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添加依赖项
前面已经提到过在 cargo 配置文件 Cargo.toml 中如何手动添加工程依赖项，cargo 同样提供了 add 指令用于快捷添加依赖项，比如添加随机数生成包 rand
$ cargo add rand
Updating crates.io index
Adding rand v0.8.5 to dependencies.
Features:
+ alloc
+ getrandom
+ libc
+ rand_chacha
+ std
+ std_rng
- log
- min_const_gen
- nightly
- packed_simd
- serde
- serde1
- simd_support
- small_rng
Updating crates.io index
$ cat Cargo.toml
[package]
name = "hello_rust"
version = "0.1.0"
edition = "2021"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]
rand = "0.8.5"
添加完依赖项 rand 后，然后看看编译执行过程
$ cargo run
Downloaded cfg-if v1.0.0
Downloaded rand_chacha v0.3.1
Downloaded ppv-lite86 v0.2.17
Downloaded rand_core v0.6.4
Downloaded rand v0.8.5
Downloaded getrandom v0.2.11
Downloaded libc v0.2.150
Downloaded 7 crates (910.0 KB) in 4.12s
Compiling libc v0.2.150
Compiling cfg-if v1.0.0
Compiling ppv-lite86 v0.2.17
Compiling getrandom v0.2.11
Compiling rand_core v0.6.4
Compiling rand_chacha v0.3.1
Compiling rand v0.8.5
Compiling hello_rust v0.1.0 (~/hello_rust)
Finished dev [unoptimized + debuginfo] target(s) in 20.75s
Running `target/debug/hello_rust`
Hello rust's world!
看到编译过程 cargo 会先从 crates.io 下载依赖项源码，然后编译依赖项源码，最后才编译本地工程代码。但是刚刚用 cargo add 指令添加的依赖项是 rand，为什么多了这么多其它内容？因为 rand 也有依赖项，为了编译 rand 也需要先把它的依赖项源码下载下来。
在工程被首次构建后，发现工程目录下多了个 Cargo.lock 文件。这个文件是为了更具体地描述 Cargo.toml 文件中所添加的依赖项，它由 cargo 自动维护，开发人员不应该手动修改它。
看看构建之后的 Cargo.lock
$ cat Cargo.lock
# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 3
[[package]]
name = "cfg-if"
version = "1.0.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
[[package]]
name = "getrandom"
version = "0.2.11"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "fe9006bed769170c11f845cf00c7c1e9092aeb3f268e007c3e760ac68008070f"
dependencies = [
"cfg-if",
"libc",
"wasi",
]
[[package]]
name = "hello_rust"
version = "0.1.0"
dependencies = [
"rand",
]
[[package]]
name = "libc"
version = "0.2.150"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "89d92a4743f9a61002fae18374ed11e7973f530cb3a3255fb354818118b2203c"
[[package]]
name = "ppv-lite86"
version = "0.2.17"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "5b40af805b3121feab8a3c29f04d8ad262fa8e0561883e7653e024ae4479e6de"
[[package]]
name = "rand"
version = "0.8.5"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "34af8d1a0e25924bc5b7c43c079c942339d8f0a8b57c39049bef581b46327404"
dependencies = [
"libc",
"rand_chacha",
"rand_core",
]
[[package]]
name = "rand_chacha"
version = "0.3.1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "e6c10a63a0fa32252be49d21e7709d4d4baf8d231c2dbce1eaa8141b9b127d88"
dependencies = [
"ppv-lite86",
"rand_core",
]
[[package]]
name = "rand_core"
version = "0.6.4"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
dependencies = [
"getrandom",
]
[[package]]
name = "wasi"
version = "0.11.0+wasi-snapshot-preview1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
为什么还需要更多的依赖项描述呢？因为 Cargo.toml 文件中填写的依赖项其自身所依赖的包可能会更新，为避免依赖项各自更新后不兼容导致的编译错误，需要更多的详细描述对依赖项进行身份固化，比如依赖项所依赖的的依赖包名、版本、来源、校验码等，确保循环依赖在后期的再次编译中不会突然报错。
同时，Cargo.lock 文件仅会在 package 工程首次编译构建时生成，如果依赖项被更新了，再次利用 cargo 重新构建编译时会自动维护 Cargo.lock 文件内容。
检查代码
开发软件过程中，往往要不断迭代升级，频繁修改，又为了在开始下一部分编码之前，先检查当前编写代码的正确性，无论是语法还是逻辑层面都需要。由于功能逻辑未必开发完整，中途检查更多的是侧重于编译完整性，也就是语法层面，只要能编译顺利通过即可，这并不需要最终的输出文件。对于逻辑功能的验证，才需要实际运行最终输出文件。
编译时输出文件的大小直接影响到投入的等待检查时间，工程代码少，编译输出的文件较小，耗时也短；但是面对庞大项目工程时，为了加快开发中途的检查，那么 cargo 有没有提供这方面的选项工具帮助跳过输出文件这最后一步呢？
Cargo 提供了 check 指令用于检查 package 工程代码和依赖项相关的问题。
为了演示，先用 cargo clean 将当前工程的中间文件清除，然后执行 cargo check 检查代码
$ cargo clean
$ cargo check
Checking hello_rust v0.1.0 (~/hello_rust)
Finished dev [unoptimized + debuginfo] target(s) in 0.07s
$ cargo clean
$ cargo build
Compiling hello_rust v0.1.0 (~/hello_rust)
Finished dev [unoptimized + debuginfo] target(s) in 0.23s
从上面的输出来看，和 cargo build 编译指令相比，cargo check 的输出更快。
为了更详细了解 cargo build 和 cargo check 的耗时，我们重新用 time 指令来对比一下
$ cargo clean
$ time cargo check
Checking hello_rust v0.1.0 (~/hello_rust)
Finished dev [unoptimized + debuginfo] target(s) in 0.80s
real 0m0.894s
user 0m0.109s
sys 0m0.297s
$ cargo clean
$ time cargo build
Compiling hello_rust v0.1.0 (~/hello_rust)
Finished dev [unoptimized + debuginfo] target(s) in 1.17s
real 0m1.260s
user 0m0.266s
sys 0m0.453s
从上面的统计数据来看，cargo check 执行速度明显要快很多，而这还只是编译构建简单的 hello world 程序。如果换做普通规模的软件工程，cargo check 的执行速度优势想必会更明显。
