readonly?" };
read(readonlyA);

Additional Info

Other implementations:

• https://github.com/iliubinskii/eslint-plugin-misc/blob/master/src/typescript/base/no-unsafe-object-assignment.ts
• (eslint 8) https://github.com/danielnixon/eslint-plugin-total-functions/blob/master/src/rules/no-unsafe-mutable-readonly-assignment.ts

Previous issue: #5954