Tracking issue for Edition 2021 support

[jonas-schievink]

This is a list of 2021 edition changes that require rust-analyzer support. The list is taken from the official blog post https://blog.rust-lang.org/2021/05/11/edition-2021.html.

• Edition-dependent prelude (not fully implemented upstream: New prelude rust#85687) – implemented in feat: Implement per-edition preludes #9097
• IntoIterator for arrays (will be supported automatically, but needs Support rustc_skip_array_during_method_dispatch #8552 to correct behavior on 2015/2018 editions)
• Panic macro consistency – implemented in Support the new panic!() macro #8304
• Reserve literal prefixes (upstream seems to not have fully agreed on an implementation strategy at this point)
• Or-patterns in macro_rules – tracked by Support 2021 Edition pat matcher in macro_rules #9055

[m-ou-se]

Edition-dependent prelude (not fully implemented upstream: rust-lang/rust#85687)

Note that the three std::prelude::rust_* modules already exist and the right one is already selected based on the edition. So, rust-analyzer could already implement picking the right prelude. It's just that right now all three of these modules have identical contents, until that PR is merged.