Function lower_expression 

pub fn lower_expression(source: &str) -> Expr

Lower a raw expression-source slice into an Expr. Errors surface as Expr::Raw with a typed reason — never panic.

The public signature is depth-agnostic; the recursion budget is threaded internally via [lower_expression_depth] so a crafted flat binary chain or deep paren/call/unary spine in untrusted input degrades to UnknownExprReason::ExprDepthLimit at MAX_EXPR_DEPTH instead of overflowing the stack.