Fix build with LTO disabled in environment

.github/workflows/tests.yml

@@ -32,6 +32,7 @@ jobs:
             python-version: 3.7
             extra-requirements: '-r requirements/testing/travis_extra.txt'
             XVFB_RUN: xvfb-run -a
+            CFLAGS: "-fno-lto"  # Ensure that disabling LTO works.
           - os: ubuntu-16.04
             python-version: 3.8
             extra-requirements: '-r requirements/testing/travis_extra.txt'

setup.py

@@ -106,20 +106,34 @@ def add_optimization_flags(self):
         """
 
         env = os.environ.copy()
-        if not setupext.config.getboolean('libs', 'enable_lto', fallback=True):
-            return env
         if sys.platform == 'win32':
             return env
-
-        cppflags = []
-        if 'CPPFLAGS' in os.environ:
-            cppflags.append(os.environ['CPPFLAGS'])
-        cxxflags = []
-        if 'CXXFLAGS' in os.environ:
-            cxxflags.append(os.environ['CXXFLAGS'])
-        ldflags = []
-        if 'LDFLAGS' in os.environ:
-            ldflags.append(os.environ['LDFLAGS'])
+        enable_lto = setupext.config.getboolean('libs', 'enable_lto',
+                                                fallback=None)
+
+        def prepare_flags(name, enable_lto):
+            """
+            Prepare *FLAGS from the environment.
+
+            If set, return them, and also check whether LTO is disabled in each
+            one, raising an error if Matplotlib config explicitly enabled LTO.
+            """
+            if name in os.environ:
+                if '-fno-lto' in os.environ[name]:
+                    if enable_lto is True:
+                        raise ValueError('Configuration enable_lto=True, but '
+                                         '{0} contains -fno-lto'.format(name))
+                    enable_lto = False
+                return [os.environ[name]], enable_lto
+            return [], enable_lto
+
+        _, enable_lto = prepare_flags('CFLAGS', enable_lto)  # Only check lto.
+        cppflags, enable_lto = prepare_flags('CPPFLAGS', enable_lto)
+        cxxflags, enable_lto = prepare_flags('CXXFLAGS', enable_lto)
+        ldflags, enable_lto = prepare_flags('LDFLAGS', enable_lto)
+
+        if enable_lto is False:
+            return env
 
         if has_flag(self.compiler, '-fvisibility=hidden'):
             for ext in self.extensions:

setupext.py

@@ -447,7 +447,6 @@ def get_extensions(self):
         ext = Extension(
             "matplotlib.backends._tkagg", [
                 "src/_tkagg.cpp",
-                "src/py_converters.cpp",
             ],
             include_dirs=["src"],
             # psapi library needed for finding Tcl/Tk at run time.

src/_tkagg.cpp

@@ -33,14 +33,17 @@
 #define dlsym GetProcAddress
 #else
 #include <dlfcn.h>
-// Suppress -Wunused-function on POSIX, but not on Windows where that would
-// lead to PY_ARRAY_UNIQUE_SYMBOL being an unresolved external.
-#define NO_IMPORT_ARRAY
 #endif
 
 // Include our own excerpts from the Tcl / Tk headers
 #include "_tkmini.h"
-#include "py_converters.h"
+
+static int convert_voidptr(PyObject *obj, void *p)
+{
+    void **val = (void **)p;
+    *val = PyLong_AsVoidPtr(obj);
+    return *val != NULL ? 1 : !PyErr_Occurred();
+}
 
 // Global vars for Tk functions.  We load these symbols from the tkinter
 // extension module or loaded Tk libraries at run-time.

src/py_converters.cpp

@@ -94,13 +94,6 @@ int convert_from_attr(PyObject *obj, const char *name, converter func, void *p)
     return 1;
 }
 
-int convert_voidptr(PyObject *obj, void *p)
-{
-    void **val = (void **)p;
-    *val = PyLong_AsVoidPtr(obj);
-    return *val != NULL ? 1 : !PyErr_Occurred();
-}
-
 int convert_double(PyObject *obj, void *p)
 {
     double *val = (double *)p;

src/py_converters.h

@@ -22,7 +22,6 @@ typedef int (*converter)(PyObject *, void *);
 int convert_from_attr(PyObject *obj, const char *name, converter func, void *p);
 int convert_from_method(PyObject *obj, const char *name, converter func, void *p);
 
-int convert_voidptr(PyObject *obj, void *p);
 int convert_double(PyObject *obj, void *p);
 int convert_bool(PyObject *obj, void *p);
 int convert_cap(PyObject *capobj, void *capp);