[FreeApophis]

Summary

• New opt-in rule λ1012 (Warning): FirstOrDefault, LastOrDefault, SingleOrDefault and ElementAtOrDefault whose result is immediately tested for null (?? fallback, ?., is null/is not null, == null/!= null, is { } x, is not { } x, is T t). The …OrNone variants make the missing element a None instead of a null every caller has to check by hand.
• Only sequences of a reference type that is not nullable-annotated are reported: for IEnumerable<string?> a null result is ambiguous between "no element" and "the element is null". The overloads with an explicit default value, the static call form and IQueryable (where FirstOrDefault translates to the query) are left alone. The rule is silent when the …OrNone method is not available.
• Code fix: xs.FirstOrDefault() ?? fallback becomes xs.FirstOrNone().GetOrElse(fallback); a throw fallback becomes the lazy GetOrElse(() => throw …). It adds using Funcky.Extensions; next to the existing usings (file level or inside the namespace) when the method does not bind yet. The other null-check shapes are reported without a fix, the rule doc lists the idiomatic replacement for each.
• New helper UsingDirectives.AddUsing in the code fix project (parsed directive, trivia copied from the last existing using, so no CRLF/LF normalization).
• Fills the free ID λ1012 (Add a code fix for CS8714 on Option.Select rewriting to SelectMany with Option.FromNullable #965 became a code fix without a rule).
• Docs: λ1012.md, rule table, SUMMARY, changelog, AnalyzerReleases.Unshipped.

Note for reviewers: the operation tree presents an extension call unreduced (TargetMethod is the static method with the receiver as first argument), so the matcher checks IsExtensionMethod and derives the extension-call form from the syntax.

Test plan

• 6 new tests in PreferOrNoneTest: fix with using added at file level and inside a namespace, every method/overload incl. predicate, index, parentheses and throw fallback, the no-fix shapes, no null check, value types / nullable elements / IQueryable / static form not reported
• 82 analyzer tests pass, Roslyn 4.0 variant builds
• dotnet build Funcky.slnx -warnaserror --no-incremental green (no hits in Funcky itself)

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

The analyzer misses intended nullable-aware cases, and code fixes can change behavior or produce compilation errors.

5 open issues

• Avoid copying preprocessor directives into generated using directives · New
• Preserve explicit generic arguments when renaming the call · New
• Validate result type compatibility before offering the rewrite · New
• Preserve lazy evaluation for non-throwing coalescing fallbacks · New
• Check LINQ method type arguments instead of return type · New

What changed in this PR

Adds λ1012 to encourage using Funcky’s option-returning LINQ methods instead of checking …OrDefault results for null.

Changes:

• Adds null-check detection and a code fix for ?? expressions.
• Adds import insertion support and analyzer/code-fix tests.
• Documents the rule and registers it in release notes.

File	Description
Funcky.Analyzers/​Funcky.Analyzers/​OrNone/​PreferOrNoneAnalyzer.cs	Implements λ1012 detection.
Funcky.Analyzers/​Funcky.Analyzers/​AnalyzerReleases.Unshipped.md	Registers the warning.
Funcky.Analyzers/​Funcky.Analyzers.Test/​OrNone/​PreferOrNoneTest.cs	Tests diagnostics and rewrites.
Funcky.Analyzers/​Funcky.Analyzers.CodeFixes/​OrNone/​PreferOrNoneCodeFix.cs	Rewrites coalescing expressions.
Funcky.Analyzers/​Funcky.Analyzers.CodeFixes/​CodeAnalysisExtensions/​UsingDirectives.cs	Adds extension namespace imports.
Documentation/​src/​SUMMARY.md	Links the rule documentation.
Documentation/​src/​analyzer-rules/​λ1012.md	Explains usage and replacements.
Documentation/​src/​analyzer-rules/​analyzer-rules.md	Adds the rule to the index.
changelog.md	Announces the analyzer and fix.

_{Review effort: Balanced}