fix(mocks): emit init accessors for init-only properties and indexers

[thomhurst]

Fixes the generated-code break reported in discussion #6829.

Problem

public interface IFoo
{
    int Value { get; init; }
}

var mock = IFoo.Mock();

CS8854: 'IFooMockImpl' does not implement interface member 'IFoo.Value.init'. 'IFooMockImpl.Value.set' cannot implement 'IFoo.Value.init'.
CS8855: Accessors 'IFooMock.IFoo.Value.set' and 'IFoo.Value.init' should both be init-only or neither

MemberDiscovery recorded only whether a property had an accessible setter, dropping IPropertySymbol.SetMethod.IsInitOnly before emission, so every setter came out as set. An implementation has to match the slot's accessor kind exactly, so any type with an init-only property or indexer could not be mocked at all — there was no consumer-side workaround.

Fix

IsInitOnly is carried through MockMemberModel and MockExplicitInterfaceSlot (the shadowed-slot model the typed wrapper forwards from), set in CreatePropertyModel, CreateIndexerModel, MergePropertyAccessors and RecordAdditionalWrapperInterface. Every path that writes a setter now emits init when the slot is init-only: the interface implementation (implicit and explicit), the partial and wrap overrides, all three indexer variants, and the wrapper's explicit forwards.

Two of those paths forward by assignment, which an init accessor cannot do — an init-only member is assignable only on this/base (CS8852):

• the wrap override drops its _wrappedInstance.X = value pass-through and dispatches to the engine, like the abstract-member branch already does;
• the wrapper's explicit forward can't assign Object.X either, so it dispatches through MockRegistry.GetEngine(this) with the same member id and member name the implementation uses. Setups and verifications therefore still observe exactly one call whichever way the accessor is reached.

The partial override keeps its base.X = value fallback: assigning base stays legal inside an init accessor, so virtual init-only properties still fall through to the real implementation when unconfigured.

Getters, plain set accessors and non-init members emit byte-identical output — all 152 existing generator snapshots are unchanged.

Tests

• tests/TUnit.Mocks.Tests/Issue6829Tests.cs — the discussion's repro plus getter configuration, setter verification, the typed wrapper, mixed init/set/get-only members on one interface, an init-only indexer, an abstract class, a virtual class (base fallback) and Mock.Wrap.
• tests/TUnit.Mocks.SourceGenerator.Tests/Issue6829Tests.cs — a snapshot locking in the emitted init accessors across the implementation and the wrapper, and a compile assertion over the interface/abstract/virtual/wrap shapes.

Run locally on net10.0: TUnit.Mocks.Tests 1314/1314, TUnit.Mocks.SourceGenerator.Tests 154/154, TUnit.Mocks.Analyzers.Tests 63/63, TUnit.Mocks.Http.Tests 58/58, TUnit.Mocks.Logging.Tests 31/31, TUnit.Mocks.InternalsAccess.Tests 29/29.

Unrelated issue noticed

Reaching one type through both T.Mock() and Mock.Wrap(new T()) in the same compilation makes the generator throw The hintName '..._MockImplFactory.g.cs' of the added source file must be unique within a generator (MockGenerator adds that hint name from both the partial and the wrap path). It predates this change, so the new tests use two types rather than one; I can open a separate issue for it.

Summary by CodeRabbit

• Bug Fixes

  • Fixed mocking for init-only properties and indexers.
  • Generated mock accessors now correctly use init.
  • Improved configuring, reading, and verifying init-only members across supported mock styles.
  • Resolved conflicts between matching members that use set and init accessors.
  • Fixed explicit interface indexer handling and preserved separate setup surfaces where needed.
  • Ensured shared set/init members use consistent setup and verification behavior.

• Tests

  • Added coverage across interfaces, abstract and virtual classes, wrappers, and generated mocks.

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📝 Walkthrough

Walkthrough

The source generator now preserves init-only properties and indexers through discovery and emits matching init accessors for interface, partial, explicit-interface, and wrapped mock implementations. Regression tests cover generated output, compilation, configuration, verification, base fallback, and wrapped-instance behavior.

Changes

Init-only mock member support

Layer / File(s)	Summary
Member discovery and model contracts src/TUnit.Mocks.SourceGenerator/Models/MockMemberModel.cs, src/TUnit.Mocks.SourceGenerator/Models/MockExplicitInterfaceSlot.cs, src/TUnit.Mocks.SourceGenerator/Discovery/MemberDiscovery.cs	Member models and explicit wrapper slots now record setter kind. Discovery preserves this state during property and indexer merging and creates explicit aliases when init and set slots conflict.
Mock and wrapper accessor generation src/TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs, src/TUnit.Mocks.SourceGenerator/Builders/MockWrapperTypeBuilder.cs, src/TUnit.Mocks.SourceGenerator/Builders/MockMembersBuilder.cs	Generated accessors use init when required. Init-only wrapper and wrap setters dispatch through MockEngine. Explicit indexers use explicit interface implementations, and shared-slot aliases do not generate duplicate extension members.
Generated output and regression coverage tests/TUnit.Mocks.SourceGenerator.Tests/Issue6829Tests.cs, tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/*Issue6829*.verified.txt, tests/TUnit.Mocks.Tests/Issue6829Tests.cs	Snapshot, compile, and runtime tests cover init-only members and conflicting setter kinds across interface, abstract, virtual, and wrapped mock shapes.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix · Severity of issue fixed: Low

Sequence Diagram(s)

sequenceDiagram
  participant TestSource
  participant MockGenerator
  participant GeneratedMock
  participant MockEngine
  TestSource->>MockGenerator: compile init-only and mixed setter-kind members
  MockGenerator->>GeneratedMock: emit matching init and explicit set accessors
  GeneratedMock->>MockEngine: dispatch getter and setter calls
  MockEngine-->>GeneratedMock: return configured or default values

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❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 21.74% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 46 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: generated mocks now emit init accessors for init-only properties and indexers.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Confidence Score: 4/5

The init-only accessor fix appears sound, but the PR is not yet safe to merge because generated extension methods can still collide for multiple same-signature explicit indexers with independent IDs.

Findings

1.  Duplicate Indexer Extension Methods ▶

Summary

This PR carries init-only accessor metadata through mock discovery and generation so properties and indexers emit valid init implementations across interface, partial, wrapped, and typed-wrapper mock paths.

• Emits init rather than set wherever required by the mocked slot.
• Dispatches init-only wrapper and wrapped-instance assignments through the mock engine.
• Splits colliding set and init interface slots into explicit implementations sharing logical member IDs.
• Adds compile, snapshot, and runtime coverage for init-only members and mixed accessor kinds.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    A[Roslyn property or indexer symbol] --> B[MemberDiscovery]
    B --> C{Setter kind collision?}
    C -- No --> D[MockMemberModel with IsInitOnly]
    C -- Yes --> E[Explicit shared-slot alias]
    D --> F[Implementation emitter]
    D --> G[Typed wrapper emitter]
    E --> F
    E --> G
    F --> H[set or init accessor]
    G --> I{Init-only?}
    I -- Yes --> J[Dispatch directly to MockEngine]
    I -- No --> K[Forward assignment]

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Actionable comments posted: 1

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Inline comments:
In `@src/TUnit.Mocks.SourceGenerator/Builders/MockMembersBuilder.cs`:
- Line 143: Update MockMembersBuilder’s indexer-member filtering and slot
grouping so same-signature explicit indexers from additional interfaces do not
generate duplicate Item and SetItem extension methods; compatible slots should
share one setup surface and IDs, while independently addressable slots must
receive distinct callable APIs. Add a regression case covering a class primary
plus two additional interfaces declaring the same get/set string indexer.

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