Updates in Rust 2021 edition

The new edition of Rust is here. The edition 2021 of Rust have many interesting changes in it. Let us have a look at some of the changes in edition 2021.

Additions to the prelude

• 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.

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, edition 2021 has not 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:

• std::convert::TryInto
• std::convert::TryFrom
• std::iter::FromIterator

Disjoint capture in closures

• || a.x + 1 now captures only a.x instead of a.
• This can cause things to be dropped at different times or affect whether closures implement traits like Send or Clone.
  • If possible changes are present, cargo fix will insert statements like let _ = &a to force a closure to capture the entire variable.

Panic macro consistency

• panic!(..) now always uses format_args!(..), just like println!().
• panic!("{") is no longer , without escaping the { as {{.
• panic!(x) is no longer accepted if x is not a string literal.
  • Use std::panic::panic_any(x) to panic with a non-string payload.
  • Or use panic!("{}", x) to use x‘s Display implementation.
• The same applies to assert!(expr, ..).

Reserving syntax

• any_identifier#, any_identifier"...", and any_identifier'...' are now reserved syntax, and no longer tokenize.
• This is mostly relevant to macros. E.g. quote!{ #a#b } is no longer accepted.
• It doesn’t treat keywords specially, so e.g. match"..." {} is no longer accepted.
• Insert whitespace between the identifier and the subsequent #, ", or ' to avoid errors.
• Edition migrations will help you insert whitespace in such cases.

To make space for new syntax in the future, we’ve decided to reserve syntax for prefixed identifiers and literals: prefix#identifier, prefix"string", prefix'c', and prefix#123, where prefix can be any identifier. (Except those prefixes that already have a meaning, such as b'...' (byte strings) and r"..." (raw strings).)

Or patterns in macro-rules

• How patterns work in macro_rules macros changes slightly:
  • $_:pat in macro_rules now matches usage of | too: e.g. A | B.
  • The new $_:pat_param behaves like $_:pat did before; it does not match (top level) |.
  • $_:pat_param is available in all editions.

This edition has extended Patterns to support | nested anywhere in the pattern. This enables you to write Some(1 | 2) instead of Some(1) | Some(2).

IntoIterator for arrays

• Arrays implement IntoIterator in all editions.
• Calls to IntoIterator::into_iter are hidden in Rust 2015 and Rust 2018 when using method call syntax (i.e., array.into_iter()). So, array.into_iter() still resolves to (&array).into_iter() as it has before.
• array.into_iter() changes meaning to be the call to IntoIterator::into_iter in Rust 2021.

Default Cargo feature resolver

edition = "2021" implies resolver = "2" in Cargo.toml.

Warnings promoted to errors

Code that triggered the bare_trait_objects and ellipsis_inclusive_range_patterns lints will error in Rust 2021. These two existing lints are becoming hard errors in Rust 2021, but these lints will remain warnings in older editions.

---

If you want to read more content like this?  Subscribe to Rust Times Newsletter and receive insights and latest updates, bi-weekly, straight into your inbox. Subscribe to Rust Times Newsletter: https://bit.ly/2Vdlld7.