fix: Fix rendering of line breaks in diagnostics (#819)

Fixes #818

guppylang/diagnostic.py

@@ -233,9 +233,8 @@ def render_diagnostic(self, diag: Diagnostic) -> None:
             # Omit the title if we don't have a span, but a long message. This case
             # should be fairly rare.
             msg = diag.rendered_message or diag.rendered_title
-            self.buffer += textwrap.wrap(
-                f"{self.level_str(diag.level)}: {msg}",
-                self.MAX_MESSAGE_LINE_LEN,
+            self.buffer += wrap(
+                f"{self.level_str(diag.level)}: {msg}", self.MAX_MESSAGE_LINE_LEN
             )
         else:
             span = to_span(diag.span)
@@ -263,16 +262,12 @@ def render_diagnostic(self, diag: Diagnostic) -> None:
                     )
             if diag.rendered_message:
                 self.buffer.append("")
-                self.buffer += textwrap.wrap(
-                    diag.rendered_message,
-                    self.MAX_MESSAGE_LINE_LEN,
-                    replace_whitespace=False,  # Keep \n's in the message
-                )
+                self.buffer += wrap(diag.rendered_message, self.MAX_MESSAGE_LINE_LEN)
         # Finally, render all sub-diagnostics that have a non-span message
         for sub_diag in diag.children:
             if sub_diag.rendered_message:
                 self.buffer.append("")
-                self.buffer += textwrap.wrap(
+                self.buffer += wrap(
                     f"{self.level_str(sub_diag.level)}: {sub_diag.rendered_message}",
                     self.MAX_MESSAGE_LINE_LEN,
                 )
@@ -368,7 +363,7 @@ def render_line(line: str, line_number: int | None = None) -> None:
 
         # Render the label next to the highlight
         if label:
-            [label_first, *label_rest] = textwrap.wrap(
+            [label_first, *label_rest] = wrap(
                 label,
                 self.MAX_LABEL_LINE_LEN,
                 # One space after the last `^`
@@ -386,3 +381,24 @@ def render_line(line: str, line_number: int | None = None) -> None:
     def level_str(level: DiagnosticLevel) -> str:
         """Returns the text used to identify the different kinds of diagnostics."""
         return level.name.lower().capitalize()
+
+
+def wrap(text: str, width: int, **kwargs: Any) -> list[str]:
+    """Custom version of `textwrap.wrap` that correctly handles text with line breaks.
+
+    Even with `replace_whitespace=False`, the original version doesn't count line breaks
+    as new paragraphs and instead would produce bad results like
+
+    ```
+    short paragraph of text
+
+    long paragraph of text xxxxx                             # broken too early :(
+    xxxx xxxxxx xxxx xxxxx xxxxxxxx xxxxxxxxxxxx xxxxx xx
+    xxxx xxxxxxx xxxxx xx xxxx xxxxxxx xxxx
+    ```
+    """
+    return [
+        line
+        for paragraph in text.splitlines()
+        for line in (textwrap.wrap(paragraph, width, **kwargs) if paragraph else [""])
+    ]

tests/diagnostics/snapshots/test_message_new_line.txt

@@ -0,0 +1,6 @@
+Error: Apple Apple Apple Apple Apple Apple Apple Apple Apple Apple Apple Apple
+Apple Apple Apple Apple Apple Apple Apple Apple
+
+Orange Orange Orange Orange Orange Orange Orange Orange Orange Orange Orange
+Orange Orange Orange Orange Orange Orange Orange Orange Orange Orange Orange
+Orange Orange Orange Orange Orange Orange Orange Orange

tests/diagnostics/test_diagnostics_rendering.py

@@ -212,3 +212,12 @@ def test_three_line_span(snapshot, request):
     span = Span(Loc(file, 1, 5), Loc(file, 3, 1))
     diagnostic = MyError(span)
     run_test(source, diagnostic, snapshot, request)
+
+
+def test_message_new_line(snapshot, request):
+    @dataclass(frozen=True)
+    class MyError(Error):
+        message: ClassVar[str] = "Apple " * 20 + "\n\n" + "Orange " * 30
+
+    diagnostic = MyError(None)
+    run_test("", diagnostic, snapshot, request)

tests/error/py_errors/python_err.err

@@ -8,8 +8,7 @@ Error: Python error (at $FILE:6:14)
 Traceback printed below:
 
 Traceback (most recent call last):
-  File "<string>",
-line 1, in <module>
+  File "<string>", line 1, in <module>
 ZeroDivisionError: division by zero
 
 Guppy compilation failed due to 1 previous error