clippy::missing_const_for_fn

warning: this could be a `const fn`
   --> src/cmd/diff.rs:190:5
    |
190 | /     fn new(csv_writer: csv::Writer<W>, no_headers: bool) -> Self {
191 | |         Self {
192 | |             csv_writer,
193 | |             no_headers,
194 | |         }
195 | |     }
    | |_____^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn
    = note: `-W clippy::missing-const-for-fn` implied by `-W clippy::nursery`
    = help: to override `-W clippy::nursery` add `#[allow(clippy::missing_const_for_fn)]`

src/cmd/diff.rs

@@ -187,7 +187,7 @@ struct CsvDiffWriter<W: Write> {
 }
 
 impl<W: Write> CsvDiffWriter<W> {
- fn new(csv_writer: csv::Writer<W>, no_headers: bool) -> Self {
+ const fn new(csv_writer: csv::Writer<W>, no_headers: bool) -> Self {
  Self {
  csv_writer,
  no_headers,