Bug #21187: Strings concatenated with `\` getting frozen with literal hashes (PRISM only)

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[LocoDelAssembly (Hernán Pereira)]

When the first elements of a literal hash are strings that are concatenated with \, those elements are flagged with PM_NODE_FLAG_STATIC_LITERAL and a special optimization that I believe was introduced in https://github.com/ruby/ruby/commit/8080de04be8e99e71309745822a9d436cc4ae37c causes the strings to be frozen.

Reproduction

test.rb

a = {
  a: 'one' \
     'two',
  b: 'three' \
     'four',
  c: 'five',
  d: 'six' \
     'seven'
}

b = {
  a: 'one',
  b: 'two' \
     'three'
}

puts "a = #{a.map { |k,v| {k => v.frozen?} }}"
puts "b = #{b.map { |k,v| {k => v.frozen?} }}"

With prism:

$ ruby test.rb                 
a = [{a: true}, {b: true}, {c: false}, {d: false}]
b = [{a: false}, {b: false}]

With parse.y:

$ ruby --parser=parse.y test.rb
a = [{a: false}, {b: false}, {c: false}, {d: false}]
b = [{a: false}, {b: false}]

(Notice b hash is unaffected in both parsers)

Not sure if this is just part of undefined behavior or this is indeed a bug. Assigning a string concatenated with \ to a variable doesn't make it frozen, to the best of my knowledge this seems to be hash-specific.