Ensure Source#offsets is set correctly in all cases

lib/prism/parse_result.rb

@@ -26,7 +26,8 @@ class Source
     # source is a subset of a larger source or if this is an eval. offsets is an
     # array of byte offsets for the start of each line in the source code, which
     # can be calculated by iterating through the source code and recording the
-    # byte offset whenever a newline character is encountered.
+    # byte offset whenever a newline character is encountered.  The first
+    # element is always 0 to mark the first line.
     #--
     #: (String source, Integer start_line, Array[Integer] offsets) -> Source
     def self.for(source, start_line, offsets)

templates/lib/prism/dsl.rb.erb

@@ -1,11 +1,13 @@
 #--
 # rbs_inline: enabled
 
+require_relative "polyfill/byteindex"
+
 module Prism
   # The DSL module provides a set of methods that can be used to create prism
   # nodes in a more concise manner. For example, instead of writing:
   #
-  #     source = Prism::Source.for("[1]", 1, [])
+  #     source = Prism::Source.for("[1]", 1, [0])
   #
   #     Prism::ArrayNode.new(
   #       source,
@@ -62,7 +64,16 @@ module Prism
     #--
     #: (String string) -> Source
     def source(string)
-      Source.for(string, 1, [])
+      Source.for(string, 1, compute_offsets(string))
+    end
+
+    private def compute_offsets(code)
+      offsets = [0]
+      start = 0
+      while i = code.byteindex("\n", start)
+        offsets << (start = i + 1)
+      end
+      offsets
     end
 
     # Create a new Location object.
@@ -136,7 +147,7 @@ module Prism
     #--
     #: () -> Source
     def default_source
-      Source.for("", 1, [])
+      Source.for("", 1, [0])
     end
 
     # The default location object that gets attached to nodes if no location is