Auto merge of #79576 - m-ou-se:2021, r=Mark-Simulacrum

Add edition 2021.

:fireworks: Happy new ~~year~~ Rust. :champagne:

This adds --edition=2021, and updates suggestions about 2018 to say "2018 *or later*".

Related Cargo PR: https://github.com/rust-lang/cargo/pull/8922

---

Edit: This adds the new edition as *unstable*. Without `-Z unstable-options`, `--edition=2021` results in:
```
$ rustc --edition=2021
error: edition 2021 is unstable and only available with -Z unstable-options.
```

1 files changed, 3 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index b147f42fada..d11db74a3bd 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -15,6 +15,7 @@ use rustc_ast::{AnonConst, BinOp, BinOpKind, FnDecl, FnRetTy, MacCall, Param, Ty
 use rustc_ast::{Arm, Async, BlockCheckMode, Expr, ExprKind, Label, Movability, RangeLimits};
 use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, DiagnosticBuilder, PResult};
+use rustc_span::edition::LATEST_STABLE_EDITION;
 use rustc_span::source_map::{self, Span, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Pos};
@@ -2108,8 +2109,8 @@ impl<'a> Parser<'a> {
 
         let mut async_block_err = |e: &mut DiagnosticBuilder<'_>, span: Span| {
             recover_async = true;
-            e.span_label(span, "`async` blocks are only allowed in the 2018 edition");
-            e.help("set `edition = \"2018\"` in `Cargo.toml`");
+            e.span_label(span, "`async` blocks are only allowed in Rust 2018 or later");
+            e.help(&format!("set `edition = \"{}\"` in `Cargo.toml`", LATEST_STABLE_EDITION));
             e.note("for more on editions, read https://doc.rust-lang.org/edition-guide");
         };