Document dotnet_prefer_system_hash_code (IDE0070) and new IDE0221 rule

[Copilot]

Two EditorConfig style rules introduced in Visual Studio 2026 18.4.0 were undocumented. This PR adds the missing option to IDE0070 and creates the full reference page for IDE0221.

IDE0070 — dotnet_prefer_system_hash_code

The rule already existed but was documented as having no options. The option was added:

• Updated ide0070.md: added dotnet_prefer_system_hash_code to frontmatter keywords, properties table, and replaced the "no options" placeholder with a proper Options section (true | false, default: true)
• Updated index.md: linked the option in the rule index table

IDE0221 — dotnet_style_prefer_non_hidden_explicit_cast_in_source

New rule that flags explicit casts where the compiler silently inserts an additional hidden explicit cast. Both casts can fail at runtime independently, so the rule recommends making all intermediate casts visible.

// Violation: compiler inserts a hidden (Base) cast
var v = (Derived)new Castable();

// Fixed: both casts are explicit in source
var v2 = (Derived)(Base)new Castable();

• Created ide0221.md with full rule reference (C#-only, option true | false, default: true)
• Added IDE0221 to index.md table, language-rules.md C# expression-level preferences list, and tools-diagnostics/toc.yml
• Added both new options to the code-style-rule-options.md example EditorConfig

---

Internal previews

📄 File	🔗 Preview link
docs/fundamentals/code-analysis/code-style-rule-options.md	docs/fundamentals/code-analysis/code-style-rule-options
docs/fundamentals/code-analysis/style-rules/ide0070.md	docs/fundamentals/code-analysis/style-rules/ide0070
docs/fundamentals/code-analysis/style-rules/ide0221.md	docs/fundamentals/code-analysis/style-rules/ide0221
docs/fundamentals/code-analysis/style-rules/index.md	docs/fundamentals/code-analysis/style-rules/index
docs/fundamentals/code-analysis/style-rules/language-rules.md	docs/fundamentals/code-analysis/style-rules/language-rules
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc

[Copilot]

Pull request overview

This PR updates the .NET code style rules documentation to cover two newly introduced EditorConfig options, including adding a new rule reference page for IDE0221 and documenting the previously-undocumented option for IDE0070.

Changes:

• Documented the dotnet_prefer_system_hash_code option for IDE0070 and linked it from the style rules index.
• Added a new reference page for IDE0221, including option details and examples.
• Wired IDE0221 into the relevant navigation/indices (rule index, language rules list, tools diagnostics TOC), and added both options to the example EditorConfig.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/navigate/tools-diagnostics/toc.yml	Adds IDE0221 to the tools diagnostics TOC for discoverability.
docs/fundamentals/code-analysis/style-rules/language-rules.md	Adds IDE0221 to the C# style rules list.
docs/fundamentals/code-analysis/style-rules/index.md	Links IDE0070 and IDE0221 to their documented options in the main rules index table.
docs/fundamentals/code-analysis/style-rules/ide0221.md	Introduces the full IDE0221 rule reference page, including option details and examples.
docs/fundamentals/code-analysis/style-rules/ide0070.md	Documents the dotnet_prefer_system_hash_code option for IDE0070.
docs/fundamentals/code-analysis/code-style-rule-options.md	Adds the two new options to the example EditorConfig snippet.

[BillWagner]

This LGTM. You can when ready.