$ dune build
Info: Creating file dune-project with this contents:
| (lang dune 2.5)
File "dune", line 3, characters 1-12:
3 |  (variant x))
     ^^^^^^^^^^^
Error: 'variant' is only available since version 0.1 of the experimental
library variants feature.. Please update your dune-project file to have
(using library_variants 0.1), but please note that this extension is
experimental and we might drop support for it in the future.

$ dune build
Info: Creating file dune-project with this contents:
| (lang dune 2.5)
File "dune", line 3, characters 1-12:
3 |  (variant x))
     ^^^^^^^^^^^
Error: 'variant' is only available since version 0.1 of the experimental
library variants feature.. Please update your dune-project file to have (lang
0.1).

$ cat > dune <<EOF
(library
 (name toto)
 (variant x))
EOF
$ dune build