Why does Ruby seem to hoist variable declarations from inside a case statement even if that code path is not executed? [duplicate]

When the Ruby parser sees the sequence identifier, equal-sign, value,
as in this expression

x = 1

it allocates space for a local variable called x. The creation of the
variable—not the assignment of a value to it, but the internal
creation of a variable—always takes place as a result of this kind of
expression, even if the code isn’t executed! Consider this example:

if false
  x = 1
end
p x # Output: nil
p y # Fatal Error: y is unknown

The assignment to x isn’t executed, because it’s wrapped in a failing
conditional test. But the Ruby parser sees the sequence x = 1, from
which it deduces that the program involves a local variable x. The
parser doesn’t care whether x is ever assigned a value. Its job is
just to scour the code for local variables for which space needs to
be allocated. The result is that x inhabits a strange kind of variable limbo.
It has been brought into being and initialized to nil.
In that respect, it differs from a variable that has no existence at
all; as you can see in the example, examining x gives you the value
nil, whereas trying to inspect the non-existent variable y results
in a fatal error. But although x exists, it has not played any role in
the program. It exists only as an artifact of the parsing process.

Well-Grounded Rubyist chapter 6.1.2