Support mutable function arguments

[dalcde]

auto_impl should support trait methods of the form

fn foo(&self, mut bar: Bar) {
    // default implementation that mutates bar
}

[KodrAus]

The current failure here is:

┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
error: argument patterns are not supported by #[auto-impl]
 --> tests/compile-pass/self_by_value_mut.rs:9:19
  |
9 |     fn foo(&self, mut data: Data) {
  |                   ^^^^^^^^
  |
  = help: please use a simple name like "foo" (but not `_`)

error[E0057]: this function takes 1 argument but 0 arguments were supplied
  --> tests/compile-pass/self_by_value_mut.rs:7:1
   |
7  | #[auto_impl(Fn)]
   | ^^^^^^^^^^^^^^^^ an argument of type `Data` is missing
   |
note: associated function defined here
  --> $RUST/core/src/ops/function.rs
   |
   |     extern "rust-call" fn call(&self, args: Args) -> Self::Output;
   |                           ^^^^
   = note: this error originates in the attribute macro `auto_impl` (in Nightly builds, run with -Z macro-backtrace for more info)
help: provide the argument
   |
7  | #[auto_impl(Fn)]({Data})
   |
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈

This is just a limitation we could work around for this style of simple pattern. In the meantime, you can write:

fn foo(&self, data: Data) {
    let mut data = data;
    data.id += 1;
}