Support linker removal of unused converters

[steveharter]

When building a standalone app, unused converters should be linked out. This helps to reduce assembly size, increase startup perf (no unnecessary JITing) and reduced private bytes (less JITing and less objects in memory). These are goals for 5.0 as documented here.

Each converter that uses a value type (e.g. Int64Converter) saves ~10K of assembly size after crossgen and each converter that uses a reference type (e.g. ArrayConverter) saves ~.5K of assembly size after crossgen.

In order to help trim references a new API is necessary:

public class JsonSerializerOptions
{
...
    public static JsonSerializerOptions CreateWithNoConverters();
...
}

This creates an options instance that has no built-in converters. It doesn't "root" any of the converters so those converters can linked out.

Converters are then added manually through the existing Converters.Add() method by the calling code that initializes the options or by applying the [JsonConverter] attribute to properties or perhaps to an Assembly (requires a change to that attribute so it can target an Assembly).

Calling Converters.Add() and\or using [JsonConverter] to root the converter types is intended to be done through a new code-gen feature but could also be leveraged by those who want the smallest possible standalone app size.

This also means the existing converters that are now internal in the System.Text.Json.Serialization.Converters namespace will need to be made public so they can be referenced. Currently there are ~46 converters.

[stephentoub]

This also means the existing converters that are now internal in the System.Text.Json.Serialization.Converters namespace will need to be made public so they can be referenced. Currently there are ~46 converters.

That's a ton of new public surface area that also exposes what is essentially implementation detail.

Is this path not going to become immediate legacy with source generators?

If this route is really important to pursue, could we instead, for example, expose a single method like:

public void AddImplicitConverter<T>()

The implementation would switch on the type, and I believe the linker should be able to trim away anything not related to the specific type used.

Also, 10K seems like a lot. Maybe it's all necessary, but have we looked to see where it's coming from and whether there are ways to reduce it, aside from this proposal?

[eerhardt]

This work isn't necessary for 5.0, so moving to 6.0.

[eerhardt]

Do we still need this issue now that we have JSON Source Generator?

[layomia]

The JSON source generator helps us achieve this - #45448.