Improve diagnostics for unclosed heredocs (#13182)

Author: viniciusmuller

lib/elixir/lib/exception.ex

@@ -794,6 +794,15 @@ defmodule Exception do
     end
   end
 
+  @doc false
+  def format_expected_delimiter(opening_delimiter) do
+    terminator = :elixir_tokenizer.terminator(opening_delimiter)
+
+    if terminator |> Atom.to_string() |> String.contains?(["\"", "'"]),
+      do: terminator,
+      else: ~s("#{terminator}")
+  end
+
   @doc false
   def format_snippet(
         {start_line, _start_column} = start_pos,
@@ -1183,10 +1192,10 @@ defmodule MismatchedDelimiterError do
     start_pos = {start_line, start_column}
     end_pos = {end_line, end_column}
     lines = String.split(snippet, "\n")
-    expected_delimiter = :elixir_tokenizer.terminator(opening_delimiter)
+    expected_delimiter = Exception.format_expected_delimiter(opening_delimiter)
 
     start_message = "└ unclosed delimiter"
-    end_message = ~s/└ mismatched closing delimiter (expected "#{expected_delimiter}")/
+    end_message = ~s/└ mismatched closing delimiter (expected #{expected_delimiter})/
 
     snippet =
       Exception.format_snippet(
@@ -1306,10 +1315,10 @@ defmodule TokenMissingError do
 
     start_pos = {line, column}
     end_pos = {end_line, end_column}
-    expected_delimiter = :elixir_tokenizer.terminator(opening_delimiter)
+    expected_delimiter = Exception.format_expected_delimiter(opening_delimiter)
 
     start_message = ~s/└ unclosed delimiter/
-    end_message = ~s/└ missing closing delimiter (expected "#{expected_delimiter}")/
+    end_message = ~s/└ missing closing delimiter (expected #{expected_delimiter})/
 
     snippet =
       Exception.format_snippet(

lib/elixir/src/elixir_tokenizer.erl

@@ -1030,7 +1030,16 @@ extract_heredoc_with_interpolation(Line, Column, Scope, Interpol, T, H) ->
           {ok, NewLine, NewColumn, tokens_to_binary(Parts2), Rest, NewScope};
 
         {error, Reason} ->
-          {error, interpolation_format(Reason, " (for heredoc starting at line ~B)", [Line])}
+          {Position, Message, List} = interpolation_format(Reason, " (for heredoc starting at line ~B)", [Line]),
+          {line, EndLine} = lists:keyfind(line, 1, Position),
+           Meta = [
+             {error_type, unclosed_delimiter},
+             {opening_delimiter, '"""'},
+             {line, Line},
+             {column, Column},
+             {end_line, EndLine}
+          ],
+          {error, {Meta, Message, List}}
       end;
 
     error ->
@@ -1481,6 +1490,7 @@ terminator('do') -> 'end';
 terminator('(')  -> ')';
 terminator('[')  -> ']';
 terminator('{')  -> '}';
+terminator('"""') -> '"""';
 terminator('<<') -> '>>'.
 
 %% Keywords checking

lib/elixir/test/elixir/kernel/diagnostics_test.exs

@@ -361,6 +361,33 @@ defmodule Kernel.DiagnosticsTest do
              """
     end
 
+    test "missing heredoc terminator" do
+      output =
+        capture_raise(
+          """
+          a = \"""
+          test string
+
+          IO.inspect(10 + 20)
+          """,
+          TokenMissingError
+        )
+
+      assert output == """
+             ** (TokenMissingError) token missing on nofile:4:20:
+                 error: missing terminator: \""" (for heredoc starting at line 1)
+                 │
+               1 │ a = \"""
+                 │     └ unclosed delimiter
+               2 │ test string
+               3 │ 
+               4 │ IO.inspect(10 + 20)
+                 │                    └ missing closing delimiter (expected \""")
+                 │
+                 └─ nofile:4:20\
+             """
+    end
+
     test "shows in between lines if EOL is not far below" do
       output =
         capture_raise(

lib/elixir/test/elixir/kernel/parser_test.exs

@@ -528,7 +528,7 @@ defmodule Kernel.ParserTest do
     test "heredoc with incomplete interpolation" do
       assert_token_missing(
         [
-          "nofile:2:1:",
+          "nofile:1:4:",
           ~s/missing interpolation terminator: "}" (for heredoc starting at line 1)/
         ],
         ~c"\"\"\"\n\#{\n"