feat(linter): implement unicorn/no-immediate-mutation rule

[tt-a1i]

Summary

Implements the unicorn/no-immediate-mutation rule that disallows mutating a variable immediately after initialization.

Closes #9534

What it detects

• Array mutations: array.push(), array.unshift()
• Object property assignments: obj.foo = bar
• Object.assign() calls
• Set mutations: set.add()
• Map mutations: map.set()

Features

• Handles both variable declarations and assignment expressions
• Correctly skips self-referencing mutations (e.g., array.push(array[0]))
• Supports all statement containers including SwitchCase
• Validates global references for Object/Set/Map to avoid false positives when these identifiers are shadowed

Examples

// Bad
const array = [1, 2];
array.push(3);

const obj = { foo: 1 };
obj.bar = 2;

const set = new Set([1]);
set.add(2);

// Good
const array = [1, 2, 3];

const obj = { foo: 1, bar: 2 };

const set = new Set([1, 2]);

Test plan

• All 94 pass cases and 118 fail cases from eslint-plugin-unicorn pass
• Clippy passes with no warnings

AI Assistance Disclosure

I used Codex to review the changes, sanity-check the implementation against existing patterns, and help spot potential edge cases.

[codspeed-hq]

CodSpeed Performance Report

Merging #17118 will not alter performance

_{Comparing tt-a1i:feat/no-immediate-mutation (e8258f2) with main (b87600a)}

Summary

✅ 4 untouched
⏩ 41 skipped¹

Footnotes

1. 41 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[camc314]

Thank you!