diff --git a/Steepfile b/Steepfile
index 5b18ffab86f..e710262e719 100644
--- a/Steepfile
+++ b/Steepfile
@@ -13,5 +13,6 @@ target :lib do
   ignore "lib/prism/lex_compat.rb"
   ignore "lib/prism/serialize.rb"
   ignore "lib/prism/ffi.rb"
+  ignore "lib/prism/polyfill/byteindex.rb"
   ignore "lib/prism/translation"
 end
diff --git a/lib/prism.rb b/lib/prism.rb
index c733baf8c92..19774538e7a 100644
--- a/lib/prism.rb
+++ b/lib/prism.rb
@@ -67,6 +67,7 @@ def self.load(source, serialized)
   end
 end
 
+require_relative "prism/polyfill/byteindex"
 require_relative "prism/node"
 require_relative "prism/node_ext"
 require_relative "prism/parse_result"
diff --git a/lib/prism/parse_result.rb b/lib/prism/parse_result.rb
index e8d77172283..63cc72a9664 100644
--- a/lib/prism/parse_result.rb
+++ b/lib/prism/parse_result.rb
@@ -347,6 +347,18 @@ def join(other)
 
       Location.new(source, start_offset, other.end_offset - start_offset)
     end
+
+    # Join this location with the first occurrence of the string in the source
+    # that occurs after this location on the same line, and return the new
+    # location. This will raise an error if the string does not exist.
+    def adjoin(string)
+      line_suffix = source.slice(end_offset, source.line_end(end_offset) - end_offset)
+
+      line_suffix_index = line_suffix.byteindex(string)
+      raise "Could not find #{string}" if line_suffix_index.nil?
+
+      Location.new(source, start_offset, length + line_suffix_index + string.bytesize)
+    end
   end
 
   # This represents a comment that was encountered during parsing. It is the
diff --git a/lib/prism/polyfill/byteindex.rb b/lib/prism/polyfill/byteindex.rb
new file mode 100644
index 00000000000..98c6089f141
--- /dev/null
+++ b/lib/prism/polyfill/byteindex.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+# Polyfill for String#byteindex, which didn't exist until Ruby 3.2.
+if !("".respond_to?(:byteindex))
+  String.include(
+    Module.new {
+      def byteindex(needle, offset = 0)
+        charindex = index(needle, offset)
+        slice(0...charindex).bytesize if charindex
+      end
+    }
+  )
+end
diff --git a/lib/prism/polyfill/string.rb b/lib/prism/polyfill/unpack1.rb
similarity index 100%
rename from lib/prism/polyfill/string.rb
rename to lib/prism/polyfill/unpack1.rb
diff --git a/prism.gemspec b/prism.gemspec
index 18d0c00a411..8a3efe826d3 100644
--- a/prism.gemspec
+++ b/prism.gemspec
@@ -86,7 +86,8 @@ Gem::Specification.new do |spec|
     "lib/prism/parse_result/comments.rb",
     "lib/prism/parse_result/newlines.rb",
     "lib/prism/pattern.rb",
-    "lib/prism/polyfill/string.rb",
+    "lib/prism/polyfill/byteindex.rb",
+    "lib/prism/polyfill/unpack1.rb",
     "lib/prism/reflection.rb",
     "lib/prism/serialize.rb",
     "lib/prism/translation.rb",
diff --git a/rbi/prism/parse_result.rbi b/rbi/prism/parse_result.rbi
index 61d125c331e..73e0be1bc70 100644
--- a/rbi/prism/parse_result.rbi
+++ b/rbi/prism/parse_result.rbi
@@ -44,7 +44,7 @@ class Prism::Source
   def code_units_column(byte_offset, encoding); end
 end
 
-class Prism::ASCIISource < Source
+class Prism::ASCIISource < Prism::Source
   sig { params(byte_offset: Integer).returns(Integer) }
   def character_offset(byte_offset); end
 
@@ -154,6 +154,9 @@ class Prism::Location
 
   sig { params(other: Prism::Location).returns(Prism::Location) }
   def join(other); end
+
+  sig { params(string: String).returns(Prism::Location) }
+  def adjoin(string); end
 end
 
 class Prism::Comment
diff --git a/sig/prism/parse_result.rbs b/sig/prism/parse_result.rbs
index c475fa597b9..3487d5412ca 100644
--- a/sig/prism/parse_result.rbs
+++ b/sig/prism/parse_result.rbs
@@ -55,6 +55,7 @@ module Prism
     def deconstruct_keys: (Array[Symbol]? keys) -> Hash[Symbol, untyped]
     def pretty_print: (untyped q) -> untyped
     def join: (Location other) -> Location
+    def adjoin: (String string) -> Location
   end
 
   class Comment
diff --git a/templates/lib/prism/node.rb.erb b/templates/lib/prism/node.rb.erb
index 4e07c397e6d..50e622a8d12 100644
--- a/templates/lib/prism/node.rb.erb
+++ b/templates/lib/prism/node.rb.erb
@@ -83,7 +83,7 @@ module Prism
     # Important to note is that the column given to this method should be in
     # bytes, as opposed to characters or code units.
     def tunnel(line, column)
-      queue = [self]
+      queue = [self] #: Array[Prism::node]
       result = []
 
       while (node = queue.shift)
diff --git a/templates/lib/prism/serialize.rb.erb b/templates/lib/prism/serialize.rb.erb
index 29ae5356ba4..578e7d2e70f 100644
--- a/templates/lib/prism/serialize.rb.erb
+++ b/templates/lib/prism/serialize.rb.erb
@@ -1,5 +1,5 @@
 require "stringio"
-require_relative "polyfill/string"
+require_relative "polyfill/unpack1"
 
 module Prism
   # A module responsible for deserializing parse results.
diff --git a/test/prism/ruby_api_test.rb b/test/prism/ruby_api_test.rb
index 538e715ca3f..a1e2592d3d4 100644
--- a/test/prism/ruby_api_test.rb
+++ b/test/prism/ruby_api_test.rb
@@ -285,6 +285,19 @@ def test_node_tunnel
       assert_equal [ProgramNode, StatementsNode, CallNode, ArgumentsNode, CallNode, ArgumentsNode], tunnel.map(&:class)
     end
 
+    def test_location_adjoin
+      program = Prism.parse("foo.bar = 1").value
+
+      location = program.statements.body.first.message_loc
+      adjoined = location.adjoin("=")
+
+      assert_kind_of Location, adjoined
+      refute_equal location, adjoined
+
+      assert_equal 4, adjoined.start_offset
+      assert_equal 9, adjoined.end_offset
+    end
+
     private
 
     def parse_expression(source)