Add analyzer for StringBuilder.Append(enum) to suggest ToStringFast() (NEEG012)

[Copilot]

StringBuilder.Append(object?) implicitly calls ToString() on enum values, which is significantly slower than the generated ToStringFast() method. This analyzer detects such usage and suggests the optimized alternative.

Implementation

• StringBuilderAppendAnalyzer: Diagnostic NEEG012 (Warning) that flags:
  • StringBuilder.Append(enumValue) calls where the enum has [EnumExtensions] or is in [EnumExtensions<T>]
• StringBuilderAppendCodeFixProvider: Automatic fix replaces enum arguments with enumValue.ToStringFast()
• Tests: 19 test cases covering standard usage, external enums, namespace handling, and edge cases

Examples

StringBuilder.Append

var sb = new StringBuilder();
MyEnum value = MyEnum.First;

// Flagged by NEEG012
sb.Append(value);

// Suggested fix
sb.Append(value.ToStringFast());

Follows the pattern established by ToStringAnalyzer (NEEG004) for string interpolation detection.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Detect usage of StringBuilder.Append() and encourage replacing with ToStringFast()</issue_title>
<issue_description>Original request:

Hello, @andrewlock
i thing, analyzer can check use enum members in StringBuilder.Append(object? value) method, because Append(object? value) invoke ToStirng()method of object

Originally posted by @tecAmoRaller in #195
</issue_description>

<agent_instructions>We're looking for code that looks like this:

var sb = new StringBuilder();
MyEnum value = MyEnum.First;
sb.Append(value);

and suggesting replacing with:

var sb = new StringBuilder();
MyEnum value = MyEnum.First;
sb.Append(value.ToStringFast());

• Only enums which are decorated with [EnumExtensions] or referenced in [EnumExtensions<T> should be considered
• The analyzer should be written using the same style as existing analyzers, for example the ToStringAnalyzer.
• A code fixer should be provided, and should similarly have tests that match the existing tests for other analyzers
• The analyzer should be written as Warning level
• The suggestion/fix should use the extension implementation of ToStringFast()</agent_instructions>

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

• Fixes Detect usage of StringBuilder.Append() and encourage replacing with ToStringFast() #229

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[andrewlock]

@copilot we should probably also handle the various StringBuilder.AppendFormat() methods too, where the first argument is a string and one of the other arguments is a target enum. We can use the same analyzer for this I think

[Copilot]

@copilot we should probably also handle the various StringBuilder.AppendFormat() methods too, where the first argument is a string and one of the other arguments is a target enum. We can use the s...

Added support for StringBuilder.AppendFormat() in commit 6548e92. The analyzer now detects enum arguments in all AppendFormat overloads (with and without IFormatProvider). Added 9 comprehensive tests covering various scenarios including multiple enums, mixed argument types, and provider overloads. All 28 tests passing.

[andrewlock]

Append format is actually way more complicated so it's probably not worth the effort

[github-actions]

Test Results

    180 files  ±  0      180 suites  ±0   6h 37m 29s ⏱️ + 1h 58m 19s
 21 876 tests + 19   21 872 ✅ + 19   4 💤 ±0  0 ❌ ±0 
389 982 runs  +414  389 910 ✅ +414  72 💤 ±0  0 ❌ ±0 

Results for commit 90bbfe4. ± Comparison against base commit 890f053.

[Copilot]

Pull request overview

This PR adds a new analyzer (NEEG012) that detects when enum values are passed directly to StringBuilder.Append(object?), which implicitly calls ToString(). The analyzer suggests using the generated ToStringFast() method instead for better performance.

Key changes:

• Implements StringBuilderAppendAnalyzer to detect StringBuilder.Append(enumValue) calls
• Provides StringBuilderAppendCodeFixProvider to automatically replace with ToStringFast()
• Adds comprehensive test coverage with 19 test cases

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/NetEscapades.EnumGenerators/Diagnostics/UsageAnalyzers/StringBuilderAppendAnalyzer.cs	New analyzer that flags StringBuilder.Append() calls with enum arguments
src/NetEscapades.EnumGenerators/Diagnostics/UsageAnalyzers/StringBuilderAppendCodeFixProvider.cs	Code fix provider that wraps enum arguments with ToStringFast()
tests/NetEscapades.EnumGenerators.Tests/StringBuilderAppendAnalyzerTests.cs	19 test cases covering various scenarios including external enums and namespace handling
tests/NetEscapades.EnumGenerators.IntegrationTests/AnalyzerTests.cs	Integration test for NEEG012 with pragma disable/restore
tests/NetEscapades.EnumGenerators.Nuget.AnalyzerTests/AnalyzerTests.cs	NuGet package analyzer test case for NEEG012
tests/NetEscapades.EnumGenerators.IntegrationTests/.editorconfig	Adds NEEG012 severity configuration
README.md	Documents NEEG012 in the analyzer configuration section
src/NetEscapades.EnumGenerators/README.md	Documents NEEG012 in the package README
build/Build.cs	Adds NEEG012 to expected analyzer errors in build verification

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[Copilot]

Extra blank line can be removed for consistency with other analyzers in the codebase.

Suggested change

[tecAmoRaller]

@andrewlock plz publish new version