Parse const_closures syntax.

Enables parsing of the syntax for `#![features(const_closures)]` introduced in rust-lang/rust#106004

Author: onestacked

crates/parser/src/grammar/expressions/atom.rs

@@ -152,7 +152,7 @@ pub(super) fn atom_expr(
             m.complete(p, BLOCK_EXPR)
         }
 
-        T![static] | T![async] | T![move] | T![|] => closure_expr(p),
+        T![const] | T![static] | T![async] | T![move] | T![|] => closure_expr(p),
         T![for] if la == T![<] => closure_expr(p),
         T![for] => for_expr(p, None),
 
@@ -255,7 +255,7 @@ fn array_expr(p: &mut Parser<'_>) -> CompletedMarker {
 // }
 fn closure_expr(p: &mut Parser<'_>) -> CompletedMarker {
     assert!(match p.current() {
-        T![static] | T![async] | T![move] | T![|] => true,
+        T![const] | T![static] | T![async] | T![move] | T![|] => true,
         T![for] => p.nth(1) == T![<],
         _ => false,
     });
@@ -265,7 +265,9 @@ fn closure_expr(p: &mut Parser<'_>) -> CompletedMarker {
     if p.at(T![for]) {
         types::for_binder(p);
     }
-
+    // test const_closure
+    // fn main() { let cl = const || _ = 0; }
+    p.eat(T![const]);
     p.eat(T![static]);
     p.eat(T![async]);
     p.eat(T![move]);

crates/parser/test_data/parser/inline/ok/0205_const_closure.rast

@@ -0,0 +1,42 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "main"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE " "
+        LET_STMT
+          LET_KW "let"
+          WHITESPACE " "
+          IDENT_PAT
+            NAME
+              IDENT "cl"
+          WHITESPACE " "
+          EQ "="
+          WHITESPACE " "
+          CLOSURE_EXPR
+            CONST_KW "const"
+            WHITESPACE " "
+            PARAM_LIST
+              PIPE "|"
+              PIPE "|"
+            WHITESPACE " "
+            BIN_EXPR
+              UNDERSCORE_EXPR
+                UNDERSCORE "_"
+              WHITESPACE " "
+              EQ "="
+              WHITESPACE " "
+              LITERAL
+                INT_NUMBER "0"
+          SEMICOLON ";"
+        WHITESPACE " "
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0205_const_closure.rs

@@ -0,0 +1 @@
+fn main() { let cl = const || _ = 0; }