Documenting Trait Bounds And Candidate Preference In Rust

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Documenting Trait Bounds and Candidate Preference in Rust

Hey Rustaceans! I'm diving deep into documenting some significant changes related to trait bounds and candidate preference in Rust, specifically from this pull request. This is a pretty intricate area, touching on many core concepts, so I wanted to get some community input on the best way to structure this documentation. Let's break down the challenge and figure out how to make this as clear as possible for everyone.

The Challenge: Interdependent Concepts

Documenting these changes involves a web of interdependent concepts. We're not just talking about one isolated feature; it's more like a network where each piece relies on others. Here’s a breakdown of what I’m adding or need to add, and what each concept depends on:

  • Items
    • Instantiating generic parameters of an item: This involves how Rust handles generic types when you're using items like functions or structs. You know, when you have a Vec<T>, how does Rust know what T is? We need to explain the process of figuring that out.
  • Types
    • Alias types (associated types + opaque types):
      • Normalizing associated types: This is where things get interesting. Associated types are like placeholders within traits, and normalizing them means resolving them to their concrete types. Think of it as figuring out what a recipe actually means in terms of ingredients.
      • The concept of a rigid alias: A rigid alias is an alias that behaves like a concrete type once it's been resolved. It's 'rigid' because it doesn't change. Understanding this is crucial for type equality.
    • Inference variables: These are the question marks in Rust's type system. When you don't explicitly specify a type, Rust uses inference variables to figure it out. It's like Rust is playing detective with your code.
    • Placeholders: Placeholders are temporary types used during type checking, especially when dealing with higher-ranked types. They're like the scaffolding that helps build the type system.
  • Type equality
    • Structural except for alias types, what does structural mean, concept of a rigid type: Structural equality means two types are equal if their structure is the same. However, alias types have their own rules, especially rigid ones. It’s like comparing two houses – are they made of the same materials and have the same layout?
    • Relating aliases: How do we determine if two alias types are equal? This involves normalization and understanding the underlying types they represent.
    • Higher ranked: This deals with types that involve lifetimes or other generics in a more complex way. Think of it as functions that can work with a variety of lifetimes, not just one.
  • Trait bounds
    • Satisfaction: how to prove/satisfy trait bounds:
      • Via trait implementations: This is the bread and butter of trait bounds. How does Rust know a type implements a trait? Through implementations, of course!
      • Via in-scope where-bound: where clauses allow you to add constraints on generic types. This is another way to satisfy trait bounds.
      • Via item bounds of rigid aliases: Rigid aliases can also carry trait bounds, adding another layer of complexity.
      • Candidate preference (what I originally set out to document): When multiple implementations could satisfy a trait bound, how does Rust choose the best one? This is the heart of candidate preference.

Interdependencies: A Tangled Web

Here's where it gets tricky. These concepts don't exist in isolation. They're all connected:

  • Equality of alias types needs to know about normalization: To compare alias types, you need to normalize them first.
  • Normalization needs to know about "satisfying trait bounds" as its behavior only makes sense in reference to that: Normalization depends on whether trait bounds are satisfied.
  • Satisfying trait bounds needs to talk about equality: To prove a trait bound is satisfied, you often need to show that types are equal.
  • Using item bounds relies on the concept of rigid aliases: You can't understand item bounds without understanding rigid aliases.

Additional Dependencies and Annoyances

To explain how we use impls to satisfy a trait bound, I'm currently relying on inference variables and the concept of "instantiating generic parameters when using an item." And to explain equality and subtyping of higher-ranked types, I'm talking about inference variables and placeholders. It's a lot to juggle!

Breaking It Down: A Modular Approach?

I'm open to splitting this up into smaller, more manageable chunks. We could leave certain things as TODO or simply not document them for now. For example, we might skip explaining what it means to "equate a type" until that gets merged separately, or we could ignore satisfying trait bounds via item bounds of rigid aliases. What do you guys think?

Key Questions

What I'd really like to nail down are these two key questions:

  1. Does this structure seem good? Before I get too deep into the concrete documentation, I want to make sure the overall organization makes sense.
  2. How do I take this structure and actually get it into the reference? What's the best way to integrate these changes into the Rust Reference?

I've got a work-in-progress branch here if you want to dive into the nitty-gritty details. But for now, let's focus on the big picture.

Diving Deeper: The Core Concepts in Detail

Let’s break down some of these core concepts even further to understand why they are so critical and how they interrelate. This will give us a better foundation for structuring the documentation.

Instantiating Generic Parameters

When we talk about instantiating generic parameters, we're essentially discussing how Rust fills in the blanks for generic types. Imagine you have a generic function like this:

fn generic_function<T>(value: T) -> T {
    value
}

When you call generic_function(5), Rust needs to figure out that T is i32. This process of figuring out the concrete type for a generic parameter is instantiation. It's fundamental because generics are a cornerstone of Rust's expressive type system. Without a clear understanding of instantiation, developers might struggle with how generic code behaves at runtime.

This process is closely tied to inference variables, those placeholders Rust uses to deduce types. When you call generic_function(5), Rust might initially represent T as an inference variable (?T). Then, by looking at the argument 5, it can infer that ?T must be i32. This interplay between instantiation and inference is crucial for understanding how Rust handles generic code.

Alias Types: Associated and Opaque

Alias types, including associated and opaque types, are powerful tools for abstraction and code organization. Associated types allow traits to define types that implementing types must specify. Opaque types, on the other hand, hide the concrete type behind a trait, offering a way to abstract over implementation details.

Associated types are like placeholders within a trait definition. Consider the Iterator trait:

trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

Here, Item is an associated type. When you implement Iterator for a specific type, you must specify what Item is. For example, for a Vec iterator, Item might be i32 if it's a Vec<i32>. Documenting this clearly is important because associated types allow traits to be more flexible and expressive.

Opaque types, introduced with the impl Trait syntax, provide a way to return a type that implements a trait without naming the concrete type. This is useful for hiding implementation details and preventing breaking changes. For example:

fn create_iterator() -> impl Iterator<Item = i32> {
    vec![1, 2, 3].into_iter()
}

Here, the return type is impl Iterator<Item = i32>, but the concrete type (in this case, the iterator for a Vec) is not exposed. Opaque types are a crucial feature for API design, so clear documentation is essential.

Normalizing Associated Types

Normalizing associated types is the process of resolving them to their concrete types. This is a critical step in type checking and trait resolution. It’s like expanding an abbreviation to its full form. If you have Self::Item in a generic function, Rust needs to figure out what concrete type Item represents for the specific type Self. This involves looking at the trait implementation and substituting the associated type.

Normalization is also closely related to the concept of a rigid alias. A rigid alias is an alias type that, once normalized, behaves like a concrete type. It doesn’t change during type checking. Understanding this rigidity is vital for reasoning about type equality, especially when trait bounds come into play.

Type Equality: Structural vs. Alias Types

Type equality is a fundamental concept in any type system. In Rust, we have to consider both structural equality and the specific rules for alias types.

Structural equality means that two types are equal if they have the same structure. For example, (i32, bool) is structurally equal to (i32, bool). However, alias types introduce a twist. Two alias types might be equal even if their underlying types are different, as long as they normalize to the same type. This is where understanding normalization becomes crucial.

Relating aliases involves checking if two alias types are equivalent. This often requires normalizing both types and then comparing the results. The rules for relating aliases are more complex than simple structural equality, and they are deeply intertwined with how Rust handles trait bounds.

Higher-ranked types add another layer of complexity to type equality. These are types that involve lifetimes or other generics in a more complex way, often using for<> syntax. Comparing higher-ranked types requires considering how they behave for all possible lifetimes, not just one. This is where placeholders and inference variables come into play, as they help Rust reason about these types.

Trait Bound Satisfaction

Trait bound satisfaction is the process of proving that a type implements a trait. This can happen in several ways:

  • Via trait implementations: The most common way is through a direct impl block. If you have impl MyTrait for MyType, then MyType satisfies the MyTrait bound.
  • Via in-scope where-bound: where clauses allow you to add constraints on generic types. For example, fn my_function<T: MyTrait>(...) where T: AnotherTrait means that T must satisfy both MyTrait and AnotherTrait.
  • Via item bounds of rigid aliases: Rigid aliases can also carry trait bounds. If you have a rigid alias like type MyAlias: MyTrait = ..., then MyAlias automatically satisfies MyTrait.

Candidate Preference

Finally, candidate preference is the process Rust uses to choose the best implementation when multiple implementations could satisfy a trait bound. This is a critical part of trait resolution and can significantly impact how generic code behaves. The rules for candidate preference are complex and depend on factors like specialization and the specificity of implementations. Documenting this clearly is essential for advanced Rust developers who want to understand the intricacies of the type system.

Structuring the Documentation: A Possible Approach

Given these interdependencies, here’s a potential structure for the documentation:

  1. Introduction to Generics and Type Parameters: Start with a high-level overview of generics and type parameters to set the stage.
  2. Alias Types:
    • Introduce associated types and opaque types.
    • Explain normalization of associated types.
    • Define rigid aliases and their significance.
  3. Type Equality:
    • Discuss structural equality.
    • Explain how alias types affect equality.
    • Cover higher-ranked type equality.
  4. Trait Bounds:
    • Explain trait bound satisfaction.
    • Detail how trait implementations, where clauses, and rigid aliases contribute to satisfaction.
  5. Candidate Preference:
    • Describe the process of choosing the best implementation when multiple candidates exist.
  6. Advanced Topics:
    • Discuss inference variables and placeholders in more detail.

This structure attempts to build knowledge incrementally, starting with the basics and gradually introducing more complex concepts. By addressing each concept in a logical order and highlighting their interdependencies, we can create documentation that is both comprehensive and accessible.

Next Steps: Let's Collaborate!

So, what do you guys think? Does this structure resonate? Are there any areas you feel need more emphasis or a different approach? How can we best integrate this into the Rust Reference? I'm eager to hear your thoughts and work together to make this documentation the best it can be. Let's make the Rust type system a little less mysterious, one concept at a time! 🔥