The CI error here looks real. Were we previously compiling cells with a different Rust version, hiding this?

   Compiling tock-cells v0.1.0 (/home/runner/work/tock/tock/libraries/tock-cells)
error[E0432]: unresolved import `map_cell`
   --> libraries/tock-cells/src/map_cell.rs:330:9
    |
330 |     use map_cell::MapCell;
    |         ^^^^^^^^ help: a similar path exists: `crate::map_cell`
    |
    = note: `use` statements changed in Rust 2018; read more at <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html>

For more information about this error, try `rustc --explain E0432`.

Were we previously compiling cells with a different Rust version, hiding this?

Even if you use a current Rust version, it still supports the behaviors dictated by the various Rust editions, and defaults to the 2015 edition. Crates compiled on different editions are inter-operable, so we haven't been doing anything wrong here. Edition 2015 just has a lot of legacy (such as less clear rules around imports), so we want to use the current edition everywhere.

This one's on me, didn't expect ci-tests to fail when ci-build succeeds. Good that the merge queue blocks on all of these. 😄

@brghena I took the liberty to fix this import.