Parse pyproject.toml statically where possible

piptools/build.py

@@ -14,6 +14,13 @@
 import pyproject_hooks
 from pip._internal.req import InstallRequirement
 from pip._internal.req.constructors import install_req_from_line, parse_req_from_line
+from pip._vendor.packaging.markers import Marker
+from pip._vendor.packaging.requirements import Requirement
+
+if sys.version_info >= (3, 11):
+    import tomllib
+else:
+    import tomli as tomllib
 
 PYPROJECT_TOML = "pyproject.toml"
 
@@ -41,6 +48,67 @@ class ProjectMetadata:
     build_requirements: tuple[InstallRequirement, ...]
 
 
+def maybe_statically_parse_project_metadata(
+    src_file: pathlib.Path,
+) -> ProjectMetadata | None:
+    """
+    Return the metadata for a project, if it can be statically parsed from pyproject.toml.
+
+    This function is typically significantly faster than invoking a build backend.
+    Returns None if the project metadata cannot be statically parsed.
+    """
+    if src_file.name != PYPROJECT_TOML:
+        return None
+
+    try:
+        with open(src_file, "rb") as f:
+            pyproject_contents = tomllib.load(f)
+    except tomllib.TOMLDecodeError:
+        pyproject_contents = {}
+
+    # Not valid PEP 621 metadata
+    if (
+        "project" not in pyproject_contents
+        or "name" not in pyproject_contents["project"]
+    ):
+        return None
+
+    # Dynamic dependencies require build invocation
+    dynamic = pyproject_contents["project"].get("dynamic", [])
+    if "dependencies" in dynamic or "optional-dependencies" in dynamic:
+        return None
+
+    package_name = pyproject_contents["project"]["name"]
+    comes_from = f"{package_name} ({src_file})"
+
+    extras = pyproject_contents["project"].get("optional-dependencies", {}).keys()
+    install_requirements = [
+        InstallRequirement(Requirement(req), comes_from)
+        for req in pyproject_contents["project"].get("dependencies", [])
+    ]
+    for extra, reqs in (
+        pyproject_contents.get("project", {}).get("optional-dependencies", {}).items()
+    ):
+        for req in reqs:
+            requirement = Requirement(req)
+            # Note we don't need to modify `requirement` to include this extra
+            marker = Marker(f"extra == '{extra}'")
+            install_requirements.append(
+                InstallRequirement(requirement, comes_from, markers=marker)
+            )
+
+    comes_from = f"{package_name} ({src_file}::build-system.requires)"
+    build_requirements = [
+        InstallRequirement(Requirement(req), comes_from)
+        for req in pyproject_contents.get("build-system", {}).get("requires", [])
+    ]
+    return ProjectMetadata(
+        extras=tuple(extras),
+        requirements=tuple(install_requirements),
+        build_requirements=tuple(build_requirements),
+    )
+
+
 def build_project_metadata(
     src_file: pathlib.Path,
     build_targets: tuple[str, ...],
@@ -51,6 +119,8 @@ def build_project_metadata(
     """
     Return the metadata for a project.
 
+    Attempts to determine the metadata statically from the pyproject.toml file.
+
     Uses the ``prepare_metadata_for_build_wheel`` hook for the wheel metadata
     if available, otherwise ``build_wheel``.
 
@@ -66,6 +136,10 @@ def build_project_metadata(
     :param quiet: Whether to suppress the output of subprocesses.
     """
 
+    project_metadata = maybe_statically_parse_project_metadata(src_file)
+    if project_metadata is not None:
+        return project_metadata
+
     src_dir = src_file.parent
     with _create_project_builder(src_dir, isolated=isolated, quiet=quiet) as builder:
         metadata = _build_project_wheel_metadata(builder)