Sorry for the potential data loss that might come out of this :(. I posted to dev-platform/firefox-dev but I'll add it here too in case someone misses those posts and ends up here:
If you don't have any |mach try| presets for the syntax selector, feel free to ignore. Otherwise, make sure you update to the latest central before running |mach try| or you might lose your presets.
If you see a message like this in your |mach try| output:
warning: unknown section 'syntax', the following presets were not migrated
Then you're too late and it means the presets listed underneath will be lost if you don't manually re-create them. To re-create them:
Create a [try] heading and copy + paste the unmigrated presets underneath. For example, the file should look like this:
foo = -b o -p all -u mochitest
bar = -b d -p none
Run: mv ~/.mozbuild/try_presets.yml temp.yml
Trigger the migration again: mach try --no-push
Copy + paste the contents of temp.yml into ~/.mozbuild/try_presets.yml
To re-iterate, these steps are only necessary if you see the bolded warning above.
In short, I recently changed the |mach try| preset format and included an automatic migration script. But due to a bug in the migration, only the first preset in the list was successfully migrated. This bug only affects presets that were saved with try syntax. Unfortunately the associated test for the migration only tested a single preset so didn't detect the bug. The bug is fixed on the latest mozilla-central
Very sorry for the inconvenience and let me know if you need help,