[v2.0] Add compile-time validation for DependsOn attributes via source generator

[thomhurst]

Summary

The DependsOnAttribute performs type validation at runtime (during attribute instantiation), missing the opportunity for compile-time safety. Developers can use the non-generic version with an incorrect type and only discover the error when running the pipeline.

Current Behavior

File: src/ModularPipelines/Attributes/DependsOnAttribute.cs

public DependsOnAttribute(Type type)
{
    if (!type.IsAssignableTo(typeof(IModule)))
        throw new Exception($"{type.FullName} is not a Module");
    Type = type;
}

This validation happens at reflection time, not compile time.

Problems

1. No compile-time safety: Errors only surface during pipeline initialization
2. Non-generic overload risk: Developers might use [DependsOn(typeof(NotAModule))]
3. Late feedback: CI pipelines fail at runtime instead of build time
4. Poor IDE experience: No squiggles or warnings for invalid types

Proposed Solution

Source Generator Diagnostics

Add a source generator that emits compile-time diagnostics:

// Generator analyzes [DependsOn(typeof(X))] attributes
// Emits error if X does not implement IModule

[DependsOn(typeof(string))]  // Compile error: MP001 - 'string' does not implement IModule
public class MyModule : Module<None> { }

Diagnostic codes:

• MP001: Type does not implement IModule
• MP002: Circular dependency detected
• MP003: Module depends on itself

API Change (Optional)

Consider marking the non-generic constructor as obsolete:

[Obsolete("Use the generic DependsOn<TModule> attribute for compile-time safety")]
public DependsOnAttribute(Type type) { ... }

Implementation

1. Extend ModularPipelines.SourceGenerator with attribute analyzer
2. Register diagnostic descriptors for module validation errors
3. Emit diagnostics during code generation phase
4. Add tests for all diagnostic scenarios

Impact

• Semi-breaking: Adds compile errors for previously runtime errors
• Improves: Developer experience, build-time safety, IDE integration

Labels

• v2.0
• enhancement
• source-generator
• developer-experience