2 files changed, 84 insertions, 2 deletions
diff --git a/test/prism/snapshots_test.rb b/test/prism/snapshots_test.rb
new file mode 100644
index 0000000000..3fbda1f86e
--- /dev/null
+++ b/test/prism/snapshots_test.rb
@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+require_relative "test_helper"
+
+# We don't want to generate snapshots when running
+# against files outside of the project folder.
+return if Prism::TestCase::Fixture.custom_base_path?
+
+module Prism
+  class SnapshotsTest < TestCase
+    # When we pretty-print the trees to compare against the snapshots, we want
+    # to be certain that we print with the same external encoding. This is
+    # because methods like Symbol#inspect take into account external encoding
+    # and it could change how the snapshot is generated. On machines with
+    # certain settings (like LANG=C or -Eascii-8bit) this could have been
+    # changed. So here we're going to force it to be UTF-8 to keep the snapshots
+    # consistent.
+    def setup
+      @previous_default_external = Encoding.default_external
+      ignore_warnings { Encoding.default_external = Encoding::UTF_8 }
+    end
+
+    def teardown
+      ignore_warnings { Encoding.default_external = @previous_default_external }
+    end
+
+    except = []
+
+    # These fail on TruffleRuby due to a difference in Symbol#inspect:
+    # :测试 vs :"测试"
+    if RUBY_ENGINE == "truffleruby"
+      except.push(
+        "emoji_method_calls.txt",
+        "seattlerb/bug202.txt",
+        "seattlerb/magic_encoding_comment.txt"
+      )
+    end
+
+    Fixture.each(except: except) do |fixture|
+      define_method(fixture.test_name) { assert_snapshot(fixture) }
+    end
+
+    private
+
+    def assert_snapshot(fixture)
+      source = fixture.read
+
+      result = Prism.parse(source, filepath: fixture.path)
+      assert result.success?
+
+      printed = PP.pp(result.value, +"", 79)
+      snapshot = fixture.snapshot_path
+
+      if File.exist?(snapshot)
+        saved = File.read(snapshot)
+
+        # If the snapshot file exists, but the printed value does not match the
+        # snapshot, then update the snapshot file.
+        if printed != saved
+          File.write(snapshot, printed)
+          warn("Updated snapshot at #{snapshot}.")
+        end
+
+        # If the snapshot file exists, then assert that the printed value
+        # matches the snapshot.
+        assert_equal(saved, printed)
+      else
+        # If the snapshot file does not yet exist, then write it out now.
+        directory = File.dirname(snapshot)
+        FileUtils.mkdir_p(directory) unless File.directory?(directory)
+
+        File.write(snapshot, printed)
+        warn("Created snapshot at #{snapshot}.")
+      end
+    end
+  end
+end
diff --git a/test/prism/test_helper.rb b/test/prism/test_helper.rb
index b848500283..c5afdc0ea5 100644
--- a/test/prism/test_helper.rb
+++ b/test/prism/test_helper.rb
@@ -38,7 +38,7 @@ module Prism
     # are used to define test methods that assert against each fixture in some
     # way.
     class Fixture
-      BASE = File.join(__dir__, "fixtures")
+      BASE = ENV.fetch("FIXTURE_BASE", File.join(__dir__, "fixtures"))
 
       attr_reader :path
 
@@ -63,9 +63,14 @@ module Prism
       end
 
       def self.each(except: [], &block)
-        paths = Dir[ENV.fetch("FOCUS") { File.join("**", "*.txt") }, base: BASE] - except
+        glob_pattern = ENV.fetch("FOCUS") { custom_base_path? ? File.join("**", "*.rb") : File.join("**", "*.txt") }
+        paths = Dir[glob_pattern, base: BASE] - except
         paths.each { |path| yield Fixture.new(path) }
       end
+
+      def self.custom_base_path?
+        ENV.key?("FIXTURE_BASE")
+      end
     end
 
     # Yield each encoding that we want to test, along with a range of the