Add build_root_singleton rule

src/python/pants/init/BUILD

@@ -11,6 +11,7 @@ python_library(
     '3rdparty/python/twitter/commons:twitter.common.collections',
     'src/python/pants/backend/python/subsystems',
     'src/python/pants/base:build_environment',
+    'src/python/pants/base:build_root',
     'src/python/pants/base:exception_sink',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:target_roots',

src/python/pants/init/engine_initializer.py

@@ -13,6 +13,7 @@
 from pants.backend.python.targets.python_library import PythonLibrary
 from pants.backend.python.targets.python_tests import PythonTests
 from pants.base.build_environment import get_buildroot
+from pants.base.build_root import BuildRoot
 from pants.base.exiter import PANTS_SUCCEEDED_EXIT_CODE
 from pants.base.file_system_project_tree import FileSystemProjectTree
 from pants.build_graph.build_configuration import BuildConfiguration
@@ -364,7 +365,11 @@ def symbol_table_singleton() -> SymbolTable:
 
     @rule
     def union_membership_singleton() -> UnionMembership:
-      return UnionMembership(build_configuration.union_rules())
+      return UnionMembership(build_configuration.union_rules())\
+
+    @rule
+    def build_root_singleton() -> BuildRoot:
+      return BuildRoot.instance
 
     # Create a Scheduler containing graph and filesystem rules, with no installed goals. The
     # LegacyBuildGraph will explicitly request the products it needs.
@@ -375,6 +380,7 @@ def union_membership_singleton() -> UnionMembership:
         build_configuration_singleton,
         symbol_table_singleton,
         union_membership_singleton,
+        build_root_singleton,
       ] +
       create_legacy_graph_tasks() +
       create_fs_rules() +

src/python/pants/rules/core/BUILD

@@ -1,6 +1,7 @@
 python_library(
     dependencies = [
       '3rdparty/python:dataclasses',
+      'src/python/pants/base:build_root',
       'src/python/pants/base:exiter',
       'src/python/pants/build_graph',
       'src/python/pants/engine:goal',

src/python/pants/rules/core/filedeps.py

@@ -3,7 +3,7 @@
 
 from pathlib import Path
 
-from pants.base.build_environment import get_buildroot
+from pants.base.build_root import BuildRoot
 from pants.engine.console import Console
 from pants.engine.goal import Goal, LineOriented
 from pants.engine.legacy.graph import TransitiveHydratedTargets
@@ -36,14 +36,14 @@ def register_options(cls, register):
 def file_deps(
   console: Console,
   filedeps_options: Filedeps.Options,
+  build_root: BuildRoot,
   transitive_hydrated_targets: TransitiveHydratedTargets
 ) -> Filedeps:
 
   absolute = filedeps_options.values.absolute
   globs = filedeps_options.values.globs
 
   unique_rel_paths = set()
-  build_root = get_buildroot()
 
   for hydrated_target in transitive_hydrated_targets.closure:
     if hydrated_target.address.rel_path:
@@ -56,7 +56,7 @@ def file_deps(
 
   with Filedeps.line_oriented(filedeps_options, console) as print_stdout:
     for rel_path in sorted(unique_rel_paths):
-      final_path = str(Path(build_root, rel_path)) if absolute else rel_path
+      final_path = str(Path(build_root.path, rel_path)) if absolute else rel_path
       print_stdout(final_path)
 
   return Filedeps(exit_code=0)