cargo build failing due to unused dependency serde_stacker

On latest revision:

$ cargo build --frozen --release --all-features
[...]
error: extern crate `serde_stacker` is unused in crate `libsurfer`
  |
  = help: remove the dependency or add `use serde_stacker as _;` to the crate root
note: the lint level is defined here
 --> libsurfer/src/lib.rs:1:9
  |
1 | #![deny(unused_crate_dependencies)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^

warning: unused variable: `t`
  --> libsurfer/src/translation/mod.rs:88:35
   |
88 |             AnyTranslator::Python(t) => {}
   |                                   ^ help: if this is intentional, prefix it with an underscore: `_t`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `t`
   --> libsurfer/src/translation/mod.rs:141:35
    |
141 |             AnyTranslator::Python(t) => None,
    |                                   ^ help: if this is intentional, prefix it with an underscore: `_t`

warning: `libsurfer` (lib) generated 2 warnings
error: could not compile `libsurfer` (lib) due to 1 previous error; 2 warnings emitted
warning: build failed, waiting for other jobs to finish...
==> ERROR: A failure occurred in build().
    Aborting...

Any idea why this isn't spotted in CI?