Investigate native AOT compatibility

[martincostello]

Is your feature request related to a specific problem? Or an existing feature? Please describe.

ASP.NET Core 8 is making investments in supporting native AOT scenarios. In the case of APIs using Polly, we should ensure that we are also native AOT-friendly.

Describe your proposed or preferred solution:

Polly is fully usable with native AOT (or with known, documented, limitations).

Potential changes we may have to make include:

• annotations for trimming
• changes to any use of reflection

Describe any alternative options you've considered:

Do nothing.

Any additional info?

ASP.NET Core blog post

[martintmk]

Currently, as far as I know the Polly.Core only uses reflection for options validation:

https://github.com/App-vNext/Polly/blob/main/src/Polly.Core/Utils/ValidationHelper.cs

However, .NET folks are planning to introduce a generator for options validation:
dotnet/runtime#85475

This can help us make Polly V8 reflection-free. The only downside is that the generator works with Microsoft.Extensions.Options package and I think we shouldn't have this dependency in Polly.Core. Ideally, the generator should also work with only System.ComponentModel.Annotations.

cc @geeknoid

[geeknoid]

The .NET 8 generator currently depends on Microsoft.Extensions.Options. This is because the geneator looks at the IValidateOptions type to determine the type to implement validation for.

Do you have any ideas how the generator could work without taking a dependency on Microsoft.Extensions.Options?

[martintmk]

@geeknoid Just thinking out loud. We could use this syntax:

[OptionsValidator]
public partial class RetryStrategyOptions
{
    // Can generate inner RetryStrategyOptionsValidator here 
}

internal static partial class PollyValidators
{
   // just implements the method
   [OptionsValidator]
   public static partial IEnumerable<ValidationResult> ValidateRetryOptions(RetryStrategyOptions options);
}

Essentially, we just need something that we pass the options instance to and it returns us the list of ValidationResult. If the list is empty, then there are no errors. Ideally, we would just reference the generator package as a build-time dependency without any other changes. It could recognize our internal OptionsValidatorAttribute and just output the code.

That way, the Polly stays dependency free on .NET3.1+ runtimes.

[geeknoid]

@martintmk I like those ideas. Not sure if I'll have time to put that in, but I'll try...