Fix lex compat when dedent should be 0

lib/prism/lex_compat.rb

@@ -357,6 +357,7 @@ def initialize
           @dedent_next = true
           @dedent = nil
           @embexpr_balance = 0
+          @ended_on_newline = false
         end
 
         # As tokens are coming in, we track the minimum amount of common leading
@@ -366,13 +367,14 @@ def <<(token)
           case token.event
           when :on_embexpr_beg, :on_heredoc_beg
             @embexpr_balance += 1
+            @dedent = 0 if @dedent_next && @ended_on_newline
           when :on_embexpr_end, :on_heredoc_end
             @embexpr_balance -= 1
           when :on_tstring_content
             if embexpr_balance == 0
               line = token.value
 
-              if !(line.strip.empty? && line.end_with?("\n")) && dedent_next
+              if dedent_next && !(line.strip.empty? && line.end_with?("\n"))
                 leading = line[/\A(\s*)\n?/, 1]
                 next_dedent = 0
 
@@ -385,11 +387,16 @@ def <<(token)
                 end
 
                 @dedent = [dedent, next_dedent].compact.min
+                @dedent_next = true
+                @ended_on_newline = line.end_with?("\n")
+                tokens << token
+                return
               end
             end
           end
 
           @dedent_next = token.event == :on_tstring_content && embexpr_balance == 0
+          @ended_on_newline = false
           tokens << token
         end
 

test/prism/fixtures/tilde_heredocs.txt

@@ -1,3 +1,9 @@
+<<~EOF
+  a
+#{1}
+  a
+EOF
+
 <<~EOF
   a
 EOF

test/prism/heredoc_dedent_test.rb

@@ -7,8 +7,13 @@ class HeredocDedentTest < TestCase
     filepath = File.expand_path("fixtures/tilde_heredocs.txt", __dir__)
 
     File.read(filepath).split(/(?=\n)\n(?=<)/).each_with_index do |heredoc, index|
+      # The first example in this file has incorrect dedent calculated by
+      # TruffleRuby so we skip it.
+      next if index == 0 && RUBY_ENGINE == "truffleruby"
+
       define_method "test_heredoc_#{index}" do
         node = Prism.parse(heredoc).value.statements.body.first
+
         if node.is_a?(StringNode)
           actual = node.unescaped
         else