Support getting files in the build tree from editable installs

src/scikit_build_core/resources/_editable_redirect.py

@@ -1,6 +1,10 @@
 from __future__ import annotations
 
 import importlib.abc
+import importlib.machinery
+
+# TODO: importlib.readers is Python version specific
+import importlib.readers  # type: ignore[import-not-found]
 import importlib.util
 import os
 import subprocess
@@ -11,6 +15,7 @@
 TYPE_CHECKING = False
 if TYPE_CHECKING:
     from importlib.machinery import ModuleSpec
+    from pathlib import Path
 
 
 DIR = os.path.abspath(os.path.dirname(__file__))
@@ -77,6 +82,27 @@ def release(self) -> None:
         os.close(self.lock_file_fd)
 
 
+# Note: This solution relies on importlib's call stack in Python 3.11. Python 3.9 looks
+# different, so might require a different solution, but I haven't gone deeper into that
+# yet since I don't have a solution for the 3.11 case yet anyway.
+class ScikitBuildRedirectingReader(importlib.readers.FileReader):  # type: ignore[misc]
+    def files(self) -> Path:
+        # ATTENTION: This is where the problem is. The expectation is that this returns
+        # a Traversable object. We could hack together an object that satisfies that
+        # API, but methods like `joinpath` don't have sensible implementations if
+        # `files` could return multiple paths instead of a single one. We could do some
+        # hackery to figure out which paths exist on the backend by hiding some internal
+        # representation that knows both possible roots and checks for existence when
+        # necessary, but that seriously violates the principle of least surprise for the
+        # user so I'd be quite skeptical.
+        return self.path  # type: ignore[no-any-return]
+
+
+class ScikitBuildRedirectingLoader(importlib.machinery.SourceFileLoader):
+    def get_resource_reader(self, module: str) -> ScikitBuildRedirectingReader:  # type: ignore[override]
+        return ScikitBuildRedirectingReader(self)
+
+
 class ScikitBuildRedirectingFinder(importlib.abc.MetaPathFinder):
     def __init__(
         self,
@@ -153,6 +179,9 @@ def find_spec(
                 submodule_search_locations=submodule_search_locations
                 if redir.endswith(("__init__.py", "__init__.pyc"))
                 else None,
+                loader=ScikitBuildRedirectingLoader(
+                    fullname, os.path.join(self.dir, redir)
+                ),
             )
         if fullname in self.known_source_files:
             redir = self.known_source_files[fullname]
@@ -162,6 +191,7 @@ def find_spec(
                 submodule_search_locations=submodule_search_locations
                 if redir.endswith(("__init__.py", "__init__.pyc"))
                 else None,
+                loader=ScikitBuildRedirectingLoader(fullname, redir),
             )
         return None