src/doc/edition-guide/src/rust-2021/warnings-promoted-to-error.md

   1 # Warnings promoted to errors
   2
   3 ## Summary
   4
   5 - Code that triggered the `bare_trait_objects` and `ellipsis_inclusive_range_patterns` lints will error in Rust 2021.
   6
   7 ## Details
   8
   9 Two existing lints are becoming hard errors in Rust 2021, but these lints will remain warnings in older editions.
  10
  11 ### `bare_trait_objects`:
  12
  13 The use of the `dyn` keyword to identify [trait objects](https://doc.rust-lang.org/book/ch17-02-trait-objects.html)
  14 will be mandatory in Rust 2021.
  15
  16 For example, the following code which does not include the `dyn` keyword in `&MyTrait`
  17 will produce an error instead of just a lint in Rust 2021:
  18
  19 ```rust
  20 pub trait MyTrait {}
  21
  22 pub fn my_function(_trait_object: &MyTrait) { // should be `&dyn MyTrait`
  23   unimplemented!()
  24 }
  25 ```
  26
  27 ### `ellipsis_inclusive_range_patterns`:
  28
  29 The [deprecated `...` syntax](https://doc.rust-lang.org/stable/reference/patterns.html#range-patterns)
  30 for inclusive range patterns (i.e., ranges where the end value is *included* in the range) is no longer
  31 accepted in Rust 2021. It has been superseded by `..=`, which is consistent with expressions.
  32
  33 For example, the following code which uses `...` in a pattern will produce an error instead of
  34 just a lint in Rust 2021:
  35
  36 ```rust
  37 pub fn less_or_eq_to_100(n: u8) -> bool {
  38   matches!(n, 0...100) // should be `0..=100`
  39 }
  40 ```
  41
  42 ## Migrations
  43
  44 If your Rust 2015 or 2018 code does not produce any warnings for `bare_trait_objects`
  45 or `ellipsis_inclusive_range_patterns` and you've not allowed these lints through the
  46 use of `#![allow()]` or some other mechanism, then there's no need to migrate.
  47
  48 To automatically migrate any crate that uses `...` in patterns or does not use `dyn` with
  49 trait objects, you can run `cargo fix --edition`.