Struct ConstParam
pub struct ConstParam {
pub attrs: Vec<Attribute>,
pub const_token: Const,
pub ident: Ident,
pub colon_token: Colon,
pub ty: Type,
pub eq_token: Option<Eq>,
pub default: Option<Expr>,
}Iterator<Item = &Expr>
node: &ConstParam
node.default.as_slice()Iterator<Item = &mut Expr>
node: &mut ConstParam
node.default.as_mut_slice()