Unsafe evolution: add MemorySafetyRules attribute

[jjonescz]

Test plan: #81207

Inspired by RefSafetyRules attribute.

Relevant speclet section: https://github.com/dotnet/csharplang/blob/main/proposals/unsafe-evolution.md#metadata

[jjonescz]

@333fred @jcouv for reviews, thanks #Closed

[jjonescz]

@333fred @jcouv for reviews, thanks #Closed

[333fred]

Didn't fully review the tests, too many questions on the first couple for me to continue.

[333fred]

Is there a version of this test that doesn't manually apply MemorySafetyRules? IE, gets the rules from a reference (as it will from the BCL), and compiles successfully?

It would also be good to have some tests with multiple visible definitions of the attribute:

1. No definition in the BCL, visible from 1 reference (I would expect this to succeed).
2. No definition in the BCL, visible from 2 references (I would expect this to error).
3. No definition in the BCL, visible from 2 references, plus defined in the current compilation (I would expect this to succeed).
4. Definition in the BCL, visible from 2 references (I would expect this to succeed). #Resolved

[jjonescz]

Is there a version of this test that doesn't manually apply MemorySafetyRules? IE, gets the rules from a reference (as it will from the BCL), and compiles successfully?

I'm not sure I understand. A compilation cannot get the rules from a reference, it only gets them if it sets compilationOptions.WithMemorySafetyRules(2). The attribute itself can come from a reference, yes, that's tested in RulesAttribute_FromMetadata.

As for the suggestions, if I understand correctly:

1. tested in RulesAttribute_FromMetadata,
2. adding RulesAttribute_FromMetadata_Multiple (it doesn't fail, we synthesize our own attribute, RefSafetyRules behaves in the same way, adding a test for it too),
3. included in RulesAttribute_FromMetadata_Multiple,
4. seems equivalent to (1) - compiler doesn't distinguish between BCL and other references in this regard.

[333fred]

I'm specifically referring to the attribute. I agree 1-3 are covered, but 4 is not equivalent to 1. WellKnownTypes will prefer from the BCL over references.

[jjonescz]

I see. Added RulesAttribute_FromMetadata_Multiple_AndCorLib. Thanks.

[jcouv]

Done with review pass (commit 6)

[jcouv]

LGTM Thanks (commit 10) with minor feedback on LINQ usage

[jjonescz]

@333fred for another look, thanks