Add `<'_>` to `Formatter` in `Visitor::expecting`

[zacknewman]

The lint group rust-2018-idioms enables elided-lifetimes-in-paths. Visitor::expecting omits the lifetime from Formatter. When relying on autocompletion, this requires one to manually add a lifetime argument to silence this warning. Would it be difficult to change this?

#![warn(rust_2018_idioms)]
use core::fmt;
use serde::de::{Error, Visitor};
pub struct Foo;
impl<'de> Visitor<'de> for Foo {
    type Value = ();
    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str("()")
    }
    fn visit_unit<E>(self) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Ok(())
    }
}

expecting was autocompleted in Helix.

[zack@laptop src]$ cargo check
    Checking bar v0.1.0 (/home/zack/projects/bar)
warning: hidden lifetime parameters in types are deprecated
 --> src/lib.rs:7:46
  |
7 |     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
  |                                         -----^^^^^^^^^
  |                                         |
  |                                         expected lifetime parameter
  |
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![warn(rust_2018_idioms)]
  |         ^^^^^^^^^^^^^^^^
  = note: `#[warn(elided_lifetimes_in_paths)]` implied by `#[warn(rust_2018_idioms)]`
help: indicate the anonymous lifetime
  |
7 |     fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
  |                                                       ++++

warning: `bar` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s

In contrast if the definition were defined with an explicit lifetime, autocompletion retains it; and no warning is issued:

#![warn(rust_2018_idioms)]
use core::fmt;
pub struct Foo;
pub trait Bar<'de>: Sized {
    type Value;
    fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result;
}
impl<'de> Bar<'de> for Foo {
    type Value = ();
    fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter.write_str("()")
    }
}

[zack@laptop src]$ cargo check
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s

[devnetsec]

I just ran the test suite successfully, that change should be trivial.