serde is not an optional dependency

[pascalkuthe]

Judging from how the Cargo.toml is written, I assumed that serde was supposed to be an optional dependency.
However, when inspecting cargo bloat for my project (which uses smol_str) I noticed a long compile time for serde.
I was able to reproduce with an empty project (just an empty src/lib.rs file):

[package]
name = "foo"
version = "0.1.0"
edition = "2021"
[dependencies]
smol_str = "0.1.21"

Running cargo check shows that serde is compiled and running cargo bloat yields

> cargo bloat --time -j 1
Time Crate
3.20s serde
0.39s smol_str

Just as an example for my larger project, serde is still among the top 7 contributors even tough I am not using it at all (and according to cargo tree only depend upon it trough smol_str).

> cargo bloat --time -j 1
 Time Crate
9.99s syn
9.93s hir_def
5.41s rowan
4.40s basedb
3.59s logos_derive
3.49s regex_syntax
3.10s serde
...

I am using the latest stable version (rust 1.56.1 on linux/nixos). Is there any way to remove the serde dependency or am I missing something?

[lnicola]

That's probably because of:

default = ["std"]
std = ["serde/std"]

You can try setting default_features = false on smol_str.

[pascalkuthe]

Ah yes that does fix the problem. Thank you!
If this is intentional it might be nice to document this because to me a feature named "std" didn't suggest a serde dependency.

[lnicola]

Is this rust-lang/cargo#3494?

[pascalkuthe]

Yeah, that seems like the same issue. Sadly, the feature to fix this isn't stable yet (although the stabilization RFC has been accepted).

After looking at the source code again, I noticed that the std feature is not used at all (in the src code) currently. So maybe just removing the "std" feature from the default features and adding a notice to the README would be an option until then?

[lnicola]

I've seen projects with a use-serde feature instead of serde, but I'm not sure how that helps here.

[pascalkuthe]

I think the real reason for this feature is that serde can also be used with `!#[no_std]~. So someone that wants to this crate with ![no_std] together with serde would disable the std feature (but enable the serde featurure).