The Fuchsia GN build machinery allows for separate components to be built in different “variants”. A variant usually just means using extra compiler options, but they can do more than that if you write some more GN code. The variants defined so far enable things like sanitizers and LTO.
The GN build argument select_variant controls which components are built in which variants. It applies automatically to every executable, loadable_module, or driver_module target in GN files. It's a flexible mechanism in which you give a list of matching rules to apply to each target to decide which variant to use (if any). To support this flexibility, the value for select_variant uses a detailed GN syntax. For simple cases, this can just be a list of strings.
Using fx set:
fx set core.x64 --variant={host_asan,asan/cat,asan/ledger}
Alternatively, you can add or modify the variants on an existing build by editing the GN args (substituting your build's GN output directory for out/default as necessary):
./buildtools/gn args out/default
That command will bring up an editor. Append to that file:
select_variant = [ "host_asan", "asan/cat", "asan/ledger" ]
The first switch applies the host_asan matching rule, which enables AddressSanitizer for all the executables built to run on the build host.
The second switch applies the asan matching rule, which enables AddressSanitizer for executables built to run on the target (i.e. the Fuchsia device). The /cat suffix constrains this matching rule only to the binary named cat.
The third switch is like the second, but matches the binary named ledger.
The GN code supports much more flexible matching rules than just the binary name, but there are no shorthands for those. See the select_variant build argument documentation for more details.
To see the list of variants available and learn more about how to define new ones, see the known_variants build argument.
