-p ast

class <emptyTree><<C <root>>> < ()
  a = "part 1 of heredoc ".concat("not a heredoc".+("eom part\n").to_s()).concat("\n").concat("eom part\n").concat("EOM\n").concat("part 2 of heredoc\n").to_s()

  b = "oweqijfoiwjefqwoefij\n"

  def foo<<C <todo sym>>>(&<blk>)
    begin
      c = "oqweijfoqwiejf\n"
      <self>.puts(c)
    end
  end

  <self>.puts(a)

  <self>.puts(b)

  <self>.foo()
end

-p parse-tree-json

{
  "type" : "Begin",
  "stmts" : [
    {
      "type" : "Assign",
      "lhs" : {
        "type" : "LVarLhs",
        "name" : "a"
      },
      "rhs" : {
        "type" : "DString",
        "nodes" : [
          {
            "type" : "DString",
            "nodes" : [
              {
                "type" : "String",
                "val" : "part 1 of heredoc "
              },
              {
                "type" : "Begin",
                "stmts" : [
                  {
                    "type" : "Send",
                    "receiver" : {
                      "type" : "DString",
                      "nodes" : [
                        {
                          "type" : "String",
                          "val" : "not a heredoc"
                        }
                      ]
                    },
                    "method" : "+",
                    "args" : [
                      {
                        "type" : "DString",
                        "nodes" : [
                          {
                            "type" : "DString",
                            "nodes" : [
                              {
                                "type" : "String",
                                "val" : "eom part\n"
                              }
                            ]
                          }
                        ]
                      }
                    ]
                  }
                ]
              },
              {
                "type" : "String",
                "val" : "\n"
              },
              {
                "type" : "String",
                "val" : "eom part\n"
              },
              {
                "type" : "String",
                "val" : "EOM\n"
              },
              {
                "type" : "String",
                "val" : "part 2 of heredoc\n"
              }
            ]
          }
        ]
      }
    },
    {
      "type" : "Assign",
      "lhs" : {
        "type" : "LVarLhs",
        "name" : "b"
      },
      "rhs" : {
        "type" : "DString",
        "nodes" : [
          {
            "type" : "DString",
            "nodes" : [
              {
                "type" : "String",
                "val" : "oweqijfoiwjefqwoefij\n"
              }
            ]
          }
        ]
      }
    },
    {
      "type" : "DefMethod",
      "name" : "foo",
      "args" : null,
      "body" : {
        "type" : "Begin",
        "stmts" : [
          {
            "type" : "Assign",
            "lhs" : {
              "type" : "LVarLhs",
              "name" : "c"
            },
            "rhs" : {
              "type" : "DString",
              "nodes" : [
                {
                  "type" : "DString",
                  "nodes" : [
                    {
                      "type" : "String",
                      "val" : "oqweijfoqwiejf\n"
                    }
                  ]
                }
              ]
            }
          },
          {
            "type" : "Send",
            "receiver" : null,
            "method" : "puts",
            "args" : [
              {
                "type" : "LVar",
                "name" : "c"
              }
            ]
          }
        ]
      }
    },
    {
      "type" : "Send",
      "receiver" : null,
      "method" : "puts",
      "args" : [
        {
          "type" : "LVar",
          "name" : "a"
        }
      ]
    },
    {
      "type" : "Send",
      "receiver" : null,
      "method" : "puts",
      "args" : [
        {
          "type" : "LVar",
          "name" : "b"
        }
      ]
    },
    {
      "type" : "Send",
      "receiver" : null,
      "method" : "foo",
      "args" : [
      ]
    }
  ]
}

source.rb

a = <<EOD
part 1 of heredoc #{ "not a heredoc" + <<EOM }
eom part
EOM
part 2 of heredoc
EOD

b = <<-EOD
oweqijfoiwjefqwoefij
EOD


def foo
  c = <<~EOD
    oqweijfoqwiejf
  EOD
  puts c
end

puts a
puts b
foo