Module expr 

The Expr union over “any R expression”, and the HasArgList shared trait for the argument-bearing nodes.

R’s leaf expressions are bare tokens, not LITERAL nodes (1 + 2 is BINARY_EXPR { INT, PLUS, INT }; a bare name is an IDENT). So Expr casts from a SyntaxElement (node or token) and carries both node variants and token-atom variants — a single match Expr::cast(el) then covers every operand a consumer meets, with no separate “is it an IDENT token” arm.

Enums

Expr

Any R expression: a compound node (BINARY_EXPR, CALL_EXPR, …) or a leaf atom token (a name or a literal).

Traits

HasArgList

A node that owns an argument list: CALL_EXPR (f(…)), SUBSET_EXPR (x[…]), and SUBSET2_EXPR (x[[…]]).