`poetry install` is unusable when `tool.poetry.build` is used

• I am on the latest Poetry version.

• I have searched the issues of this repo and believe that this is not a duplicate.

• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Fedora 33

• Poetry version: 1.1.4

• Contents of your pyproject.toml file:

[build-system]
requires = ["poetry-core>=1.0.0", "Cython>=0.29"]
build-backend = "poetry.core.masonry.api"

[tool.poetry]
[...]
build = "build.py"

where build.py contains something like this:

from distutils.command.build_ext import build_ext

def get_modules():
    # Some build logic (eg. cython)
    pass

def build(setup_kwargs):
    setup_kwargs.update({
        'ext_modules': get_modules(),
        'cmdclass': {'build_ext': build_ext}
    })

Issue

When running poetry install, poetry doesn't run the build script to install its output, resulting in missing files and a broken installed package 😢

Workaround

poetry build -f wheel
poetry run pip install dist/*.whl

Solution

poetry install should produce usable installs, even when a build script is used.

If that's not possible (or until then, if the fix will take a while), an error should be printed explaining why poetry install is unusable.

Start with the pyproject.toml configuration and build.py example, then reproduce the issue with poetry install using a project that defines tool.poetry.build. Compare the result with poetry build -f wheel followed by poetry run pip install dist/*.whl; done means poetry install includes the build output, or clearly explains why it cannot.