Two enhancements to tailor.

src/python/pants/core/goals/tailor.py

@@ -268,6 +268,10 @@ async def rename_conflicting_targets(ptgts: PutativeTargets) -> UniquelyNamedPut
     for ptgt in ptgts:
         idx = 0
         possibly_renamed_ptgt = ptgt
+        # Targets in root-level BUILD files must be named explicitly.
+        if possibly_renamed_ptgt.path == "" and possibly_renamed_ptgt.kwargs.get("name") is None:
+            possibly_renamed_ptgt = possibly_renamed_ptgt.rename("root")
+        # Eliminate any address collisions.
         while possibly_renamed_ptgt.address.spec in existing_addrs:
             possibly_renamed_ptgt = ptgt.rename(f"{ptgt.name}{idx}")
             idx += 1
@@ -342,6 +346,16 @@ def make_content_str(
 @rule(desc="Edit BUILD files with new targets")
 async def edit_build_files(req: EditBuildFilesRequest) -> EditedBuildFiles:
     ptgts_by_build_file = group_by_build_file(req.putative_targets)
+    # There may be an existing *directory* whose name collides with that of a BUILD file
+    # we want to create. This is more likely on a system with case-insensitive paths,
+    # such as MacOS. We detect such cases and use an alt BUILD file name to fix.
+    existing_paths = await Get(Paths, PathGlobs(ptgts_by_build_file.keys()))
+    existing_dirs = set(existing_paths.dirs)
+    # Technically there could be a dir named "BUILD.pants" as well, but that's pretty unlikely.
+    ptgts_by_build_file = {
+        (f"{bf}.pants" if bf in existing_dirs else bf): pts
+        for bf, pts in ptgts_by_build_file.items()
+    }
     existing_build_files_contents = await Get(DigestContents, PathGlobs(ptgts_by_build_file.keys()))
     existing_build_files_contents_by_path = {
         ebfc.path: ebfc.content for ebfc in existing_build_files_contents

src/python/pants/core/goals/tailor_test.py

@@ -141,6 +141,28 @@ def test_rename_conflicting_targets(rule_runner: RuleRunner) -> None:
     )
 
 
+def test_root_targets_are_explicitly_named(rule_runner: RuleRunner) -> None:
+    rule_runner.create_file("foo.f90", "")
+    ptgt = PutativeTarget("", "", "fortran_library", ["foo.f90"], FortranLibrarySources.default)
+    unpts = rule_runner.request(UniquelyNamedPutativeTargets, [PutativeTargets([ptgt])])
+    ptgts = unpts.putative_targets
+    assert (
+        PutativeTargets(
+            [
+                PutativeTarget(
+                    "",
+                    "root",
+                    "fortran_library",
+                    ["foo.f90"],
+                    FortranLibrarySources.default,
+                    kwargs={"name": "root"},
+                )
+            ]
+        )
+        == ptgts
+    )
+
+
 def test_restrict_conflicting_sources(rule_runner: RuleRunner) -> None:
     dir_structure = {
         "src/fortran/foo/BUILD": "fortran_library(sources=['bar/baz1.f90'])",
@@ -173,6 +195,7 @@ def test_restrict_conflicting_sources(rule_runner: RuleRunner) -> None:
 
 def test_edit_build_files(rule_runner: RuleRunner) -> None:
     rule_runner.create_file("src/fortran/foo/BUILD", 'fortran_library(sources=["bar1.f90"])')
+    rule_runner.create_dir("src/fortran/baz/BUILD")  # NB: A directory, not a file.
     req = EditBuildFilesRequest(
         PutativeTargets(
             [
@@ -200,12 +223,12 @@ def test_edit_build_files(rule_runner: RuleRunner) -> None:
     )
     edited_build_files = rule_runner.request(EditedBuildFiles, [req])
 
-    assert edited_build_files.created_paths == ("src/fortran/baz/BUILD",)
+    assert edited_build_files.created_paths == ("src/fortran/baz/BUILD.pants",)
     assert edited_build_files.updated_paths == ("src/fortran/foo/BUILD",)
 
     contents = rule_runner.request(DigestContents, [edited_build_files.digest])
     expected = [
-        FileContent("src/fortran/baz/BUILD", "fortran_library()\n".encode()),
+        FileContent("src/fortran/baz/BUILD.pants", "fortran_library()\n".encode()),
         FileContent(
             "src/fortran/foo/BUILD",
             textwrap.dedent(