为什么Rust会先用`cargo build --release`再加上`cargo run`再次编译我的项目？

Question

Rust documentation teaches us that cargo build creates a binary file after compiling, which we can execute with cargo run . cargo run will again compile the code if it notices any change after cargo build command is executed. It also says that cargo build --release command creates the final program, which will run faster.

My question is, why is that when I do cargo build --release , it compiles the code, which is fine. But when I execute cargo run , it again compiles the code, even though I haven't changed any code since. It is working normally with cargo build , followed by cargo run ie compiling one time with the former command.

naufil@naufil-Inspiron-7559:~/Desktop/rust/20April/variables$ cargo build
   Compiling variables v0.1.0 (/home/naufil/Desktop/rust/20April/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.35s
naufil@naufil-Inspiron-7559:~/Desktop/rust/20April/variables$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/variables`
Hello, world! 6
naufil@naufil-Inspiron-7559:~/Desktop/rust/20April/variables$ cargo build --release
   Compiling variables v0.1.0 (/home/naufil/Desktop/rust/20April/variables)
    Finished release [optimized] target(s) in 0.34s
naufil@naufil-Inspiron-7559:~/Desktop/rust/20April/variables$ cargo run
   Compiling variables v0.1.0 (/home/naufil/Desktop/rust/20April/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.23s
     Running `target/debug/variables`
Hello, world! 6

Answer 1

cargo run attempts to run the debug build of your project. Use cargo run --release instead. A cargo build --release followed by cargo run --release won't compile again.

Cargo maintains two pretty much completely independent sets of build artifacts:

• The debug build, stored in target/debug/
• The release build, stored in target/release/

---

All of these sub-commands allow you to specify which of these profiles to use (not necessarily an exhaustive list):

• Default: debug (switch to release mode with --release )
  • cargo build
  • cargo run
  • cargo test
  • cargo check
• Default: release (switch to debug mode with --debug )
  • cargo bench
  • cargo install