[ruby-core:97586] [Ruby master Bug#16738] RbConfig::CONFIG["SDKROOT"] not frozen on macOS

Issue #16738 has been updated by Eregon (Benoit Daloze).


The reason RbConfig entries cannot be frozen currently is because RbConfig.expand which calls String#replace.
So I think this is a bug, and we should ensure all RbConfig entries are mutable, hence the spec.

Alternatively, one could try to rewrite `RbConfig.expand`, but not sure that's possible in a compatible way
(I don't understand that code in details).

----------------------------------------
Bug #16738: RbConfig::CONFIG["SDKROOT"] not frozen on macOS
https://bugs.ruby-lang.org/issues/16738#change-84770

* Author: bjfish (Brandon Fish)
* Status: Open
* Priority: Normal
* ruby -v: 2.6.5, 2.7.0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
The `RbConfig::CONFIG["SDKROOT"]` is not frozen on macOS when the `SDKROOT` environment variable is set which causes the kernel spec (https://github.com/ruby/ruby/blob/1b3339528c0804aa0e673bd3f15be8b087d17bd8/spec/ruby/library/rbconfig/rbconfig_spec.rb#L27) to fail.

**Spec Failure** 
```
1)
RbConfig::CONFIG contains no frozen strings even with --enable-frozen-string-literal FAILED
Expected "SDKROOT Failure\n" + "Done\n" == "Done\n"
to be truthy but was false
/Users/bfish/Documents/projects/spec/library/rbconfig/rbconfig_spec.rb:28:in `block (2 levels) in <top (required)>'
/Users/bfish/Documents/projects/spec/library/rbconfig/rbconfig_spec.rb:4:in `<top (required)>'
```

**Example**
```
% unset SDKROOT
% ruby -e 'p RbConfig::CONFIG["SDKROOT"].frozen?'
false
% export SDKROOT="test"
% ruby -e 'p RbConfig::CONFIG["SDKROOT"].frozen?'
true

```

**Environment**
macOS

**Versions Tested**
2.6.5
2.7.0




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