Function count_preceding_synthetic 

fn count_preceding_synthetic(stmts: &[Stmt], i: usize, decl_start: u32) -> usize

How many Stmt::Assigns immediately before index i are synthetic chained-init lowerings — identified by a span that starts after the LocalDecl’s span starts (i.e. the assignment text sits inside the chained T x = y = …; block, but the AST lifted it ahead of the decl).