Improve help message of from_over_into lint

[flip1995]

impl Into<c_int> or Signal {
...
}

In this case [...] the fix is to change last impl to this:

impl From<Signal> for c_int {
...
}

It might be helpful if the recommendation actually printed the type so it was explicitly obvious (implied). Here is an example error:

warning: an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
   --> /mnt/host/source/src/platform/crosvm/sys_util/src/signal.rs:174:1
    |
174 | impl Into<c_int> for Signal {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider to implement `From` instead
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#from_over_into

A better [help] message might look something like:

    = help: consider to implement `From<Signal>` instead

Originally posted by @Allen-Webb in #4894 (comment)

So the help message should at least mention the generic type that should be used. Maybe even produce an non-autoapplicable suggestion.

[Allen-Webb]

Just today I converted a few cases caught by the linter. One was even for a proc_macro. I think an autoapplicable suggestion might be possible but one hurdle would be coming up with an identifier $ident for the argument of from() that doesn't collide with any of the contents of the previous into(self) implementation. You would then replace self with $ident.

It seems like there may be some rare odd cases where From<> probably wouldn't be that useful such as for tuples that aren't given a named type and the developer might just silence the warning with an #[allow()], but even in those cases automatically converting them wouldn't be harmful.

Here are links the the gerrit CLs where I made the changes in case they are helpful cases when developing the help message:

• typical cases
• proc-macro case
• not as useful case

[ABouttefeux]

@rustbot claim