From a6fa78881949753f351716ed84815edade406a7f Mon Sep 17 00:00:00 2001
From: finswimmer <finswimmer77@gmail.com>
Date: Wed, 22 Jan 2025 06:50:53 +0100
Subject: [PATCH] feat(cli): fallback to build-in poetry-core on build

---
 src/poetry/console/commands/build.py               | 12 ++++++++++++
 tests/console/commands/test_build.py               |  2 ++
 .../build_systems/no_build_backend/README.md       |  2 ++
 .../build_systems/no_build_backend/pyproject.toml  | 14 ++++++++++++++
 .../no_build_backend/simple_project/__init__.py    |  0
 .../build_systems/no_build_system/README.md        |  2 ++
 .../build_systems/no_build_system/pyproject.toml   | 11 +++++++++++
 .../no_build_system/simple_project/__init__.py     |  0
 8 files changed, 43 insertions(+)
 create mode 100644 tests/fixtures/build_systems/no_build_backend/README.md
 create mode 100644 tests/fixtures/build_systems/no_build_backend/pyproject.toml
 create mode 100644 tests/fixtures/build_systems/no_build_backend/simple_project/__init__.py
 create mode 100644 tests/fixtures/build_systems/no_build_system/README.md
 create mode 100644 tests/fixtures/build_systems/no_build_system/pyproject.toml
 create mode 100644 tests/fixtures/build_systems/no_build_system/simple_project/__init__.py

diff --git a/src/poetry/console/commands/build.py b/src/poetry/console/commands/build.py
index 0bc293b1e16..7d47a87a7a0 100644
--- a/src/poetry/console/commands/build.py
+++ b/src/poetry/console/commands/build.py
@@ -91,6 +91,15 @@ def _requires_isolated_build(self) -> bool:
 
         :returns: True if an isolated build is required, False otherwise.
         """
+        if not self._has_build_backend_defined():
+            self.io.write_error_line(
+                "<warning><b>WARNING</>: No build backend defined. Please define one in the <c1>pyproject.toml</>.\n"
+                "Falling back to using the built-in `poetry-core` version.\n"
+                "In a future release Poetry will fallback to `setuptools` as defined by PEP 517.\n"
+                "More details can be found at https://python-poetry.org/docs/libraries/#packaging</>"
+            )
+            return False
+
         if (
             self.poetry.package.build_script
             or len(self.poetry.build_system_dependencies) != 1
@@ -114,6 +123,9 @@ def _get_builder(self) -> Callable[..., None]:
 
         return self._build
 
+    def _has_build_backend_defined(self) -> bool:
+        return "build-backend" in self.poetry.pyproject.data.get("build-system", {})
+
     def build(self, options: BuildOptions) -> int:
         if not self.poetry.is_package_mode:
             self.io.write_error_line(
diff --git a/tests/console/commands/test_build.py b/tests/console/commands/test_build.py
index 18b57a3bf25..babba7421fe 100644
--- a/tests/console/commands/test_build.py
+++ b/tests/console/commands/test_build.py
@@ -269,6 +269,8 @@ def test_prepare_formats(fmt: str | None, expected_formats: list[str]) -> None:
         ("multiple_build_deps", True),
         ("no_core", True),
         ("core_from_git", True),
+        ("no_build_system", False),
+        ("no_build_backend", False),
     ],
 )
 def test_requires_isolated_build(
diff --git a/tests/fixtures/build_systems/no_build_backend/README.md b/tests/fixtures/build_systems/no_build_backend/README.md
new file mode 100644
index 00000000000..f7fe15470f9
--- /dev/null
+++ b/tests/fixtures/build_systems/no_build_backend/README.md
@@ -0,0 +1,2 @@
+My Package
+==========
diff --git a/tests/fixtures/build_systems/no_build_backend/pyproject.toml b/tests/fixtures/build_systems/no_build_backend/pyproject.toml
new file mode 100644
index 00000000000..ce7a0d64c46
--- /dev/null
+++ b/tests/fixtures/build_systems/no_build_backend/pyproject.toml
@@ -0,0 +1,14 @@
+[project]
+name = "simple-project"
+version = "1.2.3"
+description = "Some description."
+authors = [
+    { name = "Poetry Contributors", email = "no-reply@python-poetry.org" }
+]
+license = { text = "MIT" }
+readme = "README.md"
+keywords = ["packaging", "dependency", "poetry"]
+requires-python = ">=3.4"
+
+[build-system]
+requires = ["poetry-core>=1.1.0a7"]
diff --git a/tests/fixtures/build_systems/no_build_backend/simple_project/__init__.py b/tests/fixtures/build_systems/no_build_backend/simple_project/__init__.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/fixtures/build_systems/no_build_system/README.md b/tests/fixtures/build_systems/no_build_system/README.md
new file mode 100644
index 00000000000..f7fe15470f9
--- /dev/null
+++ b/tests/fixtures/build_systems/no_build_system/README.md
@@ -0,0 +1,2 @@
+My Package
+==========
diff --git a/tests/fixtures/build_systems/no_build_system/pyproject.toml b/tests/fixtures/build_systems/no_build_system/pyproject.toml
new file mode 100644
index 00000000000..6c3dd468b9f
--- /dev/null
+++ b/tests/fixtures/build_systems/no_build_system/pyproject.toml
@@ -0,0 +1,11 @@
+[project]
+name = "simple-project"
+version = "1.2.3"
+description = "Some description."
+authors = [
+    { name = "Poetry Contributors", email = "no-reply@python-poetry.org" }
+]
+license = { text = "MIT" }
+readme = "README.md"
+keywords = ["packaging", "dependency", "poetry"]
+requires-python = ">=3.4"
diff --git a/tests/fixtures/build_systems/no_build_system/simple_project/__init__.py b/tests/fixtures/build_systems/no_build_system/simple_project/__init__.py
new file mode 100644
index 00000000000..e69de29bb2d