Commits on Apr 26, 2021

1. Add expr202x macro pattern …

   This makes it possible to use `inline_const` (rust-lang#76001) and `let_chains`
   (rust-lang#53667) inside macros' `expr` patterns in a future edition by
   bifurcating the `expr` nonterminal in a similar way to `pat2021` to
   remove some backwards compatibility exceptions that disallow
   `const`/`let` at the beginning of an `expr` match.
   
   Fixes rust-lang#84155 and relaxes the backward compat restriction from rust-lang#80135 for
   a future edition. This is not intended to go into 2021 as it I don't
   think it's simple to write an automatic fix, and certainly not now that
   it's past the soft deadline for inclusion in 2021 by a long shot.
   
   Here is a pathological case of rust-lang#79908 that forces this to be an edition
   change:
   
   ```rust
   macro_rules! evil {
       ($e:expr) => {
           // or something else
           const {$e-1}
       };
       (const $b:block) => {
           const {$b}
       }
   }
   fn main() {
       let x = 5;
       match x {
           evil!(const { 5 }) => panic!("oh no"),
           _ => (),
       };
   }
   ```

   

   lf- committed Apr 26, 2021
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