better errors on compilation failure or rust setuptools import (#395)

* better errors on compilation failure or rust setuptools import

* pep8

setup.py

@@ -1,10 +1,27 @@
 #!/usr/bin/env python
 import platform
+import re
+import shutil
+import subprocess
 import sys
 
 from setuptools import setup
 
-from setuptools_rust import RustExtension
+try:
+    from setuptools_rust import RustExtension
+except ImportError:
+    print(
+        """
+        =============================DEBUG ASSISTANCE==========================
+        If you are seeing an error here please try the following to
+        successfully install cryptography:
+
+        Upgrade to the latest pip and try again. This will fix errors for most
+        users. See: https://pip.pypa.io/en/stable/installing/#upgrading-pip
+        =============================DEBUG ASSISTANCE==========================
+        """
+    )
+    raise
 
 if platform.python_implementation() == "PyPy":
     if sys.pypy_version_info < (2, 6):
@@ -14,19 +31,72 @@
         )
 
 
-setup(
-    rust_extensions=[
-        RustExtension(
-            "_bcrypt",
-            "src/_bcrypt/Cargo.toml",
-            py_limited_api=True,
-            # Enable abi3 mode if we're not using PyPy.
-            features=(
-                []
-                if platform.python_implementation() == "PyPy"
-                else ["pyo3/abi3-py36"]
+try:
+    setup(
+        rust_extensions=[
+            RustExtension(
+                "_bcrypt",
+                "src/_bcrypt/Cargo.toml",
+                py_limited_api=True,
+                # Enable abi3 mode if we're not using PyPy.
+                features=(
+                    []
+                    if platform.python_implementation() == "PyPy"
+                    else ["pyo3/abi3-py36"]
+                ),
+                rust_version=">=1.56.0",
             ),
-            rust_version=">=1.56.0",
-        ),
-    ],
-)
+        ],
+    )
+except:  # noqa: E722
+    # Note: This is a bare exception that re-raises so that we don't interfere
+    # with anything the installation machinery might want to do. Because we
+    # print this for any exception this msg can appear (e.g. in verbose logs)
+    # even if there's no failure. For example, SetupRequirementsError is raised
+    # during PEP517 building and prints this text. setuptools raises SystemExit
+    # when compilation fails right now, but it's possible this isn't stable
+    # or a public API commitment so we'll remain ultra conservative.
+
+    import pkg_resources
+
+    print(
+        """
+    =============================DEBUG ASSISTANCE=============================
+    If you are seeing a compilation error please try the following steps to
+    successfully install bcrypt:
+    1) Upgrade to the latest pip and try again. This will fix errors for most
+       users. See: https://pip.pypa.io/en/stable/installing/#upgrading-pip
+    2) Ensure you have a recent Rust toolchain installed. bcrypt requires
+       rustc >= 1.56.0.
+    """
+    )
+    print(f"    Python: {'.'.join(str(v) for v in sys.version_info[:3])}")
+    print(f"    platform: {platform.platform()}")
+    for dist in ["pip", "setuptools", "setuptools_rust"]:
+        try:
+            version = pkg_resources.get_distribution(dist).version
+        except pkg_resources.DistributionNotFound:
+            version = "n/a"
+        print(f"    {dist}: {version}")
+    version = "n/a"
+    if shutil.which("rustc") is not None:
+        try:
+            # If for any reason `rustc --version` fails, silently ignore it
+            rustc_output = subprocess.run(
+                ["rustc", "--version"],
+                capture_output=True,
+                timeout=0.5,
+                encoding="utf8",
+                check=True,
+            ).stdout
+            version = re.sub("^rustc ", "", rustc_output.strip())
+        except subprocess.SubprocessError:
+            pass
+    print(f"    rustc: {version}")
+
+    print(
+        """\
+    =============================DEBUG ASSISTANCE=============================
+    """
+    )
+    raise