[ruby-core:118965] [Ruby master Bug#20701] Hash argument passed as keyword splat can be mutated inside method

Issue #20701 has been reported by pabloh (Pablo Herrero).

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Bug #20701: Hash argument passed as keyword splat can be mutated inside method
https://bugs.ruby-lang.org/issues/20701

* Author: pabloh (Pablo Herrero)
* Status: Open
* ruby -v: ruby 3.4.0dev (2024-08-27T00:39:24Z master e5295644c9) [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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If you call a method with a splatted kwargs that also uses a splatted array followed by at least one positional argument, the called method can mutate the splatted kwargs.

``` ruby
def foo(a, h)
  h[:splat_modified] = true
end

a = []
b = {splat_modified: false}

foo(*a, :x, **b) 
b[:splat_modified] # Should be 'false' but is 'true'

# It makes no difference if the called method expects a kwargs
def foo2(a, **h)
  h[:splat_modified] = true
end

b[:splat_modified] = false
foo2(*a, :x, **b)
b[:splat_modified] # Should be 'false' but is 'true'


# The case when a block is also pass is broken too

c = proc { }
b[:splat_modified] = false
foo(*a, :x, **b, &c) 
b[:splat_modified] # Should be 'false' but is 'true'


# When the extra positional argument after the array splat is removed the correct behavior is restored
b[:splat_modified] = false
foo(*a, **b)
foo(*a, :x, **b, &c)
b[:splat_modified] # Is 'false'
```

Apparently, this problem is due to the `KW_SPLAT_MUT` flag being set at the method's call site:


`ruby --dump=insns,-optimize -e 'a, b, c = [4,6,4],{a: 3},:ds; foo(*a,9,**c)` outputs
```
0000 duparray                               [4, 6, 4]                 (   1)[Li]
0002 duphash                                {:a=>3}
0004 putobject                              :ds
0006 setlocal                               c@2, 0
0009 setlocal                               b@1, 0
0012 setlocal                               a@0, 0
0015 putself
0016 getlocal                               a@0, 0
0019 splatarray                             true
0021 putobject                              9
0023 pushtoarray                            1
0025 getlocal                               c@2, 0
0028 send                                   <calldata!mid:foo, argc:2, ARGS_SPLAT|ARGS_SPLAT_MUT|FCALL|KW_SPLAT|KW_SPLAT_MUT>, nil
0031 leave
```

The error is present in both 3.3 and 3.4:
- ruby 3.4.0dev (2024-08-27T00:39:24Z master e5295644c9) [x86_64-linux]
- ruby 3.3.4 (2024-07-09 revision be1089c8ec) [x86_64-linux]

PR for test: https://github.com/ruby/ruby/pull/11467



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