Add expr202x macro pattern

This makes it possible to use `inline_const` (#76001) and `let_chains`
(#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 #84155 and relaxes the backward compat restriction from #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 #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"),
        _ => (),
    };
}
```

Author: lf-

Diff for: compiler/rustc_ast/src/token.rs

@@ -698,7 +698,13 @@ pub enum NonterminalKind {
         /// edition of the span. This is used for diagnostics.
         inferred: bool,
     },
-    Expr,
+    /// Expr202x with exceptions for const and let not being allowed at the start
+    Expr2015 {
+        inferred: bool,
+    },
+    Expr202x {
+        inferred: bool,
+    },
     Ty,
     Ident,
     Lifetime,
@@ -728,7 +734,9 @@ impl NonterminalKind {
             },
             sym::pat2015 => NonterminalKind::Pat2015 { inferred: false },
             sym::pat2021 => NonterminalKind::Pat2021 { inferred: false },
-            sym::expr => NonterminalKind::Expr,
+            sym::expr => NonterminalKind::Expr2015 { inferred: true },
+            sym::expr2015 => NonterminalKind::Expr2015 { inferred: false },
+            sym::expr202x => NonterminalKind::Expr202x { inferred: false },
             sym::ty => NonterminalKind::Ty,
             sym::ident => NonterminalKind::Ident,
             sym::lifetime => NonterminalKind::Lifetime,
@@ -749,7 +757,10 @@ impl NonterminalKind {
             NonterminalKind::Pat2021 { inferred: false } => sym::pat2021,
             NonterminalKind::Pat2015 { inferred: true }
             | NonterminalKind::Pat2021 { inferred: true } => sym::pat,
-            NonterminalKind::Expr => sym::expr,
+            NonterminalKind::Expr2015 { inferred: false } => sym::expr2015,
+            NonterminalKind::Expr202x { inferred: false } => sym::expr202x,
+            NonterminalKind::Expr2015 { inferred: true }
+            | NonterminalKind::Expr202x { inferred: true } => sym::expr,
             NonterminalKind::Ty => sym::ty,
             NonterminalKind::Ident => sym::ident,
             NonterminalKind::Lifetime => sym::lifetime,

Diff for: compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -1095,7 +1095,9 @@ fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
                 // maintain
                 IsInFollow::Yes
             }
-            NonterminalKind::Stmt | NonterminalKind::Expr => {
+            NonterminalKind::Stmt
+            | NonterminalKind::Expr2015 { .. }
+            | NonterminalKind::Expr202x { .. } => {
                 const TOKENS: &[&str] = &["`=>`", "`,`", "`;`"];
                 match tok {
                     TokenTree::Token(token) => match token.kind {

Diff for: compiler/rustc_expand/src/mbe/quoted.rs

@@ -63,16 +63,27 @@ pub(super) fn parse(
                                     let span = token.span.with_lo(start_sp.lo());
 
                                     match frag.name {
-                                        sym::pat2015 | sym::pat2021 => {
-                                            if !features.edition_macro_pats {
-                                                feature_err(
-                                                    sess,
-                                                    sym::edition_macro_pats,
-                                                    frag.span,
-                                                    "`pat2015` and `pat2021` are unstable.",
-                                                )
-                                                .emit();
-                                            }
+                                        sym::pat2015 | sym::pat2021
+                                            if !features.edition_macro_pats =>
+                                        {
+                                            feature_err(
+                                                sess,
+                                                sym::edition_macro_pats,
+                                                frag.span,
+                                                "`pat2015` and `pat2021` are unstable.",
+                                            )
+                                            .emit();
+                                        }
+                                        sym::expr2015 | sym::expr202x
+                                            if !features.edition_macro_expr =>
+                                        {
+                                            feature_err(
+                                                sess,
+                                                sym::edition_macro_expr,
+                                                frag.span,
+                                                "`expr2015` and `expr202x` are unstable.",
+                                            )
+                                            .emit();
                                         }
                                         _ => {}
                                     }

Diff for: compiler/rustc_feature/src/active.rs

@@ -532,7 +532,7 @@ declare_features! (
     /// Allows the use of `no_sanitize` attribute.
     (active, no_sanitize, "1.42.0", Some(39699), None),
 
-    // Allows limiting the evaluation steps of const expressions
+    /// Allows limiting the evaluation steps of const expressions
     (active, const_eval_limit, "1.43.0", Some(67217), None),
 
     /// Allow negative trait implementations.
@@ -655,6 +655,9 @@ declare_features! (
     /// Allows unsizing coercions in `const fn`.
     (active, const_fn_unsize, "1.53.0", Some(64992), None),
 
+    /// `:expr2015` and `:expr202x` macro matchers.
+    (active, edition_macro_expr, "1.53.0", None, None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

Diff for: compiler/rustc_parse/src/parser/nonterminal.rs

@@ -24,13 +24,14 @@ impl<'a> Parser<'a> {
         }
 
         match kind {
-            NonterminalKind::Expr => {
+            NonterminalKind::Expr2015 { .. } => {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Const)
             }
+            NonterminalKind::Expr202x { .. } => token.can_begin_expr(),
             NonterminalKind::Ty => token.can_begin_type(),
             NonterminalKind::Ident => get_macro_ident(token).is_some(),
             NonterminalKind::Literal => token.can_begin_literal_maybe_minus(),
@@ -128,7 +129,9 @@ impl<'a> Parser<'a> {
                 })?)
             }
 
-            NonterminalKind::Expr => token::NtExpr(self.parse_expr_force_collect()?),
+            NonterminalKind::Expr2015 { .. } | NonterminalKind::Expr202x { .. } => {
+                token::NtExpr(self.parse_expr_force_collect()?)
+            }
             NonterminalKind::Literal => {
                 // The `:literal` matcher does not support attributes
                 token::NtLiteral(

Diff for: compiler/rustc_span/src/symbol.rs

@@ -496,6 +496,7 @@ symbols! {
         dylib,
         dyn_metadata,
         dyn_trait,
+        edition_macro_expr,
         edition_macro_pats,
         eh_catch_typeinfo,
         eh_personality,
@@ -526,6 +527,8 @@ symbols! {
         expf64,
         export_name,
         expr,
+        expr2015,
+        expr202x,
         extended_key_value_attributes,
         extern_absolute_paths,
         extern_crate_item_prelude,

Diff for: src/test/ui/feature-gates/feature-gate-edition_macro_expr.rs

@@ -0,0 +1,8 @@
+// feature gate test for edition_macro_expr
+
+macro_rules! mac {
+    ($x:expr2015) => {}; //~ERROR `expr2015` and `expr202x` are unstable
+    ($x:expr202x) => {}; //~ERROR `expr2015` and `expr202x` are unstable
+}
+
+fn main() {}

Diff for: src/test/ui/feature-gates/feature-gate-edition_macro_expr.stderr

@@ -0,0 +1,19 @@
+error[E0658]: `expr2015` and `expr202x` are unstable.
+  --> $DIR/feature-gate-edition_macro_expr.rs:4:9
+   |
+LL |     ($x:expr2015) => {};
+   |         ^^^^^^^^
+   |
+   = help: add `#![feature(edition_macro_expr)]` to the crate attributes to enable
+
+error[E0658]: `expr2015` and `expr202x` are unstable.
+  --> $DIR/feature-gate-edition_macro_expr.rs:5:9
+   |
+LL |     ($x:expr202x) => {};
+   |         ^^^^^^^^
+   |
+   = help: add `#![feature(edition_macro_expr)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

Diff for: src/test/ui/macros/macro-expr2015-matching.rs

@@ -0,0 +1,34 @@
+// run-pass
+
+#![feature(edition_macro_expr)]
+#![feature(inline_const)]
+#![allow(incomplete_features)]
+
+macro_rules! new_const {
+    ($e:expr202x) => {
+        $e
+    };
+    (const $e:block) => {
+        1
+    };
+}
+
+macro_rules! old_const {
+    ($e:expr2015) => {
+        $e
+    };
+    (const $e:block) => {
+        1
+    };
+}
+
+fn main() {
+    match 1 {
+        old_const!(const { 2 }) => (),
+        _ => unreachable!(),
+    }
+    match 1 {
+        new_const!(const { 2 }) => unreachable!(),
+        _ => (),
+    }
+}