Function parse_if_expr 

fn parse_if_expr(cx: &mut ParseCtxt<'_>) -> ParseResult<Expr>

⟨if_expr⟩ := if ⟨expr⟩ ⟨block⟩ ⟨ else if ⟨expr⟩ ⟨block⟩ ⟩* else ⟨block⟩

The ⟨expr⟩ in conditions is parsed with allow_struct = false