Fix source root stripping to not strip requirements.txt files in plugins

src/python/pants/backend/python/target_types.py

@@ -641,7 +641,8 @@ def parse_requirements_file(content: str, *, rel_path: str) -> Iterator[Requirem
 
 
 class PythonRequirementsFileSources(Sources):
-    pass
+    required = True
+    uses_source_roots = False
 
 
 class PythonRequirementsFile(Target):

src/python/pants/core/target_types.py

@@ -40,6 +40,7 @@
 
 class FilesSources(Sources):
     required = True
+    uses_source_roots = False
 
 
 class Files(Target):

src/python/pants/core/util_rules/source_files.py

@@ -4,7 +4,6 @@
 from dataclasses import dataclass
 from typing import Iterable, Set, Tuple, Type
 
-from pants.core.target_types import FilesSources
 from pants.engine.fs import MergeDigests, Snapshot
 from pants.engine.rules import Get, MultiGet, collect_rules, rule
 from pants.engine.target import HydratedSources, HydrateSourcesRequest
@@ -63,7 +62,7 @@ async def determine_source_files(request: SourceFilesRequest) -> SourceFiles:
     )
 
     for hydrated_sources, sources_field in zip(all_hydrated_sources, request.sources_fields):
-        if isinstance(sources_field, FilesSources):
+        if not sources_field.uses_source_roots:
             unrooted_files.update(hydrated_sources.snapshot.files)
 
     digests_to_merge = tuple(

src/python/pants/core/util_rules/source_files_test.py

@@ -90,13 +90,23 @@ def test_address_specs(rule_runner: RuleRunner) -> None:
     )
 
 
-def test_file_sources(rule_runner: RuleRunner) -> None:
+def test_unrooted_sources(rule_runner: RuleRunner) -> None:
+    """Any Sources field with `uses_source_roots=False`, such as `FilesSources`, should be marked as
+    unrooted sources."""
     sources = TargetSources("src/python", ["README.md"])
     field = mock_sources_field(rule_runner, sources, sources_field_cls=FilesSources)
     assert_sources_resolved(
         rule_runner, [field], expected=[sources], expected_unrooted=sources.full_paths
     )
 
+    class CustomSources(SourcesField):
+        uses_source_roots = False
+
+    field = mock_sources_field(rule_runner, sources, sources_field_cls=CustomSources)
+    assert_sources_resolved(
+        rule_runner, [field], expected=[sources], expected_unrooted=sources.full_paths
+    )
+
 
 def test_gracefully_handle_no_sources(rule_runner: RuleRunner) -> None:
     sources_field = mock_sources_field(rule_runner, SOURCES1, include_sources=False)

src/python/pants/engine/target.py

@@ -1212,8 +1212,9 @@ def compute_value(
 
 class Sources(StringSequenceField, AsyncFieldMixin):
     alias = "sources"
-    expected_file_extensions: ClassVar[Optional[Tuple[str, ...]]] = None
-    expected_num_files: ClassVar[Optional[Union[int, range]]] = None
+    expected_file_extensions: ClassVar[tuple[str, ...] | None] = None
+    expected_num_files: ClassVar[int | range | None] = None
+    uses_source_roots: ClassVar[bool] = True
     help = (
         "A list of files and globs that belong to this target.\n\nPaths are relative to the BUILD "
         "file's directory. You can ignore files/globs by prefixing them with `!`.\n\nExample: "