Use pants.toml as default config if it exists

Without this change, users would need to explicitly set `--pants-config-files=pants.toml` every time.

If `pants.toml` exists, we will ignore the default of `pants.ini` unless the user explicitly adds `pants.ini` via `--pants-config-files`.

src/python/pants/base/build_environment.py

@@ -3,6 +3,7 @@
 
 import logging
 import os
+from pathlib import Path
 from typing import Optional
 
 from pants.base.build_root import BuildRoot
@@ -51,7 +52,10 @@ def get_pants_configdir() -> str:
 
 
 def get_default_pants_config_file() -> str:
-  """Return the default location of the pants config file."""
+  """Return the default location of the Pants config file."""
+  default_toml = Path(get_buildroot(), "pants.toml")
+  if default_toml.is_file():
+    return str(default_toml)
   return os.path.join(get_buildroot(), 'pants.ini')
 
 

src/python/pants/option/options_bootstrapper.py

@@ -7,6 +7,7 @@
 import stat
 import sys
 from dataclasses import dataclass
+from pathlib import Path
 from typing import Dict, Iterable, List, Mapping, Optional, Sequence, Set, Tuple, Type
 
 from pants.base.build_environment import get_default_pants_config_file
@@ -53,8 +54,9 @@ def get_config_file_paths(env, args) -> List[str]:
     evars = ['PANTS_GLOBAL_PANTS_CONFIG_FILES', 'PANTS_PANTS_CONFIG_FILES', 'PANTS_CONFIG_FILES']
 
     path_list_values = []
-    if os.path.isfile(get_default_pants_config_file()):
-      path_list_values.append(ListValueComponent.create(get_default_pants_config_file()))
+    default = get_default_pants_config_file()
+    if Path(default).is_file():
+      path_list_values.append(ListValueComponent.create(default))
     for var in evars:
       if var in env:
         path_list_values.append(ListValueComponent.create(env[var]))