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Default auto traits

This text is a description of #120706 PR which is part of "MCP: Low level components for async drop"

Intro

Sometimes we want to use type system to express specific behavior and provide safety guarantees. This behavior can be specified by various "marker" traits. For example, we use Send and Sync to keep track of which types are thread safe. As the language develops, there are more problems that could be solved by adding new marker traits:

All the traits proposed above are supposed to be auto traits implemented for most types, and usually implemented automatically by compiler.

For backward compatibility these traits have to be added implicitly to all bound lists in old code (see below). Adding new default bounds involves many difficulties: many standard library interfaces may need to opt out of those default bounds, and therefore be infected with confusing ?Trait syntax, migration to a new edition may contain backward compatibility holes, supporting new traits in the compiler can be quite difficult and so forth. Anyway, it's hard to evaluate the complexity until we try the system on a practice.

In this PR we introduce new optional lang items for traits that are added to all bound lists by default, similarly to existing Sized. The examples of such traits could be Leak, Move, SyncDrop or something else, it doesn't matter much right now (further I will call them DefaultAutoTrait's). We want to land this change into rustc under an option, so it becomes available in bootstrap compiler. Then we'll be able to do standard library experiments with the aforementioned traits without adding hundreds of #[cfg(not(bootstrap))]s. Based on the experiments, we can come up with some scheme for the next edition, in which such bounds are added in a more targeted way, and not just everywhere.

Most of the implementation is basically a refactoring that replaces hardcoded uses of Sized with iterating over a list of traits including both Sized and the new traits when -Zexperimental-default-bounds is enabled (or just Sized as before, if the option is not enabled).

Default bounds for old editions

All existing types, including generic parameters, are considered Leak/Move/SyncDrop and can be forgotten, moved or destroyed in generic contexts without specifying any bounds. New types that cannot be, for example, forgotten and do not implement Leak can be added at some point, and they should not be usable in such generic contexts in existing code.

To both maintain this property and keep backward compatibility with existing code, the new traits should be added as default bounds everywhere in previous editions. Besides the implicit Sized bound contexts that includes supertrait lists and trait lists in trait objects (dyn Trait1 + ... + TraitN). Compiler should also generate implicit DefaultAutoTrait implementations for foreign types (extern { type Foo; }) because they are also currently usable in generic contexts without any bounds.

Supertraits

Adding the new traits as supertraits to all existing traits is potentially necessary, because, for example, using a Self param in a trait's associated item may be a breaking change otherwise:

trait Foo: Sized {
    fn new() -> Option<Self>; // ERROR: `Option` requires `DefaultAutoTrait`, but `Self` is not `DefaultAutoTrait`
}

// desugared `Option`
enum Option<T: DefaultAutoTrait + Sized> {
    Some(T),
    None,
}

However, default supertraits can significantly affect compiler performance. For example, if we know that T: Trait, the compiler would deduce that T: DefaultAutoTrait. It also implies proving F: DefaultAutoTrait for each field F of type T until an explicit impl is be provided.

If the standard library is not modified, then even traits like Copy or Send would get these supertraits.

In this PR for optimization purposes instead of adding default supertraits, bounds are added to the associated items:

// Default bounds are generated in the following way:
trait Trait {
   fn foo(&self) where Self: DefaultAutoTrait {}
}

// instead of this:
trait Trait: DefaultAutoTrait {
   fn foo(&self) {}
}

It is not always possible to do this optimization because of backward compatibility:

pub trait Trait<Rhs = Self> {}
pub trait Trait1 : Trait {} // ERROR: `Rhs` requires `DefaultAutoTrait`, but `Self` is not `DefaultAutoTrait`

or

trait Trait {
   type Type where Self: Sized;
}
trait Trait2<T> : Trait<Type = T> {} // ERROR: `DefaultAutoTrait` required for `Trait2`, by implicit  `Self: DefaultAutoTrait` in `Trait::Type`

Therefore, DefaultAutoTrait's are still being added to supertraits if the Self params or type bindings were found in the trait header.

Trait objects

Trait objects requires explicit + Trait bound to implement corresponding trait which is not backward compatible:

fn use_trait_object(x: Box<dyn Trait>) {
   foo(x) // ERROR: `foo` requires `DefaultAutoTrait`, but `dyn Trait` is not `DefaultAutoTrait`
}

// implicit T: DefaultAutoTrait here
fn foo<T>(_: T) {}

So, for a trait object dyn Trait we should add an implicit bound dyn Trait + DefaultAutoTrait to make it usable, and allow relaxing it with a question mark syntax dyn Trait + ?DefaultAutoTrait when it's not necessary.

Foreign types

If compiler doesn't generate auto trait implementations for a foreign type, then it's a breaking change if the default bounds are added everywhere else:

// implicit T: DefaultAutoTrait here
fn foo<T: ?Sized>(_: &T) {}

extern "C" {
    type ExternTy;
}

fn forward_extern_ty(x: &ExternTy) {
    foo(x); // ERROR: `foo` requires `DefaultAutoTrait`, but `ExternTy` is not `DefaultAutoTrait`
}

We'll have to enable implicit DefaultAutoTrait implementations for foreign types at least for previous editions:

// implicit T: DefaultAutoTrait here
fn foo<T: ?Sized>(_: &T) {}

extern "C" {
    type ExternTy;
}

impl DefaultAutoTrait for ExternTy {} // implicit impl

fn forward_extern_ty(x: &ExternTy) {
    foo(x); // OK
}

Unresolved questions

New default bounds affect all existing Rust code complicating an already complex type system.

  • Proving an auto trait predicate requires recursively traversing the type and proving the predicate for it's fields. This leads to a significant performance regression. Measurements for the stage 2 compiler build show up to 3x regression.
    • We hope that fast path optimizations for well known traits could mitigate such regressions at least partially.
  • New default bounds trigger some compiler bugs in both old and new trait solver.
  • With new default bounds we encounter some trait solver cycle errors that break existing code.
    • We hope that these cases are bugs that can be addressed in the new trait solver.

Also migration to a new edition could be quite ugly and enormous, but that's actually what we want to solve. For other issues there's a chance that they could be solved by a new solver.

Literature

Most of the content in this explainer is based on the following posts:

But the literature from MPC is also relevant.