Additions to the prelude

Summary

The TryInto, TryFrom and FromIterator traits are now part of the prelude.
This might make calls to trait methods ambiguous which could make some code fail to compile.

Details

The prelude of the standard library is the module containing everything that is automatically imported in every module. It contains commonly used items such as Option, Vec, drop, and Clone.

The Rust compiler prioritizes any manually imported items over those from the prelude, to make sure additions to the prelude will not break any existing code. For example, if you have a crate or module called example containing a pub struct Option;, then use example::*; will make Option unambiguously refer to the one from example; not the one from the standard library.

However, adding a trait to the prelude can break existing code in a subtle way. For example, a call to x.try_into() which comes from a MyTryInto trait might fail to compile if std's TryInto is also imported, because the call to try_into is now ambiguous and could come from either trait. This is the reason we haven't added TryInto to the prelude yet, since there is a lot of code that would break this way.

As a solution, Rust 2021 will use a new prelude. It's identical to the current one, except for three new additions:

The tracking issue can be found here.

Migration

As a part of the 2021 edition a migration lint, rust_2021_prelude_collisions, has been added in order to aid in automatic migration of Rust 2018 codebases to Rust 2021.

In order to migrate your code to be Rust 2021 Edition compatible, run:

cargo fix --edition

The lint detects cases where functions or methods are called that have the same name as the methods defined in one of the new prelude traits. In some cases, it may rewrite your calls in various ways to ensure that you continue to call the same function you did before.

If you'd like to migrate your code manually or better understand what cargo fix is doing, below we've outlined the situations where a migration is needed along with a counter example of when it's not needed.

Migration needed

Conflicting trait methods

When two traits that are in scope have the same method name, it is ambiguous which trait method should be used. For example:

trait MyTrait<A> {
  // This name is the same as the `from_iter` method on the `FromIterator` trait from `std`.  
  fn from_iter(x: Option<A>);
}

impl<T> MyTrait<()> for Vec<T> {
  fn from_iter(_: Option<()>) {}
}

fn main() {
  // Vec<T> implements both `std::iter::FromIterator` and `MyTrait` 
  // If both traits are in scope (as would be the case in Rust 2021),
  // then it becomes ambiguous which `from_iter` method to call
  <Vec<i32>>::from_iter(None);
}

We can fix this by using fully qualified syntax:

fn main() {
  // Now it is clear which trait method we're referring to
  <Vec<i32> as MyTrait<()>>::from_iter(None);
}

Inherent methods on `dyn Trait` objects

Some users invoke methods on a dyn Trait value where the method name overlaps with a new prelude trait:

#![allow(unused)]
fn main() {
mod submodule {
  pub trait MyTrait {
    // This has the same name as `TryInto::try_into`
    fn try_into(&self) -> Result<u32, ()>;
  }
}

// `MyTrait` isn't in scope here and can only be referred to through the path `submodule::MyTrait`
fn bar(f: Box<dyn submodule::MyTrait>) {
  // If `std::convert::TryInto` is in scope (as would be the case in Rust 2021),
  // then it becomes ambiguous which `try_into` method to call
  f.try_into();
}
}

Unlike with static dispatch methods, calling a trait method on a trait object does not require that the trait be in scope. The code above works as long as there is no trait in scope with a conflicting method name. When the TryInto trait is in scope (which is the case in Rust 2021), this causes an ambiguity. Should the call be to MyTrait::try_into or std::convert::TryInto::try_into?

In these cases, we can fix this by adding an additional dereferences or otherwise clarify the type of the method receiver. This ensures that the dyn Trait method is chosen, versus the methods from the prelude trait. For example, turning f.try_into() above into (&*f).try_into() ensures that we're calling try_into on the dyn MyTrait which can only refer to the MyTrait::try_into method.

No migration needed

Inherent methods

Many types define their own inherent methods with the same name as a trait method. For instance, below the struct MyStruct implements from_iter which shares the same name with the method from the trait FromIterator found in the standard library:

#![allow(unused)]
fn main() {
use std::iter::IntoIterator;

struct MyStruct {
  data: Vec<u32>
}

impl MyStruct {
  // This has the same name as `std::iter::FromIterator::from_iter`
  fn from_iter(iter: impl IntoIterator<Item = u32>) -> Self {
    Self {
      data: iter.into_iter().collect()
    }
  }
}

impl std::iter::FromIterator<u32> for MyStruct {
    fn from_iter<I: IntoIterator<Item = u32>>(iter: I) -> Self {
      Self {
        data: iter.into_iter().collect()
      }
    }
}
}

Inherent methods always take precedent over trait methods so there's no need for any migration.

Implementation Reference

The lint needs to take a couple of factors into account when determining whether or not introducing 2021 Edition to a codebase will cause a name resolution collision (thus breaking the code after changing edition). These factors include:

Is the call a fully-qualified call or does it use dot-call method syntax?
- This will affect how the name is resolved due to auto-reference and auto-dereferencing on method call syntax. Manually dereferencing/referencing will allow specifying priority in the case of dot-call method syntax, while fully-qualified call requires specification of the type and the trait name in the method path (e.g. <Type as Trait>::method)
Is this an inherent method or a trait method?
- Inherent methods that take self will take priority over TryInto::try_into as inherent methods take priority over trait methods, but inherent methods that take &self or &mut self won't take priority due to requiring a auto-reference (while TryInto::try_into does not, as it takes self)
Is the origin of this method from core/std? (As the traits can't have a collision with themselves)
Does the given type implement the trait it could have a collision against?
Is the method being called via dynamic dispatch? (i.e. is the self type dyn Trait)
- If so, trait imports don't affect resolution, and no migration lint needs to occur

The Rust Edition Guide