Parse const effect block

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

@@ -46,6 +46,7 @@ pub(super) const ATOM_EXPR_FIRST: TokenSet =
         T![continue],
         T![async],
         T![try],
+        T![const],
         T![loop],
         T![for],
         LIFETIME_IDENT,
@@ -115,6 +116,14 @@ pub(super) fn atom_expr(p: &mut Parser, r: Restrictions) -> Option<(CompletedMar
             block_expr(p);
             m.complete(p, EFFECT_EXPR)
         }
+        // test const_block
+        // fn f() { const { } }
+        T![const] if la == T!['{'] => {
+            let m = p.start();
+            p.bump(T![const]);
+            block_expr(p);
+            m.complete(p, EFFECT_EXPR)
+        }
         T!['{'] => {
             // test for_range_from
             // fn foo() {

crates/parser/src/grammar/items.rs

@@ -96,7 +96,10 @@ pub(super) fn maybe_item(p: &mut Parser, m: Marker) -> Result<(), Marker> {
     let mut has_mods = false;
 
     // modifiers
-    has_mods |= p.eat(T![const]);
+    if p.at(T![const]) && p.nth(1) != T!['{'] {
+        p.eat(T![const]);
+        has_mods = true;
+    }
 
     // test_err async_without_semicolon
     // fn foo() { let _ = async {} }
@@ -167,7 +170,7 @@ pub(super) fn maybe_item(p: &mut Parser, m: Marker) -> Result<(), Marker> {
             m.complete(p, TRAIT);
         }
 
-        T![const] => {
+        T![const] if p.nth(1) != T!['{'] => {
             consts::konst(p, m);
         }