Handle captured primary constructor parameters in readonly context.

[AlekseyTs]

dotnet/csharplang#6938

[AlekseyTs]

@cston, @jjonescz, @dotnet/roslyn-compiler Please review.

[jjonescz]

Ever read-only?

Would it be possible to also make the field read-only if it's never mutated? For example in this case:

public class C(string p) {
    public void M() {
        System.Console.WriteLine(p);
    }
}
``` #Resolved

[AlekseyTs]

Would it be possible to also make the field read-only if it's never mutated?

Theoretically, yes. But we don't mark fields readonly based on how they are used.

[AlekseyTs]

@cston, @dotnet/roslyn-compiler For the second review

[AlekseyTs]

@cston, @dotnet/roslyn-compiler For the second review

[cston]

Consider removing this comment since the logic (and the comment) have moved to CanModifyReadonlyField().

[AlekseyTs]

Consider removing this comment since the logic (and the comment) have moved to CanModifyReadonlyField().

I intentionally left this comment here as well. For the benefit of the reader.

[cston]

}

Do we allow capture of ref struct parameters in ref struct primary constructors?

ref struct R1
{
    public int F1;
    public ref int F2;
}
ref struct R2(R1 r)
{
    int M1() => r.F1;
    ref int M2() => ref r.F2;
}
``` #Pending

[AlekseyTs]

Do we allow capture of ref struct parameters in ref struct primary constructors?

No. We disallow capturing of in, ref and out parameters regardless of context. Lambdas don't allow that too. Here is a quote from the speclet: "Capturing is not allowed for ref, in and out parameters. This is similar to a limitation for capturing in lambdas."

[cston]

Is capturing allowed for ref struct parameters passed by value though? If not, the spec and the error message should mention that explicitly.

[AlekseyTs]

Is capturing allowed for ref struct parameters passed by value though? If not, the spec and the error message should mention that explicitly.

Similar to lambdas, ref structs cannot be captured when they are used as primary constructor parameters. This is covered by IllegalCapturingDueToRefness_01 and there is a PROTOTYPE comment there to adjust the wording for primary constructors. I will mention that fact in the speclet as well.