Add std feature and make it the default

This enables `serde_json` dependency, which is needed for deserializing
response messages. This will mean that client-side API will not be
available for `no_std` environments. This isn't as big an issue as it
would seem since typically on micrcontrolers, one would implement
services, not clients. Morever, this limitation will hopefully be lifted
once [`serde-json-core` supports complex enums][ce].

On the bright side, we can now make `serde-json-core` and `heapless`
optional dependencies, which will help `std` users avoid these
dependencies that they are unlikely to otherwise depend on.

This also adds a `embedded` feature, which will enable `heapless` and
`serde-json-core` dependencies. Users must enable either `std` or
`embedded` feature to use this crate.

[ce]: rust-embedded-community/serde-json-core#94

Author: zeenix

Cargo.lock

@@ -4,7 +4,17 @@ version = "0.1.0"
 edition = "2021"
 
 [features]
-default = ["alloc"]
-alloc = []
+default = ["std"]
+std = ["dep:serde_json", "memchr/std"]
+embedded = ["dep:serde-json-core", "dep:heapless"]
 
 [dependencies]
+serde = { version = "1.0.218", default-features = false, features = ["derive"] }
+serde_json = { version = "1.0.139", features = [
+    "arbitrary_precision",
+], optional = true }
+serde-json-core = { version = "0.6.0", default-features = false, features = [
+    "heapless",
+], optional = true }
+heapless = { version = "0.8.0", optional = true }
+memchr = { version = "2.7.4", default-features = false }

zarlink/Cargo.toml

@@ -1,4 +1,4 @@
-#![no_std]
+#![cfg_attr(not(feature = "std"), no_std)]
 #![deny(
     missing_debug_implementations,
     nonstandard_style,
@@ -8,8 +8,8 @@
 #![warn(unreachable_pub)]
 #![doc = include_str!("../../README.md")]
 
-#[cfg(not(feature = "alloc"))]
-compile_error!("Currently the `alloc` feature is required");
+#[cfg(all(not(feature = "std"), not(feature = "embedded")))]
+compile_error!("Either 'std' or 'embedded' feature must be enabled.");
 
 pub mod connection;
 mod error;