Enable std/alloc without other features like chrono

[M1cha]

If you're fixed to an incompatible version of chrono or just want to save on code size, you don't really want any of that enabled. You might still want to use the utils for (de-)serializing things like Vecs though.

[jonasbb]

Is is already the case. The only things enabled by alloc and std are the features of the same name in serde. For the other crates the features use "?" to not pull new dependencies in.

serde_with/serde_with/Cargo.toml

Lines 29 to 48 in a079e9c

	alloc = [
	"serde/alloc",
	"base64?/alloc",
	"chrono_0_4?/alloc",
	"hex?/alloc",
	"serde_json?/alloc",
	"time_0_3?/alloc",
	]
	std = [
	"alloc",
	"serde/std",
	# Enables `Local` type
	"chrono_0_4?/clock",
	"chrono_0_4?/std",
	"indexmap_1?/std",
	"time_0_3?/serde-well-known",
	"time_0_3?/std",
	]

[M1cha]

Unfortunately this doesn't seem to be working though.
A binary with this dependency section:

[dependencies]
chrono = "= 0.4.19"
serde_with = { version = "2.2", default_features = false, features = ["std"] }

fails to compile with:

error: failed to select a version for `chrono`.
    ... required by package `serde_with v2.2.0`
    ... which satisfies dependency `serde_with = "^2.2"` (locked to 2.2.0) of package `tmp v0.1.0 (/tmp/tmp)`
versions that meet the requirements `^0.4.20` are: 0.4.23, 0.4.22, 0.4.21, 0.4.20

all possible versions conflict with previously selected packages.

  previously selected package `chrono v0.4.19`
    ... which satisfies dependency `chrono = "=0.4.19"` (locked to 0.4.19) of package `tmp v0.1.0 (/tmp/tmp)`

failed to select a version for `chrono` which could resolve this conflict

[jonasbb]

This is caused by rust-lang/cargo#10801 and pinning chrono to a single version. Pinning to a specific version works very poorly, and you will run into more problems like this. If you only add serde_with as a dependency, you will see that chrono will not be compiled.

You should drop the version pinning of chrono. Alternatively, you can use a different version of serde_with like <=2.0.0.

[M1cha]

I can't unpin chrono right now due to this issue.
Thanks for the hint about Cargo.lock.

serde_with 2.0 seems to work just fine.
Even though this situation is not ideal, it's not caused by serde_with directly and thus not something that can be fixed by this project.

Thanks a lot for the clarification.