[Platform Compatibility Analyzer] Nested APIs shouldn't be allowed to expand the platform set

[terrajobst]

Repro

[SupportedOSPlatform("ios")]
class Some
{    
    [SupportedOSPlatform("windows")]
    public static void Api() {}
}

Expected

'Api' cannot widen the set of supported platforms of its containing symbol Some.

Work Items

Each of these should produce separate diagnostic ids:

• Inconsistent List section of the Advanced Scenarios of Combining Attributes, when conflicting attributes are applied at the same level
• Nested APIs shouldn't be allowed to expand the platform set and version support

[carlossanlop]

Talked with @buyaa-n and since this proposal would require a new ruleID, we think it should go through API review, so I'm moving it to the runtime repo.

[carlossanlop]

@buyaa-n will help group this issue with these two others into one proposal:
https://github.com/dotnet/roslyn-analyzers/issues/4014
dotnet/roslyn-analyzers#4015

[buyaa-n]

Category: Interoperability
Suggested Severity: Info

This analyzer might be implemented as part of #45851 and should cover the following main scenarios:

• Nested APIs shouldn't be allowed to expand the platform set and version support

  • Note: this is happening in runtime for multitargeted projects when SDK adds [SupportedOSPlatform] attribute for the targeted builds

• It should flag Inconsistent list determined by the following rule:

  • If either [SupportedOSPlatform] or [UnsupportedOSPlatform] attributes are present, we group all attributes by OS platform identifier:
    • Allow list. If the lowest version for each OS platform is a [SupportedOSPlatform] attribute, the API is considered to only be supported by the listed platforms and unsupported by all other platforms.
    • Deny list. If the lowest version for each OS platform is a [UnsupportedOSPlatform] attribute, then the API is considered to only be unsupported by the listed platforms and supported by all other platforms.
    • Inconsistent list. If for some platforms the lowest version attribute is [SupportedOSPlatform] while for others it is [UnsupportedOSPlatform], the analyzer will produce a warning on the API definition because the API is attributed inconsistently.

• We can add multiple attributes for an API with the same platform, with the following rule:

  • Allow list could have only one [SupportedOSPlatform] attribute and an optional [UnsupportedOSPlatform] attribute with > version. This implies that if an API was supported on a platform then could have unsupported later, but it cannot be re-supported back
  • Deny list. could have only two [UnsupportedOSPlatform] attributes and one [SupportedOSPlatform] attribute. This implies that an API was unsupported for a platform and later added a support, then can be unsupported back. But after that, it cannot be re-supported back

[buyaa-n]

Examples:

• Nested APIs shouldn't be allowed to expand the platform set and version support

[SupportedOSPlatform("ios")]
class Some
{    
    [SupportedOSPlatform("windows")] // Warn: 'Api' cannot widen the set of supported platforms of its containing symbol `Some`.
    public static void Api() {}
}

• Inconsistent list

[UnsupportedOSPlatform("browser")] // Warn: `Inconsistent platform 'browser' found in the Supported platforms list`
[SupportedOSPlatform("windows")]
[UnsupportedOSPlatform("windows10.0.19041")]
public void Api1();
// Fixer might suggest remove `[UnsupportedOSPlatform("browser")] `

 Valid consistent list example

[SupportedOSPlatform("browser")] // No warning
[SupportedOSPlatform("windows")]
[UnsupportedOSPlatform("windows10.0.19041")]
public void Api1();

• Multiple attributes for same platform
  • Allow list

  [SupportedOSPlatform("ios12.0")] // If two SupportedOSPlatform attributes applied, the one with lowest version accepted
  [SupportedOSPlatform("ios15.0")] // Warns:  `Platform 'ios' is already supported from version 12.0`
  public void OnlyWorksOniOS();
  
  [SupportedOSPlatform("ios12.0")] // If two SupportedOSPlatform attributes applied, the one with lowest version accepted
  [UnsupportedOSPlatform("ios14.0")]
  [SupportedOSPlatform("ios15.0")] // Warns:  `Platform 'ios' is unsupported from version 14.0 cannot re supported back`
  public void OnlyWorksOniOS();

  • Deny list

  [UnsupportedOSPlatform("windows")]
  [SupportedOSPlatform("windows8.0")]
  [UnsupportedOSPlatform("windows10.0.19041")]
  [SupportedOSPlatform("windows10.0.2004")] // Warns: `Platform 'windows' is unsupported from version 10.0.19041 cannot re supported back`
  public void WorksOnWindows8To10();
  
  [UnsupportedOSPlatform("windows")]
  [UnsupportedOSPlatform("windows10.0.19041")] // Warns: `Platform 'windows' is unsupported for all versions`
  [SupportedOSPlatform("windows10.0.2004")]
  public void WorksOnWindows10();

Fixer can suggest removing the invalid attribute