Analyzer-Idea: `Un/SupportedOSPlatformAttribute` should be consistent with OSConditionAttribute

[LukasGelke]

Summary

CA1416 Helps with identifying platform-API problems. If you decorate a TestMethod/Class with another Un/SupportedOSPlatformAttribute, CA1416 is happy, but the test still runs via MsTest

Background and Motivation

the Attributes are the "correct way"

Proposed Feature

Analyzer+CodeFix who finds dependencies like this:

[SupportedOSPlatform("linux")] -> [OSCondition(OperatingSystems.Linux)]
[UnupportedOSPlatform("linux")] -> [OSCondition(ConditionMode.Exclude, OperatingSystems.Linux)]

in my short thought process, this 1-to-1 mapping should work. But OSCondition is AllowMultiple=False. so there needs to be some clever thinking, or just some restrictions, ie only-exclude or only-include

[SupportedOSPlatform("linux"), SupportedOSPlatform("OSX")] -> [OSCondition(OperatingSystems.Linux | OperatingSystems.OSX)]
[SupportedOSPlatform("linux"), UnsupportedOSPlatform("OSX")] -> can't compute codefix, unless you read containg symbols attributes (class/assembly)

• also, consider the SupportedOSPlatformAttributes may contain a version a la "windows10.0".
• Analyzer should also check existing pairs, so that something like this can't happen:
  • [SupportedOSPlatform("windows"), OSCondition(OperatingSystems.Linux)]

Alternative Designs

respect SupportedOSPlatformAttribute in MsTest

• bad, because then OSConditionAttributes are invalid on the same method/class

[Evangelink]

Thanks for the suggestion Lukas Gelke (@LukasGelke)! I think that's a strong addition, adding this for our next milestone.