Rust项目中Cargo.toml里依赖设为可选的考量因素有哪些？

我是Rust新手，正在通过阅读项目进行学习。在这款Binance API连接器Rust库中，部分依赖被设置为可选，其余则不是。具体配置如下：

[features]
default = ["enable-ureq", "enable-tungstenite"]
enable-hyper = [ "hyper", "hyper-tls", "serde_json", "futures-util", "tokio" ]
enable-ureq = [ "ureq", "serde_json" ]
enable-tungstenite = ["tungstenite"]
enable-tokio-tungstenite = ["tokio-tungstenite"]
full = ["enable-hyper", "enable-tungstenite", "enable-ureq", "enable-tokio-tungstenite"]

[dependencies]
hmac = "0.12.0"
log = "0.4.14"
serde = { version = "1.0.136", features = ["derive"] }
sha2 = { version = "0.10.6", default-features = false, features = ["oid"] }
url = "2.2.2"
rust_decimal = "1.24.0"
http = "0.2.7"
strum = { version = "0.24", features = ["derive"] }
rand = "0.8.5"
signature = "2.2.0"
base64 = "0.13.1"
ed25519-dalek = { version = "2.1.0", features = ["serde", "zeroize", "rand_core", "digest", "pkcs8", "pem"] }

# enable-ureq
ureq = { version = "2.4.0", optional = true }

# enable-hyper
hyper = { version = "0.14.16", features = ["full"], optional = true }
serde_json = { version = "1.0.78", optional = true }
hyper-tls = {version = "0.5.0", optional = true }
futures-util = {version = "0.3.21", optional = true }
tokio = { version = "1", features = ["time"], optional = true }

# enable-tungstenite
tungstenite = {version = "0.20.1", features = ["native-tls"], optional = true}

# enable-tokio-tungstenite
tokio-tungstenite = {version = "0.17.1", features = ["native-tls"], optional = true}

依赖可选性的核心考量因素 #

• 核心功能必须性：非可选依赖都是Binance API交互的基础刚需——像hmac/sha2用于API请求签名（Binance强制要求），serde处理核心数据的序列化/反序列化，rust_decimal处理高精度交易金额，这些是任何使用场景下都离不开的，所以默认强制引入。
• 适配不同运行时场景：可选依赖都是网络通信相关的实现，比如ureq是同步HTTP客户端，hyper是异步HTTP客户端，tokio-tungstenite是基于tokio的异步WebSocket客户端。设为可选后，用户可以根据自己项目的同步/异步架构选择对应依赖，避免引入冗余的运行时，缩小编译产物体积、加快编译速度。
• 降低默认使用门槛：默认只启用enable-ureq和enable-tungstenite，因为同步客户端的依赖链更简单，对新手更友好，不用处理异步编程的额外复杂度。需要异步能力的用户再手动开启enable-hyper这类feature即可。
• 减少依赖冲突风险：不同网络客户端可能依赖不同版本的基础库（比如tokio的版本差异），设为可选后用户只引入自己需要的那一套，能有效降低版本冲突的概率。
• 按需加载功能模块：比如serde_json只在需要JSON解析的网络请求场景下才用，所以跟着对应的HTTP/WebSocket feature设为可选，避免默认引入不需要的JSON处理依赖。

内容的提问来源于stack exchange，提问作者Jianqiu Ye