## Motivation

The closure generated by `#[instrument(err)]` for non-async functions is
not mutable, preventing any captured variables from being mutated. If a
function has any mutable parameters that are modified within the
function, adding the `#[instrument(err)]` attribute will result in a
compiler error.

## Solution

This change makes it so the closure is executed directly as a temporary
variable instead of storing it in a named variable prior to its use,
avoiding the need to explicitly declare it as mutable altogether or to
add an `#[allow(unused_mut)]` annotation on the closure declaration to
silence lint warnings for closures that do not need to be mutable.

The clippy::unit_arg warning is triggered for the Ok branch in the match
statements generated for the #[instrument(err)] attribute against the
function result if that function returns a unit Ok value. This commit
disables the unit_arg lint check for the Ok match arms and cleans up
other attempts to isolate the lint error.