I'd guess -flto is throwing away the special section where we collect the test headers. The section is used to avoid having to create a master list of all tests in addition to writing the tests.

Is there a way to mark that section to not be deleted by LTO?

We rely on the compiler to automatically generate __start_test_section and __stop_test_section when putting things into the section.

Gains size? You mean it's bigger with LTO?

Sure:

diff --git a/tests/test-runner.h b/tests/test-runner.h
index 64f8d4c6aa4f..600d77c93833 100644
--- a/tests/test-runner.h
+++ b/tests/test-runner.h
@@ -41,7 +41,7 @@ struct test {
        static void name(void);                                 \
                                                                \
        const struct test test##name                            \
-                __attribute__ ((section ("test_section"))) = { \
+                __attribute__ ((used, section ("test_section"))) = {   \
                #name, name, 0                                  \
        };                                                      \
                                                                \
@@ -51,7 +51,7 @@ struct test {
        static void name(void);                                 \
                                                                \
        const struct test test##name                            \
-                __attribute__ ((section ("test_section"))) = { \
+                __attribute__ ((used, section ("test_section"))) = {   \
                #name, name, 1                                  \
        };                                                      \
                                                                \

Same error here. I've created a bug on the Gentoo tracker, with a patch for Weston 3.0.0:
* bug: https://bugs.gentoo.org/634572
* patch: https://634572.bugs.gentoo.org/attachment.cgi?id=499022

Please consider applying this if it's suitable. :)

Oh and btw this is with GCC 7.2.0.

Hi, thanks for your patch, I also sent one to the mailing list back in April, see https://patchwork.freedesktop.org/series/23087/

Pushed to the list now; __attribute__((used)) seems to DTRT on both GCC and clang. Thanks!