Install static deps before requesting dynamic deps

piptools/scripts/compile.py

@@ -12,6 +12,7 @@
 import click
 import pyproject_hooks
 from build import BuildBackendException, ProjectBuilder
+from build.env import IsolatedEnvBuilder
 from build.util import project_wheel_metadata
 from click.utils import LazyFile, safecall
 from pip._internal.commands import create_command
@@ -53,18 +54,24 @@
 def _build_requirements(
     src_dir: str, src_file: str, distributions: Iterable[str], package_name: str
 ) -> list[InstallRequirement]:
-    builder = ProjectBuilder(src_dir, runner=pyproject_hooks.quiet_subprocess_runner)
-    # It is not clear that it should be possible to use `get_requires_for_build` with
-    # "editable" but it seems to work in practice.
     result = collections.defaultdict(set)
-    for req in builder.build_system_requires:
-        result[req].add(f"{package_name} ({src_file}::build-system.requires)")
-        # TODO: Follow PEP and install deps
-    for dist in distributions:
-        for req in builder.get_requires_for_build(dist):
-            result[req].add(
-                f"{package_name} ({src_file}::build-system.backend::{dist})"
-            )
+
+    with IsolatedEnvBuilder() as env:
+        builder = ProjectBuilder(
+            srcdir=src_dir,
+            python_executable=env.executable,
+            scripts_dir=env.scripts_dir,
+            runner=pyproject_hooks.quiet_subprocess_runner,
+        )
+        env.install(builder.build_system_requires)
+
+        for req in builder.build_system_requires:
+            result[req].add(f"{package_name} ({src_file}::build-system.requires)")
+        for dist in distributions:
+            for req in builder.get_requires_for_build(dist):
+                result[req].add(
+                    f"{package_name} ({src_file}::build-system.backend::{dist})"
+                )
 
     return [
         install_req_from_line(k, comes_from=v) for k, vs in result.items() for v in vs

tests/conftest.py

@@ -319,6 +319,7 @@ def _make_package(name, version="0.1", install_requires=None, extras_require=Non
                         url="https://github.com/jazzband/pip-tools",
                         install_requires={install_requires_str},
                         extras_require={extras_require},
+                        py_modules=[{name!r}],
                     )
                     """
                 )
@@ -327,6 +328,9 @@ def _make_package(name, version="0.1", install_requires=None, extras_require=Non
         # Create a README to avoid setuptools warnings.
         (package_dir / "README").touch()
 
+        # Create a module to make the package importable.
+        (package_dir / name).with_suffix(".py").touch()
+
         return package_dir
 
     return _make_package

tests/test_data/packages/small_fake_with_build_deps/backend/backend.py

@@ -1,15 +1,13 @@
 from __future__ import annotations
 
+# A dependency of the build backend that is not installed is equivalent to a build
+# backend that is not installed so we don't have to test both cases.
+import fake_static_build_dep
 import setuptools.build_meta
 
 # Re-export all names in case more hooks are added in the future
 from setuptools.build_meta import *  # noqa: F401, F403
 
-# FIXME: install static build deps somehow
-# Here the backend imports another dep but the same problem applies if the backend
-# itself is not installed in the environment where pip-compile runs.
-# import fake_static_build_dep
-
 build_wheel = setuptools.build_meta.build_wheel
 build_sdist = setuptools.build_meta.build_sdist