rustc --explain E0117 confusingly refers to Drop trait

[davepacheco]

The detailed help for compiler error E0117 opens with this summary:

The Drop trait was implemented on a non-struct type.

The rest of the help output correctly (and clearly -- thank you!) explains what happened -- namely, that the user tried to violate the orphan rules for trait implementations. However that first line is confusing since it's apparently unrelated and wrong.

Here's a simple chunk of code that generates error E0117 (as expected):

impl From<String> for Result<String, String>
{
    fn from(s: String) -> Result<String, String>
    {
        Ok(s)
    }
}

Here it is on the playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=427d7c85706b598fa8232108769a8552

The error is expected. It's the help output that's confusing:

$ rustc +nightly --explain E0117 
The `Drop` trait was implemented on a non-struct type.

...

The content is also here: https://doc.rust-lang.org/error-index.html#E0117
and I think comes from here: https://github.com/rust-lang/rust/blob/master/src/librustc_error_codes/error_codes/E0117.md

[contrun]

I just checked the only place out of sync is the description in the markdown file. I created a pr for that. #69991

[davepacheco]

Thanks!