Add tailor support for Scala (pantsbuild#13565)

Only creates `scala_sources` targets for now. Also moves `pants.backend.java.tailor` to `pants.backend.java.goals.tailor` to align with other backends.

Fixes pantsbuild#13529.

[ci skip-rust]
[ci skip-build-wheels]

src/python/pants/backend/experimental/java/register.py

@@ -1,7 +1,6 @@
 # Copyright 2021 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from pants.backend.java import tailor
 from pants.backend.java.compile import javac
 from pants.backend.java.dependency_inference import (
     import_parser,
@@ -10,7 +9,7 @@
     package_mapper,
 )
 from pants.backend.java.dependency_inference import rules as dependency_inference_rules
-from pants.backend.java.goals import check
+from pants.backend.java.goals import check, tailor
 from pants.backend.java.package import deploy_jar
 from pants.backend.java.target_types import (
     DeployJar,

src/python/pants/backend/experimental/scala/register.py

@@ -10,7 +10,7 @@
 from pants.backend.java.test import junit  # TODO: Should move to the JVM package.
 from pants.backend.scala.compile import scalac
 from pants.backend.scala.dependency_inference import scala_parser
-from pants.backend.scala.goals import check
+from pants.backend.scala.goals import check, tailor
 from pants.backend.scala.target_types import ScalaSourcesGeneratorTarget, ScalaSourceTarget
 from pants.backend.scala.target_types import rules as target_types_rules
 from pants.jvm import classpath, jdk_rules
@@ -36,6 +36,7 @@ def rules():
     return [
         *scalac.rules(),
         *check.rules(),
+        *tailor.rules(),
         *classpath.rules(),
         *junit.rules(),
         *deploy_jar.rules(),

src/python/pants/backend/java/BUILD

@@ -2,5 +2,3 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 python_sources()
-
-python_tests(name="tests")

src/python/pants/backend/java/goals/BUILD

@@ -2,3 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 python_sources()
+
+python_tests(
+    name="tests",
+)

src/python/pants/backend/java/tailor_test.py → src/python/pants/backend/java/goals/tailor_test.py

@@ -3,8 +3,8 @@
 
 import pytest
 
-from pants.backend.java import tailor
-from pants.backend.java.tailor import PutativeJavaTargetsRequest, classify_source_files
+from pants.backend.java.goals import tailor
+from pants.backend.java.goals.tailor import PutativeJavaTargetsRequest, classify_source_files
 from pants.backend.java.target_types import JavaSourcesGeneratorTarget, JunitTestsGeneratorTarget
 from pants.core.goals.tailor import (
     AllOwnedSources,

src/python/pants/backend/scala/goals/tailor.py

@@ -0,0 +1,65 @@
+# Copyright 2021 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import annotations
+
+import os
+from dataclasses import dataclass
+from typing import Iterable
+
+from pants.backend.scala.target_types import ScalaSourcesGeneratorTarget
+from pants.core.goals.tailor import (
+    AllOwnedSources,
+    PutativeTarget,
+    PutativeTargets,
+    PutativeTargetsRequest,
+    group_by_dir,
+)
+from pants.engine.fs import PathGlobs, Paths
+from pants.engine.internals.selectors import Get
+from pants.engine.rules import collect_rules, rule
+from pants.engine.target import Target
+from pants.engine.unions import UnionRule
+from pants.util.logging import LogLevel
+
+
+@dataclass(frozen=True)
+class PutativeScalaTargetsRequest(PutativeTargetsRequest):
+    pass
+
+
+def classify_source_files(paths: Iterable[str]) -> dict[type[Target], set[str]]:
+    """Returns a dict of target type -> files that belong to targets of that type."""
+    # TODO: Until https://github.com/pantsbuild/pants/issues/13332 is fixed, we classify all scala
+    # files as `scala_sources`, and none as tests.
+    return {ScalaSourcesGeneratorTarget: set(paths)}
+
+
+@rule(level=LogLevel.DEBUG, desc="Determine candidate Scala targets to create")
+async def find_putative_targets(
+    req: PutativeScalaTargetsRequest,
+    all_owned_sources: AllOwnedSources,
+) -> PutativeTargets:
+    all_scala_files_globs = req.search_paths.path_globs("*.scala")
+    all_scala_files = await Get(Paths, PathGlobs, all_scala_files_globs)
+    unowned_scala_files = set(all_scala_files.files) - set(all_owned_sources)
+    classified_unowned_scala_files = classify_source_files(unowned_scala_files)
+
+    putative_targets = []
+    for tgt_type, paths in classified_unowned_scala_files.items():
+        for dirname, filenames in group_by_dir(paths).items():
+            name = os.path.basename(dirname)
+            putative_targets.append(
+                PutativeTarget.for_target_type(
+                    tgt_type, dirname, name, sorted(filenames), kwargs={}
+                )
+            )
+
+    return PutativeTargets(putative_targets)
+
+
+def rules():
+    return [
+        *collect_rules(),
+        UnionRule(PutativeTargetsRequest, PutativeScalaTargetsRequest),
+    ]