Expand Generator Diagnostics

[j-d-ha]

Priority: P3 - Nice to have
Tier: 4 - Polish
Effort: Large (10-12 hours)

Already In Codebase

• Diagnostics exist but are currently minimal:
  • DM0001 CannotConvertFromAttributeValue
  • DM0101 NoMapperMethodsFound
  • DM0102 MismatchedPocoTypes
  • Defined in src/LayeredCraft.DynamoMapper.Generators/Diagnostics/DiagnosticDescriptors.cs

Goal

Expand generator diagnostics to cover common misconfigurations with clear, actionable messages and correct source locations.

Scope (examples)

• Duplicate DynamoDB field names (after convention + overrides)
• Member name mismatches / unknown member references in [DynamoField] and [DynamoIgnore]
• Conflicting ignore vs field config
• Kind override validation (ties into Story 3.2 / issue Validate Kind Override Compatibility #20)
• Hook signature validation (ties into Story 2.x)
• Custom method signature validation (ties into Story 3.5 / issue Validate DynamoField Custom Methods (ToMethod/FromMethod) #23)
• Converter diagnostics (ties into Story 3.4 / issue Add Converter Type Support (IDynamoConverter) #22)

Implementation notes

• Add new DiagnosticDescriptors with stable IDs.
• Attach diagnostics to the most relevant location (attribute argument, method name token, etc.).
• Prefer failing fast per member while still generating other valid mappings.

Acceptance criteria

• Key configuration errors are reported at compile time with good messages.
• Diagnostics point to the correct source span.
• Tests cover at least a representative set of diagnostics.