[ruby-core:101782] [Ruby master Feature#17479] Enable to get "--backtrace-limit" value by "$-B"

Issue #17479 has been updated by neg_hide (Hidenori Negishi).


I am making a library that makes error messages easier to read. Therefore, instead of using `Exception#full_message`, I need to analyze the Exception and output the error message myself.

For example, suppose I have a script like this:

```ruby
def foo
  require "typo_library_name"
end

def bar
  foo
end

bar
```

When I execute `ruby --backtrace-limit=1 script.rb` in a normal environment, only the backtrace of kernel_require.rb is displayed as shown below, which makes it difficult to understand the true cause of the error. (The same is true for the string obtained from `Exception#full_message`.)

```
/home/myname/.rbenv/versions/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb:85:in `require': cannot load such file -- typo_library_name (LoadError)
        from /home/myname/.rbenv/versions/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb:85:in `require'
         ... 3 levels...
```

When I use my library, it omits the backtraces of kernel_require.rb and outputs them, so the error message looks like this.

```
ruby.rb:2:in `foo': cannot load such file -- typo_library_name (LoadError)
        from ruby.rb:2:in `foo'
        from ruby.rb:6:in `bar'
        from ruby.rb:9:in `<main>'
```

This process is impossible for `Exception#full_message`. So I have to output the error message myself.

However, I want the output to look like this:

```
ruby.rb:2:in `foo': cannot load such file -- typo_library_name (LoadError)
        from ruby.rb:2:in `foo'
         ... 2 levels...
```

That's why I need a value for "--backtrace-limit".

----------------------------------------
Feature #17479: Enable to get "--backtrace-limit" value by "$-B"
https://bugs.ruby-lang.org/issues/17479#change-89610

* Author: neg_hide (Hidenori Negishi)
* Status: Open
* Priority: Normal
----------------------------------------
# Background

There is currently no way to get "--backtrace-limit" value when outputing backtraces from my script, so I have to ignore this value and output backtraces.

In order to be able to output according to "--backtrace-limit" value, I need a API to get this value from the Ruby side.

# Proposal

I propose a readonly built-in variable "$-B" to get "--backtrace-limit" value.

``` ruby
# ruby --backtrace-limit=3 script.rb

puts $-B # => 3
```



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