Add NEEG004 analyzer to detect ToString() on enums with [EnumExtensions] or EnumExtensions<T>

[Copilot]

Implements analyzer to detect ToString() calls on enums decorated with [EnumExtensions] or referenced in EnumExtensions<T> assembly attributes and suggest ToStringFast() replacement for better performance.

Changes

• ToStringAnalyzer (NEEG004): Information-level diagnostic for ToString() calls (with or without format specifiers) on enums with [EnumExtensions] attribute or external enums referenced in EnumExtensions<T> assembly attributes
  • Handles format specifiers: only suggests replacement for compatible formats (empty string, "G", "g")
  • Does not flag incompatible format specifiers like "x", "X", "d", "D"
  • Detects external enum types declared via [assembly: EnumExtensions<T>()] attributes (e.g., DateTimeKind, FileShare)
• ToStringCodeFixProvider: Batch fixer to replace ToString() → ToStringFast() with trivia preservation
  • Removes format parameters when replacing with ToStringFast()
• Test coverage: 32 test cases covering various contexts (expressions, return statements, multiple enums, attribute configurations, format specifiers, external enum types)

Example

// Direct attribute on enum
[EnumExtensions]
public enum Status { Active, Inactive }

// External enum via assembly attribute
[assembly: EnumExtensions<System.DateTimeKind>()]

// Before: NEEG004 info diagnostic
var text1 = status.ToString();
var text2 = status.ToString("G");
var text3 = dateTimeKind.ToString();

// After fix applied
var text1 = status.ToStringFast();
var text2 = status.ToStringFast();
var text3 = dateTimeKind.ToStringFast();

// Not flagged (incompatible format specifiers)
var hex = status.ToString("x");  // No diagnostic

Analyzer triggers on:

• Enums with [EnumExtensions] attribute
• External enums referenced in EnumExtensions<T> assembly attributes
• Only for compatible format specifiers

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create analyzer to detect usages of ToString() and suggest replacing with ToStringFast()</issue_title>
<issue_description>- Only enums which are decorated with [EnumExtensions] should be considered

• The analyzer should be written using the same style as existing analyzers.
• A code fixer should be provided, and should similarly have tests
• The analyzer should be written as Information level
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create analyzer to detect usages of ToString() and suggest replacing with ToStringFast() #187

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[github-actions]

Test Results

    180 files  ±  0      180 suites  ±0   1h 8m 45s ⏱️ + 19m 40s
 21 694 tests + 33   21 690 ✅ + 33   4 💤 ±0  0 ❌ ±0 
386 160 runs  +666  386 088 ✅ +666  72 💤 ±0  0 ❌ ±0 

Results for commit 48573cd. ± Comparison against base commit aabb4d1.

♻️ This comment has been updated with latest results.

[andrewlock]

@copilot when checking if an enum is valid to be replaced with ToStringFast(), we need to check if it's referenced in an EnumExtensions<T> attribute as well. Add tests and update the implementation

[Copilot]

Fixed in commit b291228. The analyzer now checks for EnumExtensions<T> assembly attributes and collects all external enum types during compilation start. Added 8 new tests covering external enum scenarios (DateTimeKind, FileShare) with various format specifiers. All 198 tests pass.