[truncate_explanation] Cut short by checking if we're going to truncate

Better check the theory and the len of the string to know if we're going
to truncate rather than checking the full length of the result after the
fact.

Author: Pierre-Sassoulas

src/_pytest/assertion/truncate.py

@@ -66,8 +66,6 @@ def _truncate_explanation(
     When this function is launched we know max_lines > 0 or max_chars > 0
     because _get_truncation_parameters was called first.
     """
-    # Check if truncation required
-    input_char_count = sum(len(line) for line in input_lines)
     # The length of the truncation explanation depends on the number of lines
     # removed but is at least 68 characters:
     # The real value is
@@ -82,11 +80,11 @@ def _truncate_explanation(
     tolerable_max_chars = (
         max_chars + 70  # 64 + 1 (for plural) + 2 (for '99') + 3 for '...'
     )
-    # The truncation explanation add two lines to the output
-    tolerable_max_lines = max_lines + 2
     if (
-        len(input_lines) <= tolerable_max_lines
-        and input_char_count <= tolerable_max_chars
+        # The truncation explanation add two lines to the output
+        max_lines == 0 or len(input_lines) <= max_lines + 2
+    ) and (
+        max_chars == 0 or sum(len(line) for line in input_lines) <= tolerable_max_chars
     ):
         return input_lines
     # Truncate first to max_lines, and then truncate to max_chars if necessary
@@ -103,11 +101,6 @@ def _truncate_explanation(
         truncated_explanation = _truncate_by_char_count(
             truncated_explanation, max_chars
         )
-
-    if truncated_explanation == input_lines:
-        # No truncation happened, so we do not need to add any explanations
-        return truncated_explanation
-
     # Something was truncated, adding '...' at the end to show that
     truncated_explanation[-1] += "..."
     truncated_line_count = (