Fix parsing of _ = x in closure body

crates/parser/src/grammar/expressions.rs

@@ -288,7 +288,7 @@ fn expr_bp(
 }
 
 const LHS_FIRST: TokenSet =
-    atom::ATOM_EXPR_FIRST.union(TokenSet::new(&[T![&], T![*], T![!], T![.], T![-]]));
+    atom::ATOM_EXPR_FIRST.union(TokenSet::new(&[T![&], T![*], T![!], T![.], T![-], T![_]]));
 
 fn lhs(p: &mut Parser<'_>, r: Restrictions) -> Option<(CompletedMarker, BlockLike)> {
     let m;

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

@@ -278,6 +278,8 @@ fn closure_expr(p: &mut Parser<'_>) -> CompletedMarker {
         // fn main() { || -> i32 { 92 }(); }
         block_expr(p);
     } else if p.at_ts(EXPR_FIRST) {
+        // test closure_body_underscore_assignment
+        // fn main() { || _ = 0; }
         expr(p);
     } else {
         p.error("expected expression");

crates/parser/test_data/parser/inline/ok/0203_closure_body_underscore_assignment.rast

@@ -0,0 +1,32 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "main"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE " "
+        EXPR_STMT
+          CLOSURE_EXPR
+            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/0203_closure_body_underscore_assignment.rs

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