Warnings promoted to errors

Summary

• Code that triggered the bare_trait_objects and ellipsis_inclusive_range_patterns lints will error in Rust 2021.

Details

Two existing lints are becoming hard errors in Rust 2021, but these lints will remain warnings in older editions.

bare_trait_objects:

The use of the dyn keyword to identify trait objects will be mandatory in Rust 2021.

For example, the following code which does not include the dyn keyword in &MyTrait will produce an error instead of just a lint in Rust 2021:

#![allow(unused)]
fn main() {
pub trait MyTrait {}

pub fn my_function(_trait_object: &MyTrait) { // should be `&dyn MyTrait`
  unimplemented!()
}
}

ellipsis_inclusive_range_patterns:

The deprecated ... syntax for inclusive range patterns (i.e., ranges where the end value is included in the range) is no longer accepted in Rust 2021. It has been superseded by ..=, which is consistent with expressions.

For example, the following code which uses ... in a pattern will produce an error instead of just a lint in Rust 2021:

#![allow(unused)]
fn main() {
pub fn less_or_eq_to_100(n: u8) -> bool {
  matches!(n, 0...100) // should be `0..=100`
}
}

Migrations

If your Rust 2015 or 2018 code does not produce any warnings for bare_trait_objects or ellipsis_inclusive_range_patterns and you've not allowed these lints through the use of #![allow()] or some other mechanism, then there's no need to migrate.

To automatically migrate any crate that uses ... in patterns or does not use dyn with trait objects, you can run cargo fix --edition.