Zero-allocation* redesign

[clarkmcc]

This issue is meant to be a scratchpad for ideas and feedback on improving how types and values are handled in the interpreter. There have been several different feature requests recently that could be solved by the a fundamental shift in how CEL values are managed.

• https://github.com/clarkmcc/cel-rust/pull/58 - Proposes a way to avoid having to serialize Rust types into CEL Value when only some fields in those types are actually referenced.
• https://github.com/clarkmcc/cel-rust/issues/73 - Requests a way to create actual Rust types in CEL expressions presumably without the intermediate ser/de using a specific format like JSON.
• https://github.com/clarkmcc/cel-rust/pull/68#discussion_r1695592657 - Requests a way to deserialize the result of a CEL expression to a serde_json::Value.

Today any value referenced in a CEL expression is owned by either the Program or the Context. The Program owns values that were created directly in the CEL expression, for example in the expression ("foo" == "bar") == false, there are three values owned by the Program, "foo", "bar", and false. Values that are owned by the Context are values that are dynamically provided at execution time to a Program as variables, like foo == bar, both foo and bar are values owned by the Context.

• I like the idea of fundamentally changing Context so that it does not own any data, meaning that you do not need to clone your types to use them in CEL. Instead I'd like the Context to have references to that data.
• In the case of deeply nested structs, we would provide a derive macro to generate field accessors that the interpreter would call when doing variable resolution. When referencing a property on a struct, CEL would operate on a reference to that data.

Questions:

• Would we even need Arc/Rc's anymore or could we get away with just this since we would assume the caller owned all the data. RIght now, an Arc is required for values owned by Program because a Program can be executed one or more times simultaneously.

  pub enum Value<'a> {
    String(&'a str)
  }

• We can easily get references to, and represent primitive types in the interpreter, but what if an expression returned a property whose type was some other user-defined struct? How would we work with that? Perhaps instead of a Value enum, we have a Value trait that exposes every behavior supported in a CEL expression, i.e.:

  pub trait Value {
    fn get_property(&self, key: &str) -> Box<dyn Value>;
  
    fn add(&self, other: &dyn Value) -> Box<dyn Value>;
  
    fn sub(&self, other: &dyn Value) -> Box<dyn Value>;
  }

[kyle-mccarthy]

While not zero-allocation, have you considered integrating with the protobuf ecosystem? There is prost-reflect which provides a DynamicMessage trait allowing field access via get_field. Program could then accept a DescriptorPool to help with field and type resolution.

[Caellian]

I like the idea of fundamentally changing Context so that it does not own any data, meaning that you do not need to clone your types to use them in CEL. Instead I'd like the Context to have references to that data.

This only works if you're not passing temporaries into Context, otherwise end-user is forced to keep (moving) them around for as long as the Context is used which is annoying and can be inefficient. I think simply using Cow for values would be simplest and most versatile, while also removing any extra copies/clones.

EDIT 1:

Also, if Value trait is added, and has add function, an additional add_to is required. In math, commutation is proven for numbers via induction and can't be assumed to hold true for every Value.

So the default implementation should be:

/// Called when other [`Value`] is added to this type.
#[inline]
fn add(&self, second: Value) -> ResolveResult {
    Err(ExecutionError::UnsupportedBinaryOperatorExternal(
        "add", std::any::type_name::<Self>(),
    ))
}

/// Called when this type is added to other [`Value`].
/// 
/// Override if addition is non-commutative.
#[inline]
fn add_to(&self, first: Value) -> ResolveResult {
    Self::add(self, first)
}