Compiler support for IgnoresAccessChecksToAttribute

[aelij]

The IgnoresAccessChecksToAttribute is the reverse of the InternalsVisibleToAttribute - it allows an assembly to declare assemblies whose internals would be visible to it. The attribute class isn't declared in the BCL but is recognized by the CLR (Desktop >= 4.6 and Core), i.e. you can declare it in your code and it would work.

This PR adds compiler support for this attribute, just like IVT.

Motivation:

• It would help a lot of scenarios where we usually turn to Reflection (which is severely limited and slow) or, if we're lucky and the sources are available, copy the code.
• A lot of developers are willing to rely on unpublished APIs, taking the risk they might break without notice in future versions. Compiler support means that this would be much safer.
• There's so much useful code (even within Roslyn) that remains internal because publishing and supporting a stable API is a very hard and slow process. Entity Framework went so far as to publish their internal types as public, in a dedicated namespace, while providing no forward compatibility guarantee.

Alternatives:

• Create a build task to generate reference assemblies for the compiler which transforms the internal types and members to public (e.g. using Cecil), and then apply the attribute.

PS - I now realize I probably should've started with an issue. I wanted to get a POC working and it turned out to be pretty simple. Let me know if I should open one instead.

[jcouv]

Thanks for the PR.

FYI @gafter @dotnet/roslyn-compiler This is a community contribution to allow a compilation to have full access to types and members in a specific assembly ([IgnoresAccessChecksToAttribute("assemblyName")]).

Two things:

• we'll have to check with LDM on the feature itself, since this raises language design questions. The next meeting is 2 weeks from now, if I recall correctly. The process is described on the csharplang repo: basically, (1) write a proposal, (2) identify an LDM member who supports/champions it, (3) review it in LDM.
• regarding the CI checks: the CI infrastructure actually builds the compiler once, and the resulting compiler (called "bootstrap") is used to build the solution. It looks like the bootstrap compiler is crashing. I think the way to reproduce this process locally is build/scripts/cibuild.cmd.

14:44:12 
D:\j\workspace\windows_debug---f1572675\Binaries\Bootstrap\Microsoft.CSharp.Core.targets(84,5): error MSB6006: "csc.exe" exited with code -1073741571. [D:\j\workspace\windows_debug---f1572675\src\VisualStudio\IntegrationTest\TestSetup\VisualStudioIntegrationTestSetup.csproj]

Note, there is an IgnoresAccessChecksToAttribute in CoreRT, which is not part of the public API at this point.

Had a quick chat with Neal, he mentioned an alternative way we could approach this: using a new kind of reference instead (from the command-line or Compilation API), which would mean "this reference is part of my program" (since the language cares about what is inside versus external to a "program").

[jcouv]

Just to clarify, don't worry about the CI errors until we hear back from LDM.

[jcouv]

Notes from LDM https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-11-06.md

[Denis535]

May be you could just allow to use typeof and nameof with non-public types? This would be the golden mean.