feat(analyzers): REACTOR_DSL_003 — constant / param-ignoring keySelector

[azchohfi]

REACTOR_DSL_003 (Reactor.Dsl, Warning) — spec 060 Batch 2

A typed Reactor collection factory (ListView<T>, GridView<T>, FlipView<T>, TreeView<T>, LazyVStack<T>, LazyHStack<T>, ItemsRepeater<T>, ItemsView<T>) called with a keySelector that returns a constant / literal / null, or that never reads its item parameter, maps every item to the same (or a null) key.

ListView(items, _ => "row", (item, i) => Row(item));   // ⚠ every row collides
ListView(items, x => null,  (item, i) => Row(item));   // ⚠ null keys bail the diff
ListView(items, i => i.Id,  (item, i) => Row(item));   // ✓ stable, unique per-item key

Why it matters (premise, verified against source)

• keySelector is the 2nd positional parameter (Func<T,string>) of the typed collection factories — src/Reactor/Elements/Dsl.cs (ListView<T> at :1464, GridView<T> :1495, FlipView<T> :1525, TreeView<T> :1017, LazyVStack<T> :1560, LazyHStack<T> :1590, ItemsRepeater<T> :1623, ItemsView<T> :1810).
• Duplicate and null keys force a correctness-preserving bailout in KeyedListDiff (the DuplicateKey / NullKey ReportBailout paths, src/Reactor/Core/Internal/KeyedListDiff.cs:194-246), which re-realizes the entire list on every change — losing focus, selection, and animation state.
• docs/guide/collections.md:74-92: keys must be stable, unique, non-null.
• Distinct from REACTOR_DSL_001 (.WithKey on Select-projected layout children).

Detection (low false-positive)

Fires only when the keySelector is a single-parameter lambda whose body provably does not depend on the item — the parameter is never referenced and the body contains no invocation / object-creation / await (any of which could vary per item). It then semantically confirms the argument binds to a keySelector parameter of type Func<T,string> on Microsoft.UI.Reactor.Factories. This rejects:

• selectors that read the item (directly, in interpolation, or via a helper),
• opaque helper calls like _ => Guid.NewGuid().ToString() (could be unique),
• the untyped selection overloads' onSelectedIndexChanged (e.g. ListView(0, _ => {}, …)),
• the IReactorKeyed two-parameter viewBuilder overload,
• method-group selectors, and same-named methods on unrelated types.

No mechanical code-fix (~): there is no obviously-correct key to substitute — the author must choose a stable per-item property. The diagnostic says so.

Changes

• src/Reactor.Analyzers/ConstantKeySelectorAnalyzer.cs (new)
• src/Reactor.Analyzers/AnalyzerReleases.Unshipped.md — descriptor + release row added together (RS2000/RS2002 coupled; build is 0-warning)
• plugins/reactor/skills/reactor-build-and-check/SKILL.md — app-author cheat-table row
• docs/_pipeline/templates/analyzer-architecture.md.dt — rules-table row (template, not generated docs)
• tests/Reactor.Tests/AnalyzerTests/ConstantKeySelectorAnalyzerTests.cs (new, 15 tests)

Verification

• dotnet test tests/Reactor.Tests -p:Platform=x64 --filter FullyQualifiedName~AnalyzerTests → 230 passed (15 new).
• Samples sweep: every real keySelector reads its item; the IReactorKeyed overloads are correctly not keySelector calls → no false positives. Firing nowhere in samples is expected for a hygiene rule (spec-060 §2.4).

[Copilot]

Pull request overview

Adds a new Roslyn analyzer rule (REACTOR_DSL_003) to the Reactor analyzer pack to detect typed collection factory calls where the keySelector is provably constant / null / or otherwise ignores the item parameter, which can cause duplicate/null keys and force keyed diff bailout (full re-realization of the list).

Changes:

• Introduces ConstantKeySelectorAnalyzer (REACTOR_DSL_003) with a cheap syntactic pre-gate and semantic confirmation against Microsoft.UI.Reactor.Factories keySelector: Func<T,string>.
• Adds a dedicated analyzer test suite (15 tests) using a minimal stubbed Factories surface to validate both positive hits and key false-positive exclusions.
• Updates analyzer release notes + contributor-facing docs (skill cheat table + analyzer architecture template) to include the new diagnostic.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Reactor.Tests/AnalyzerTests/ConstantKeySelectorAnalyzerTests.cs	Adds coverage for the new diagnostic, including overload-shape and “unrelated same-name method” negatives.
src/Reactor.Analyzers/ConstantKeySelectorAnalyzer.cs	Implements REACTOR_DSL_003 (syntactic pre-gate + semantic binding confirmation) and reports a warning on the keySelector lambda.
src/Reactor.Analyzers/AnalyzerReleases.Unshipped.md	Registers the new diagnostic in the unshipped analyzer release table.
plugins/reactor/skills/reactor-build-and-check/SKILL.md	Documents REACTOR_DSL_003 in the “common build errors” cheat table with guidance.
docs/_pipeline/templates/analyzer-architecture.md.dt	Adds the new rule to the analyzer architecture rules table (template source).

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

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Pull request overview

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