Close the gaps in the documentation generator's configuration key extractor

[meziantou]

What

GetRuleConfigurationKeys in the documentation generator only recognized fields whose type was exactly ConfigurationDefinition<T>, and TryGetRuleIdPrefix gave up unless the key's first two characters were MA. Three consequences:

• dotnet_diagnostic.MA0048.excluded_symbol_names — the one option written with a dotnet_diagnostic. prefix — was never attributed to MA0048, so ValidateRuleDocumentationContainsConfigurationKeys never checked that MA0048.md mentions it, and the generated "Configurable" tooltip listed 8 keys instead of 9.
• The definitions held in the MA0220 registry (InvalidRegexConfigurationAnalyzer.RegexConfigurations, an array) were not walked at all.
• A key that could not be attributed to any rule was silently dropped.

Every one of those keys is in fact documented today, by hand. The documentation check is the only automated guard that a configurable rule documents its options, and it had holes exactly where a rule deviates from the norm — which is when documentation is most likely to be missed.

Changes

Extractor (src/DocumentationGenerator/Program.cs)

• Arrays and collections of ConfigurationDefinition<T> are walked in addition to bare fields. The type filter still runs before GetValue, so no unrelated static constructors are triggered. A container whose shape the walker cannot traverse now throws rather than dropping its keys quietly.
• A leading dotnet_diagnostic. is stripped before matching the rule id.
• A key that cannot be attributed to any rule fails the run, with an allow list for options not owned by a rule (currently just max_line_length).

Option key casing

The keys used three casings (.exclude_tostring_methods, MA0007.IgnoreCatchAllArm, MA0032.allowOverloadsWithOptionalParameters), which worked because Roslyn's key comparer is case-insensitive but made the option surface unguessable. ConfigurationDefinition<T> now holds the current name of an option followed by its legacy names, the first key set in the configuration winning:

private static readonly ConfigurationDefinition<bool> IgnoreCatchAllArmConfiguration =
    new([Rule.Id + ".ignore_catch_all_arm", Rule.Id + ".IgnoreCatchAllArm"], defaultValue: false);

current name	legacy name, still supported
MA0007.ignore_catch_all_arm	MA0007.IgnoreCatchAllArm
MA0032.allow_overloads_with_optional_parameters	MA0032.allowOverloadsWithOptionalParameters
MA0115.report_pascal_case_unmatched_parameter	MA0115.ReportPascalCaseUnmatchedParameter

This replaces the IsHidden property, whose only use was hiding the legacy name of an option from the documentation. MA0053 (class_with_virtual_member_shoud_be_sealed) and MA0104 (namepaces_regex) no longer need a second definition and a "read the legacy key, pass it as the default" lookup, and GetConfigurationValue(ISymbol, definition, defaultValue) is removed with its last caller. The generator documents the current name only, while still checking that every name — legacy included — resolves to a rule.

Notes for the reviewer

• The single-key constructors delegate to this([key], …), so the ~60 one-key definitions are untouched.
• Keys is an ImmutableArray<string> built from a string[] constructor parameter rather than a collection-expression target: the netstandard2.0 / roslyn4.8 build's System.Collections.Immutable predates CollectionBuilderAttribute (CS9210). This keeps call sites written as collection expressions and the lookup's foreach allocation-free.
• MA0220 iterates configuration.Keys, so an invalid regex is still reported against the key the user actually wrote.
• MA0042/MA0045 and the other rules that share a setting across two rule ids keep separate definitions — those are shared options, not aliases, and each is documented under its own rule.
• The generator produces no markdown change from the Keys refactor itself: the documented key set is identical before and after.

Testing

• dotnet build clean across all five Roslyn versions.
• dotnet run --project src/DocumentationGenerator exits 0 and is idempotent. I verified the new guard fires by temporarily emptying the allow list, and that the array walking finds the MA0220 registry's 3 definitions where it previously found none.
• Full roslyn5.9 suite: 3913 passed, 0 failed. The 349–351 configuration-related tests pass on 4.8 / 4.14 / 5.0 / 5.6 / 5.9; they include the pre-existing tests that set MA0053.class_with_virtual_member_shoud_be_sealed and MA0104.namepaces_regex, so the older aliases are covered through the new mechanism. The three renamed options got their existing tests parameterized over both the new and the legacy key.