error[E0432]: unresolved import `serde`

[Techplex]

Hello I'm new to rust. I've been reading the rust book and doing lots of research.

I'm attempting to add a rest API to Mozilla's Project Mentat. I think I've got most of the structure working but I'm running into issues in the query-projector crate.

Here is the first error

error[E0432]: unresolved import `serde`
  --> query-projector/src/lib.rs:74:9
   |
74 | pub use serde::Serialize;
   |         ^^^^^ maybe a missing crate `serde`?

The error index: Rust Compiler Error Index indicates that there may be a spelling mestake (checked) some difference due to rust edition (tried setting edition to 2021) no luck. I also double checked the cargo.toml in the query-projector crate.

query-projector/Cargo.toml

[package]
name = "mentat_query_projector"
version = "0.0.2"
workspace = ".."

[features]
sqlcipher = ["rusqlite/sqlcipher"]

[dependencies]
failure = "~0.1"
indexmap = "~1.7"
serde = { version = "~1.0", optional = true }
serde_derive = { version = "~1.0", optional = true }
serde_json = "~1.0"

[dependencies.rusqlite]
version = "~0.26"
features = ["limits", "bundled"]

[dependencies.core_traits]
path = "../core-traits"

[dependencies.edn]
path = "../edn"

[dependencies.mentat_core]
path = "../core"

[dependencies.mentat_db]
path = "../db"

[dependencies.db_traits]
path = "../db-traits"

[dependencies.mentat_query_algebrizer]
path = "../query-algebrizer"

[dependencies.mentat_query_pull]
path = "../query-pull"

[dependencies.query_pull_traits]
path = "../query-pull-traits"

[dependencies.query_projector_traits]
path = "../query-projector-traits"

[dependencies.mentat_query_sql]
path = "../query-sql"

[dev-dependencies.mentat_sql]
path = "../sql"

I've tried clearing the build cache removing cargo.lock files and continue to get the same error. Ideas?

Full source code here: GitHub - TechplexEngineer/mentat at web-server

[steffahn]

Serde is an optional dependency; maybe you’ll want to put #[cfg(feature = "serde")] on that pub use … line (or on some module containing it).

---

Make sure to test whether the crate still works either with or without that feature (specify the feature by including or not including --features serde argument to cargo check or other cargo commands).

In case serde isn’t supposed to be an optional dependency at all, the Cargo.toml should be modified accordingly.

[Techplex]

If I change it to be a required dependency, here is the new cargo.toml:

[dependencies]
failure = "~0.1"
indexmap = "~1.7"
serde = { version = "~1.0", features = ["derive"] }
serde_derive = { version = "~1.0" }
serde_json = "~1.0"

I get the same error as above, all errors reproduced below:

error[E0432]: unresolved import `serde`
  --> query-projector/src/lib.rs:75:9
   |
75 | pub use serde::Serialize;
   |         ^^^^^ maybe a missing crate `serde`?


error: cannot determine resolution for the derive macro `Serialize`
  --> query-projector/src/lib.rs:77:39
   |
77 | #[derive(Clone, Debug, PartialEq, Eq, Serialize)]
   |                                       ^^^^^^^^^
   |
   = note: import resolution is stuck, try simplifying macro imports

error: cannot determine resolution for the derive macro `Serialize`
  --> query-projector/src/lib.rs:83:39
   |
83 | #[derive(Clone, Debug, PartialEq, Eq, Serialize)]
   |                                       ^^^^^^^^^
   |
   = note: import resolution is stuck, try simplifying macro imports

[steffahn]

In case the code is still edition 2015, you’d also need to put extern crate serde; somewhere (I believe typically in the root module of the crate). In case of optional dependency, that would also need to be cfg’d, so

#[cfg(feature = "serde")]
extern crate serde;

I suppose. Or without the cfg line if you made the dependency non-optional.