[ruby-core:109188] [Ruby master Bug#18905] :"@=".inspect is non-evaluatable

Issue #18905 has been updated by ufuk (Ufuk Kayserilioglu).


I was bitten by the same thing last week and was actually going to open an issue myself. Thanks for raising this @qnighy.

The thing that caught me out was that the following discrepancy:
```ruby
"4_to_5".to_sym.inspect
#=> :"4_to_5"
"4_to_5=".to_sym.inspect
#=> :4_to_5=
```
I realize that Ruby spends a considerable amount of effort to not print symbols in a format where they could not be roundtripped, and the discprancy between `:"4_to_5"` and `:"4_to_5="` was surprising.

I bisected the change to the released versions of Ruby and found that the behaviour changed somewhere between `ruby-2.2.0-preview1` and `ruby-2.2.0-preview2`:

```
$ docker run --rm -e "ALL_RUBY_SINCE=ruby-2.0" rubylang/all-ruby ./all-ruby -e "puts '4_to_5='.to_sym.inspect"
ruby-2.0.0-p0       :"4_to_5="
...
ruby-2.2.0-preview1 :"4_to_5="
ruby-2.2.0-preview2 :4_to_5=
...
ruby-3.2.0-preview1 :4_to_5=
```

Looking at the commit log, I can see that there is a commit that adds special casing for a trailing `=` but I don't understand the reason behind why the change was made: https://github.com/ruby/ruby/commit/986a893d7a420b8472c06b2e36b71b8bf00e9e21 

@nobu I understand that Ruby does not guarantee that `inspect` methods are for `eval` but Ruby seems to consistently quote invalid `Symbol`s in `inspect`, except for when they end with `=`. I think this is not consistent and Ruby might as well not quote `Symbol`s in `inspect` at all, in this case. I think something has to change to keep consistency.

----------------------------------------
Bug #18905: :"@=".inspect is non-evaluatable
https://bugs.ruby-lang.org/issues/18905#change-98329

* Author: qnighy (Masaki Hara)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
There is an edge case where `Symbol#inspect` returns a non-evaluatable expression:

```ruby
:"@=".inspect # => :@=
:"[][]=".inspect # => :[][]=
:"$$$$=" # => :$$$$=
```

More specifically, the quotations are stripped if the following conditions are met (but the converse does not hold):

- It starts with `@`, `@@`, `$` or `[`,
- the prefix is not followed by an identifier, and
- it ends with `=`.

But the rule should only apply to `:$=` and `:[]=`, because (in my understanding) we should generally have `eval(sym.inspect) == sym` for the programmers' convenience.



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