Give a more helpful warning when test compilation fails

Mention CC and CXX in particular for OS X

brian2/codegen/runtime/cython_rt/cython_rt.py

@@ -123,8 +123,12 @@ def main():
             compiled.main()
             return True
         except Exception as ex:
-            logger.warn(f'Cannot use Cython, a test compilation failed: {str(ex)} ({ex.__class__.__name__})',
-                        'failed_compile_test')
+            logger.warn(
+                f"Cannot use Cython, a test compilation failed: {str(ex)} ({ex.__class__.__name__})\n"
+                f"Certain compiler configurations (e.g. clang in a conda environment on OS X) are "
+                f"known to be problematic. Note that you can switch the compiler by setting the "
+                f"'CC' and 'CXX' environment variables. For example, you may want to try 'CC=gcc' "
+                f"and 'CXX=g++'.", 'failed_compile_test')
             return False
 
     def compile_block(self, block):

brian2/codegen/runtime/cython_rt/extension_manager.py

@@ -81,16 +81,21 @@ def create_extension(self, code, force=False, name=None,
             os.makedirs(lib_dir)
         except OSError:
             if not os.path.exists(lib_dir):
-                raise IOError("Couldn't create Cython cache directory '%s', try setting the "
-                              "cache directly with prefs.codegen.runtime.cython.cache_dir." % lib_dir)
+                raise IOError(
+                    f"Couldn't create Cython cache directory '{lib_dir}', try setting the "
+                    f"cache directly with prefs.codegen.runtime.cython.cache_dir.")
 
         numpy_version = '.'.join(numpy.__version__.split('.')[:2])  # Only use major.minor version
-        key = code, sys.version_info, sys.executable, Cython.__version__, numpy_version
+        # avoid some issues when manually switching compilers
+        CC = os.environ.get('CC', None)
+        CXX = os.environ.get('CXX', None)
+        key = (code, sys.version_info, sys.executable, Cython.__version__,
+               numpy_version, CC, CXX)
 
         if force:
             # Force a new module name by adding the current time to the
             # key which is hashed to determine the module name.
-            key += time.time(),            
+            key += time.time(),  # Note the trailing comma (this is a tuple)
 
         if key in self._code_cache:
             return self._code_cache[key]

brian2/tests/test_codegen.py

@@ -10,6 +10,7 @@
 from brian2 import prefs, clear_cache, _cache_dirs_and_extensions
 from brian2.codegen.cpp_prefs import compiler_supports_c99, get_compiler_and_args
 from brian2.codegen.optimisation import optimise_statements
+from brian2.codegen.runtime.cython_rt import CythonCodeObject
 from brian2.codegen.translation import (analyse_identifiers,
                                         get_identifiers_recursively,
                                         parse_statement,
@@ -445,6 +446,28 @@ def test_clear_cache():
 
         os.remove(fname)
 
+def test_compiler_error():
+    # In particular on OSX with clang in a conda environment, compilation might fail.
+    # Switching to a system gcc might help in such cases. Make sure that the error
+    # message mentions that.
+    old_CC = os.environ.get('CC', None)
+    old_CXX = os.environ.get('CXX', None)
+    os.environ.update({'CC': 'non-existing-compiler',
+                       'CXX': 'non-existing-compiler++'})
+    try:
+        with catch_logs() as l:
+            assert not CythonCodeObject.is_available()
+        assert len(l) > 0  # There are additional warnings about compiler flags
+        last_warning = l[-1]
+        assert last_warning[1].endswith('.failed_compile_test')
+        assert 'CC' in last_warning[2] and 'CXX' in last_warning[2]
+
+    finally:
+        if old_CC:
+            os.environ['CC'] = old_CC
+        if old_CXX:
+            os.environ['CXX'] = old_CXX
+
 
 def test_compiler_c99():
     # On a user's computer, we do not know whether the compiler actually