OpenAPI equivalent to Swagger's NonNullableReferenceTypesAsRequired

[IGx89]

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Is your feature request related to a problem? Please describe the problem.

I have APIs that are mostly providing data to frontends written in TypeScript (mostly GETs). I generate TypeScript types from the OpenAPI specs. Properties that aren't nullable I'd expect to be modeled as required in the spec since the fields will always be present. When using Swashbuckle.AspNetCore's generator with its NonNullableReferenceTypesAsRequired option set to true that happened, but there's no equivalent setting using the Microsoft.OpenAPI generator here.

As an example, I'd like both properties here to be listed as required:

public class Foo
{
	public int Bar { get; set; }
	public List<string> MyList { get; set; } = [];
}

Presently both are optional, and additionally MyList is nullable (due to a different issue, documented in #58192)

Describe the solution you'd like

It would be helpful to have an equivalent setting in OpenAPI to Swashbuckle.AspNetCore's NonNullableReferenceTypesAsRequired. See domaindrivendev/Swashbuckle.AspNetCore#2036 for details about the problem and solution done there.

Of course an alternative solution is marking everything as required/[JsonRequired]/[Required], which I've done in some places, but that's a lot of work when you have a lot of existing DTOs.

Additional context

No response

[IGx89]

Here's a schema transformer came up with to mostly match that option's behavior:

/// <summary>
/// Transforms OpenAPI schemas to mark non-nullable reference types as required.
/// </summary>
internal class RequiredTransformer : IOpenApiSchemaTransformer
{
	public Task TransformAsync(OpenApiSchema schema, OpenApiSchemaTransformerContext context, CancellationToken cancellationToken)
	{
		if (schema.Properties != null)
		{
			foreach (var property in schema.Properties)
			{
				var jsonProperty = context.JsonTypeInfo.Properties.FirstOrDefault(x => x.Name == property.Key);
				if (jsonProperty == null) continue;

				if (!jsonProperty.PropertyType.IsValueType && !jsonProperty.IsGetNullable)
				{
					schema.Required ??= new HashSet<string>();
					schema.Required.Add(property.Key);
				}
			}
		}

		return Task.CompletedTask;
	}
}

Note however that enabling NonNullableReferenceTypesAsRequired also unintuitively marks non-nullable value types as required too, which I did not implement in the above transformer since they're not actually required (no validation error when POSTing to an endpoint when one isn't present). That behavior was apparently done intentionally (see domaindrivendev/Swashbuckle.AspNetCore#3054) but I'm guessing that here most wouldn't want that behavior. You can easily remove the IsValueType condition if you want it though.