$ cargo test --no-default-features --features yaml
   Compiling config v0.11.0 (/tmp/config-rs)
error[E0599]: no variant or associated item named `Json` found for enum `FileFormat` in the current scope
  --> examples/async_source/main.rs:39:33
   |
39 |             format: FileFormat::Json,
   |                                 ^^^^ variant or associated item not found in `FileFormat`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `config` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed