Disable 80 col hard wrapping in mypy output

src/python/pants/backend/python/typecheck/mypy/rules.py

@@ -319,6 +319,11 @@ async def mypy_typecheck_partition(
     env = {
         "PEX_EXTRA_SYS_PATH": ":".join(all_used_source_roots),
         "MYPYPATH": ":".join(all_used_source_roots),
+        # Similarly, force a fixed terminal width. This is effectively infinite, disabling mypy's
+        # builtin truncation and line wrapping. Terminals do an acceptable job of soft-wrapping
+        # diagnostic text and source code is typically already hard-wrapped to a limited width.
+        # (Unique random number to make it easier to search for the source of this setting.)
+        "MYPY_FORCE_TERMINAL_WIDTH": "642092230765939",
     }
 
     process = await Get(

src/python/pants/backend/python/typecheck/mypy/rules_integration_test.py

@@ -3,6 +3,7 @@
 
 from __future__ import annotations
 
+import re
 from textwrap import dedent
 
 import pytest
@@ -872,3 +873,31 @@ def test_determine_python_files() -> None:
     assert determine_python_files(["f.py", "f.pyi"]) == ("f.pyi",)
     assert determine_python_files(["f.pyi", "f.py"]) == ("f.pyi",)
     assert determine_python_files(["f.json"]) == ()
+
+
+def test_colors_and_formatting(rule_runner: RuleRunner) -> None:
+    rule_runner.write_files(
+        {
+            f"{PACKAGE}/f.py": dedent(
+                """\
+                class incredibly_long_type_name_to_force_wrapping_if_mypy_wrapped_error_messages_12345678901234567890123456789012345678901234567890:
+                    pass
+
+                x = incredibly_long_type_name_to_force_wrapping_if_mypy_wrapped_error_messages_12345678901234567890123456789012345678901234567890()
+                x.incredibly_long_attribute_name_to_force_wrapping_if_mypy_wrapped_error_messages_12345678901234567890123456789012345678901234567890
+                """
+            ),
+            f"{PACKAGE}/BUILD": "python_sources()",
+        }
+    )
+    tgt = rule_runner.get_target(Address(PACKAGE, relative_file_path="f.py"))
+
+    result = run_mypy(rule_runner, [tgt], extra_args=["--colors=true", "--mypy-args=--pretty"])
+
+    assert len(result) == 1
+    assert result[0].exit_code == 1
+    # all one line
+    assert re.search(
+        "error:.*incredibly_long_type_name.*incredibly_long_attribute_name", result[0].stdout
+    )
+    assert result[0].report == EMPTY_DIGEST