feat(eslint-plugin): [restrict-plus-operands] add allow* options (#6161)

* feat(eslint-plugin): [restrict-plus-operands] add allow* options from restrict-template-expressions

* Warn explicitly

* Update packages/eslint-plugin/docs/rules/restrict-plus-operands.md

Co-authored-by: Brad Zacher <brad.zacher@gmail.com>

* Brad suggestion

* Fix lint-markdown

---------

Co-authored-by: Brad Zacher <brad.zacher@gmail.com>

Author: JoshuaKGoldberg

packages/eslint-plugin/docs/rules/restrict-plus-operands.md

@@ -18,70 +18,180 @@ This rule reports when a `+` operation combines two values of different types, o
 ### ❌ Incorrect
 
 ```ts
-var foo = '5.5' + 5;
-var foo = 1n + 1;
+let foo = '5.5' + 5;
+let foo = 1n + 1;
 ```
 
 ### ✅ Correct
 
 ```ts
-var foo = parseInt('5.5', 10) + 10;
-var foo = 1n + 1n;
+let foo = parseInt('5.5', 10) + 10;
+let foo = 1n + 1n;
 ```
 
 ## Options
 
-### `checkCompoundAssignments`
+:::caution
+We generally recommend against using these options, as they limit which varieties of incorrect `+` usage can be checked.
+This in turn severely limits the validation that the rule can do to ensure that resulting strings and numbers are correct.
+
+Safer alternatives to using the `allow*` options include:
+
+- Using variadic forms of logging APIs to avoid needing to `+` values.
+  ```ts
+  // Remove this line
+  console.log('The result is ' + true);
+  // Add this line
+  console.log('The result is', true);
+  ```
+- Using `.toFixed()` to coerce numbers to well-formed string representations:
+  ```ts
+  const number = 1.123456789;
+  const result = 'The number is ' + number.toFixed(2);
+  // result === 'The number is 1.12'
+  ```
+- Calling `.toString()` on other types to mark explicit and intentional string coercion:
+  ```ts
+  const arg = '11';
+  const regex = /[0-9]/;
+  const result =
+    'The result of ' +
+    regex.toString() +
+    '.test("' +
+    arg +
+    '") is ' +
+    regex.test(arg).toString();
+  // result === 'The result of /[0-9]/.test("11") is true'
+  ```
+
+:::
 
-Examples of code for this rule with `{ checkCompoundAssignments: true }`:
+### `allowAny`
+
+Examples of code for this rule with `{ allowAny: true }`:
 
 <!--tabs-->
 
 #### ❌ Incorrect
 
 ```ts
-/*eslint @typescript-eslint/restrict-plus-operands: ["error", { "checkCompoundAssignments": true }]*/
+let fn = (a: number, b: []) => a + b;
+let fn = (a: string, b: []) => a + b;
+```
 
-let foo: string | undefined;
-foo += 'some data';
+#### ✅ Correct
 
-let bar: string = '';
-bar += 0;
+```ts
+let fn = (a: number, b: any) => a + b;
+let fn = (a: string, b: any) => a + b;
+```
+
+### `allowBoolean`
+
+Examples of code for this rule with `{ allowBoolean: true }`:
+
+<!--tabs-->
+
+#### ❌ Incorrect
+
+```ts
+let fn = (a: number, b: unknown) => a + b;
+let fn = (a: string, b: unknown) => a + b;
 ```
 
 #### ✅ Correct
 
 ```ts
-/*eslint @typescript-eslint/restrict-plus-operands: ["error", { "checkCompoundAssignments": true }]*/
+let fn = (a: number, b: boolean) => a + b;
+let fn = (a: string, b: boolean) => a + b;
+```
 
-let foo: number = 0;
-foo += 1;
+### `allowNullish`
 
-let bar = '';
-bar += 'test';
+Examples of code for this rule with `{ allowNullish: true }`:
+
+<!--tabs-->
+
+#### ❌ Incorrect
+
+```ts
+let fn = (a: number, b: unknown) => a + b;
+let fn = (a: number, b: never) => a + b;
+let fn = (a: string, b: unknown) => a + b;
+let fn = (a: string, b: never) => a + b;
 ```
 
-### `allowAny`
+#### ✅ Correct
 
-Examples of code for this rule with `{ allowAny: true }`:
+```ts
+let fn = (a: number, b: undefined) => a + b;
+let fn = (a: number, b: null) => a + b;
+let fn = (a: string, b: undefined) => a + b;
+let fn = (a: string, b: null) => a + b;
+```
+
+### `allowNumberAndString`
+
+Examples of code for this rule with `{ allowNumberAndString: true }`:
 
 <!--tabs-->
 
 #### ❌ Incorrect
 
 ```ts
-var fn = (a: any, b: boolean) => a + b;
-var fn = (a: any, b: []) => a + b;
-var fn = (a: any, b: {}) => a + b;
+let fn = (a: number, b: unknown) => a + b;
+let fn = (a: number, b: never) => a + b;
 ```
 
 #### ✅ Correct
 
 ```ts
-var fn = (a: any, b: any) => a + b;
-var fn = (a: any, b: string) => a + b;
-var fn = (a: any, b: bigint) => a + b;
-var fn = (a: any, b: number) => a + b;
+let fn = (a: number, b: string) => a + b;
+let fn = (a: number, b: number | string) => a + b;
+```
+
+### `allowRegExp`
+
+Examples of code for this rule with `{ allowRegExp: true }`:
+
+<!--tabs-->
+
+#### ❌ Incorrect
+
+```ts
+let fn = (a: number, b: RegExp) => a + b;
+```
+
+#### ✅ Correct
+
+```ts
+let fn = (a: string, b: RegExp) => a + b;
+```
+
+### `checkCompoundAssignments`
+
+Examples of code for this rule with `{ checkCompoundAssignments: true }`:
+
+<!--tabs-->
+
+#### ❌ Incorrect
+
+```ts
+let foo: string | undefined;
+foo += 'some data';
+
+let bar: string = '';
+bar += 0;
+```
+
+#### ✅ Correct
+
+```ts
+let foo: number = 0;
+foo += 1;
+
+let bar = '';
+bar += 'test';
 ```
 
 ## When Not To Use It