exampleの実行 "cargo run --example"

cargo run --example "ch04_02_bool" で、直接実行できた。

PS > cargo run --example "ch04_02_bool"
warning: file found to be present 
in multiple build targets:
 \examples\ch04_01_units.rs
   Compiling ex04 v0.1.0 
(rust_project\rustbook\ch04\ex04)
error: expected item, found keyword `let`
 --> examples\ch04_02_bool.rs:1:1
  |
1 | let b1 = true;
  | ^^^ expected item

error: aborting due to previous error

error: could not compile `ex04`.

To learn more, run the command again with --verbose.

> tree /f
C:.
│  .gitignore
│  Cargo.lock
│  Cargo.toml
│
├─examples
│      ch04_01_units.rs
│      ch04_02_bool.rs

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