Decide on the bar for IDE suggestions and refactorings offered for code style (IDE analyzers) and quality (CA analyzers)

Starting with .NET5 SDK, both code style (IDE analyzers) and code quality analyzer (CA analyzers) packages are installed with the .NET SDK, which makes the user experience consistent inside Visual Studio for .NET users:

• .NET users will get code style and code quality suggestions/refactorings by default when targeting .net5 or later.
• Any CA or IDE diagnostic ID set to warning/error in editorconfig will run on build/CI and break user’s build (for IDE analyzers, MSBuild property EnforceCodeStyleOnBuild needs to be explicitly set to true, in future it might be turned on by default)

Given that the end user would no longer distinguish between a suggestion and a refactoring coming from an IDE rule (Roslyn repo) or CA rule (Roslyn-analyzers repo), we need to make sure that we have consistent bar set for both repos in terms of whether a rule should be enabled by default as a suggestion, refactoring or disabled by default. We are getting lot of new CA analyzer suggestions off late, and it would be good to have a set bar for the default severity/enabled state for these analyzers.

Proposal

Below is my proposal for bucketing:

IDE refactoring

This includes both hidden diagnostic analyzer + code fix AND code refactorings

1. Bar should not differ for hidden diagnostic + code fix versus a code refactoring. End users do not see a difference between them and even though FixAll is only applicable for code fixes today, we plan to add this support soon for code refactorings, so both of these should have the same bar.
2. A refactoring is allowed to change the public API surface of code. Given there is no visual cue for refactorings (no ... in the editor or an error list entry), we are not driving the user towards making this change, but just offering them quick actions to avoid manual work. For example:
   1. Code refactorings to change public API signatures, adds/removes/changes parameters, etc.
   2. CA1802: Use literals where appropriate
3. A refactoring is allowed to introduce new compiler warnings/errors in existing code. Such a refactoring should explicitly not support FixAll. We try our very best to not break user code in refactorings, but that is not a strict requirement for a refactoring. Code refactoring should show a warning annotation in preview for such scenarios. We have few code IDE code refactorings that do this.

IDE suggestion

Info severity diagnostic with a ... in the editor to draw user’s attention and an explicit message entry in error list

1. An analyzer would be a candidate for a suggestion when we know for sure that existing code quality will be improved. For example:
   1. CA1012: Abstract type constructors should not have public modifier, instead it should be protected
   2. CA1813: Public, non-abstract, attribute types should be marked as sealed
   3. IDE0060: Remove unused parameter or IDE0051: Remove unused private member
2. An analyzer would be a candidate for a suggestion if it flags a potential functional bug in user code, but is not a strong enough assertion to be a build warning by default to break build. For this case, it will be fine even if the analyzer does not have an associated code fix, because it might need non-trivial changes to user code to fix the issue. Goal is to draw user’s attention that something is wrong with the current code, they may have to read up more and identify the correct fix themselves. For example:
   1. CA1012: Use ValueTasks correctly
   2. CA2008: Do not create tasks without passing a TaskScheduler.

I believed this bucket would only apply to CA rules, it is highly unlikely that above will apply for any IDE style analyzers. We can take couple of routes for this bucket of rules:

1. Conservative: Do not enable such rules by default in the IDE, primary concern would be users might get confused on how to fix these and these might add noise to their editor experience.
2. Aggressive: It is fine to let users know about potential mistakes/improvements in their code, even if the fix is not trivial. They have the option to ignore the suggestion or suppress it in editorconfig if they do not value it. If it turns out too noisy, they will report an issue.

I would personally recommend the aggressive route as I feel this is the core benefit from having CA rules in the box - help user write better code or introduce them to new code quality areas that they are unaware of.

Disabled by default

1. If an analyzer needs expensive analysis, say a dataflow analysis, it must be disabled by default. This likely applies only to CA rules. For example, all DFA based security rules.
2. If an analyzer can produce known set of false positives, where the reported diagnostic is invalid, say for example it is a heuristic based analyzer, then it must be disabled by default. For example, few ported CA rules use naming heuristics like look for specific symbol names.

I believed disabled by default bucket only applies to CA rules, it is highly unlikely that above will apply for any IDE style analyzers.

[CyrusNajmabadi]

I’d like to tweak:

If an analyzer can produce known set of false positives,

Almost all our analyzers can hit some false positives. The bar we’ve had here is that we have a reasonable belief that these false positives should be very rare and unlikely to matter in reasonably written programs.

As an example, many of our analyzers assume that a property call is idempotent. Calling it one time versus many should have no effect on almost all reasonably written programs. That said, there are cases where we might produce a false positive for cases we do not feel are either realistic, or are so rare they are vastly outbalanced by the value to the majority.

[mavasani]

We now have a pretty good triage/approval process in place for .NET SDK CA analyzers (runtime team) and IDE CodeStyle analyzers (roslyn team). Going to close this out unless we hit these issues again.