Disable 80 col hard wrapping in mypy output (#16488)

This fixes part of #16451 by ensuring that the mypy output isn't truncated/hard-wrapped at all. In particular: the terminal width is forcibly set to be very wide ("infinite") because soft-wrapped display in a wide terminal is nicer than it being hard-wrapped/truncated to 80 cols.

This seems to only be possible via an undocumented environment variable: `MYPY_FORCE_TERMINAL_WIDTH`: https://github.com/python/mypy/blob/03638dd670373db0b8f00cc3bcec256d09729d06/mypy/util.py#L468

[ci skip-rust]

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),
+ # 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