[SR-15271] Improve Codable Diagnostics When CodingKeys Do Not Match Properties

[CodaFi]

Previous ID	SR-15271
Radar	rdar://83771311
Original Reporter	@CodaFi
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, StarterBug
Assignee	None
Priority	Medium

md5: 3c54547b3d23d4952661aaf14230a55d

Issue Description:

Consider

struct Foo: Codable {
  var bar: String

  private enum CodingKeys: String, CodingKey {
    case ba
  }
}

What we should have is some kind of notes that explain that the manually-provided CodingKeys are insufficient to satisfy the ad-hoc "nested type requirement" of Codable because of the typo. Instead, all we say is that Foo doesn't conform to Codable.

[CodaFi]

Marking as a starter bug, but this one probably requires a little bit of up-front knowledge of the conformance derivation machinery. I'd be happy to bring somebody up to speed here if need be.

[jirid]

WIP: #39825

[swift-ci]

Comment by Waheed Afolabi (JIRA)

@CodaFi I'd be happy to take this up and hoping you bring me up to speed!