feat(cli): fallback to build-in poetry-core on build

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>WARNING: No build backend defined. Please define one in the pyproject.toml.\n"
+                "Fallback to the build-in `poetry-core` version.\n"
+                "This warning will turn into an error in the future.\n"
+                "More details can be found at https://python-poetry.org/docs/main/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(

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(

tests/fixtures/build_systems/no_build_backend/README.md

@@ -0,0 +1,2 @@
+My Package
+==========

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 = "[email protected]" }
+]
+license = { text = "MIT" }
+readme = "README.md"
+keywords = ["packaging", "dependency", "poetry"]
+requires-python = ">=3.4"
+
+[build-system]
+requires = ["poetry-core>=1.1.0a7"]

tests/fixtures/build_systems/no_build_system/README.md

@@ -0,0 +1,2 @@
+My Package
+==========

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 = "[email protected]" }
+]
+license = { text = "MIT" }
+readme = "README.md"
+keywords = ["packaging", "dependency", "poetry"]
+requires-python = ">=3.4"