[type-declaration-immutability] add support for union types when they are interfaces

[m4thieulavoie]

Bug Report

eslint config:

'functional/type-declaration-immutability': [
          'error',
          {
            rules: [
              {
                identifiers: '^(?!I?Mutable).+',
                immutability: 'ReadonlyShallow',
                comparator: 'AtLeast',
                fixer: false,
              },
            ],
            ignoreInterfaces: true,
          },
        ],

Given the above config, I'm basically wanting anything to be at least ReadonlyShallow except for interfaces, which works great. However, given this example

interface A {
  prop: string;
}

interface B {
  prop2: string;
}

type UnionedType = A | B;

I'm getting the following error

This type is declare to have an immutability of at least "ReadonlyShallow" (actual: "Mutable")

I don't think that my config is off. It seems to me like this is more of a cornercase for this new rule.

Expected behavior

Union type of > 1 interfaces doesn't error out if ignoreInterfaces is true

Actual behavior

Union type of > 1 interfaces errors out if ignoreInterfaces is true

Steps to reproduce

Add above config and use above example

Proposed changes

[RebeccaStevens]

I've been thinking about this proposal quite a bit, but I think ultimately, this not something I want to implement into the rule.

As an alternative, you can use a naming convention.
For example, you could prefix such types with IU (short for "interface union") and then ignore this pattern with: "ignorePattern": "^IU[A-Z]\\S+"

[m4thieulavoie]

I've been thinking about this proposal quite a bit, but I think ultimately, this not something I want to implement into the rule.

As an alternative, you can use a naming convention. For example, you could prefix such types with IU (short for "interface union") and then ignore this pattern with: "ignorePattern": "^IU[A-Z]\\S+"

Unfortunately that would require us renaming thousands of union types. I think we won't be able to leverage this rule on our project for the time being 😢

[RebeccaStevens]

You could use this rule in an op-in way.

If you use a config similar to the override for the recommended and lite presets, you'll need to prefix type declarations with Readonly for the rule to enforce they are readonly.