EqualityGenerator: Generated code is uncompilable for readonly structs

[DanielSchuessler]

Hello, very interesting library, thank you :)

I'm submitting a...

Bug report / Feature request

Current behavior

To reproduce, just apply GeneratedEquality to any readonly struct. The generated mutable int? _computedHashCode field is not allowed for readonly structs.

Expected behavior

Possible solutions:

• Just recompute the hash on every GetHashCode call.
• Initialize _computedHashCode in the constructor. I don't think this is possible by adding additional members to the struct; would need cooperation from the user's constructor code.
• (Bad, but for completeness:) Mark _computedHashCode readonly and write to it in an unsafe way.

Complication: There doesn't seem to be a public Roslyn API to detect readonly structs yet: https://github.com/dotnet/roslyn/issues/23792

I have a branch here (I was hesitant to just barge in with a pull request) that resolves the issue by adding an AddHashCodeField property to GeneratedEqualityAttribute. If it is false, _computedHashCode is not generated and the hash is recomputed on every call to GetHashCode.

IMHO, this is a useful feature even without the readonly struct problem because caching the hash code is potentially detrimental to performance if the calculation is just a few arithmetic instructions (since fewer of the objects to be hashed will fit into processor caches).

Minimal reproduction of the problem with instructions

See above

Environment

Nuget Package: Uno.CodeGen

Package Version(s): 1.29.0; master branch

Affected platform(s), Visual Studio: Presumably all platforms and Visual Studio versions that support readonly structs.