Record structs: emit readonly modifier for synthesized members

[controlflow]

Version Used:

main branch 26.06.2021

Steps to Reproduce:

Compile the following code:

public record struct A(int P) {
  public readonly void ReadOnlyMethod() {
    ToString(); // CS8656
    PrintMembers(null); // CS8656
    Equals(this); // CS8656
    Deconstruct(out _); // CS8656
    GetHashCode(); // CS8656
  }
}

Expected Behavior:

No compiler warning produced.

Actual Behavior:

Warnings like:

CS8656: Call to non-readonly member 'A.ToString()' from a 'readonly' member results in an implicit copy of 'this'.

[RikkiGibson]

We discussed this internally and decided we do want to make the change to make the synthesized record members readonly when possible. I drafted a spec change in dotnet/csharplang#4890.

[RikkiGibson]

tagging @Youssef1313 just in case you find this issue interesting.