Function lower_first_expr 

pub fn lower_first_expr(
    parsed: &ParsedSource,
    cst_kind: &str,
) -> Option<Spanned<Expr>>

Lowers the first CST node of cst_kind (depth-first) as an expression.

The node-walking entry point for tests and tools that target a specific sub-expression (FunctionCall, Block, BinaryExpression, …) without reaching for tree-sitter themselves.