Provide better names for builtin deriving-generated attributes
First attempt at fixing #49967
Not in love with any choices here, don't be shy if you aren't happy with anything :)
I've tested that this produces nicer names in documentation, and that it no longer has issues conflicting with constants with the same name. (I guess we _could_ make a test for that... unsure if that would be valuable)
In all cases I took the names from the methods as declared in the relevant trait.
In some cases I had to prepend the names with _ otherwise there were errors about un-used variables. I'm uneasy with the inconsistency... do they all need to be like that? Is there a way to generate an alternate impl or use a different name (`_`?) in the cases where the arguments are not used?
Lastly the gensym addition to Ident I implemented largely as suggested, but I want to point out it's a little circuitous (at least, as far as I understand it). `cx.ident_of(name)` is just `Ident::from_str`, so we create an Ident then another Ident from it. `Ident::with_empty_ctxt(Symbol::gensym(string))` may or may not be equivalent, I don't know if it's important to intern it _then_ gensym it. It seems like either we could use that, or if we do want a new method to make this convenient, it could be on Ident instead (`from_str_gensymed`?)
Removed 'proc' from the reserved keywords list
Remove 'proc' from the reserved keywords list.
'proc' is a very useful identifier name for a lot of things. It's especially useful when dealing with processes, operating system internals, and kernel development.
Modify message for keyword as identifier name
This is a temporary solution to #46311.
The message is generic enough to cover both cases and is probably a fine enough solution to the specific problem described in the task. However, the underlying reason for this to be wrong is that `next_token_inner` returns `Lifetime` even if the token is a label. That's not simple, as the syntax for both can be quite similar and it may need to take a look to the next token to make a decision. I'm not sure I have enough knowledge about the project to be able to solve that (yet!), so I thought I'll fix the immediate problem first.
It's just not useful. It also makes it hard to have tests that probe
internal state, since the interning number is very sensitive.
Dumping the number in the case of gensym is not ideal but will do for
now.
