Add MA0191: Detect null! and default! usage

[Copilot]

The null-forgiving operator (!) suppresses nullable warnings and is often a code smell — especially when used as null! or default! to defer initialization. This adds a new opt-in rule to flag such usages.

New rule: MA0191 — Do not use the null-forgiving operator

• Detects null!, default!, and default(T)! via SyntaxKind.SuppressNullableWarningExpression
• Other uses of ! on non-null/default expressions are not reported (e.g. someExpression! is valid)
• Disabled by default — enable via .editorconfig
• Severity: Warning, Category: Design

// MA0191 reported on these
HttpClient _httpClient = null!;
string TempDir { get; set; } = default!;
string _value = default(string)!;

// MA0191 not reported (other uses of ! are valid)
_ = model.Value!.Length;

dotnet_diagnostic.MA0191.severity = warning

Files

• RuleIdentifiers.cs — adds MA0191 constant
• DoNotUseNullForgivenessAnalyzer.cs — syntax-based analyzer
• docs/Rules/MA0191.md — rule documentation
• Auto-generated: README, editorconfig pack files updated via DocumentationGenerator

[meziantou]

@copilot It should only report usage of null! and default!. Other usages are valid

[Copilot]

Updated in 2a79bf7 — the analyzer now only reports null!, default!, and default(T)!. Other uses of the null-forgiving operator (e.g. model.Value!) are no longer flagged.