eRFC: single-file packages ("cargo script") integration

text/3424-cargo-script.md

@@ -137,44 +137,26 @@ To depend on a library hosted on [crates.io], you modify `hello_world.rs`:
 
 //! ```cargo
 //! [dependencies]
-//! time = "0.1.12"
+//! regex = "1.8.0"
 //! ```
 
 fn main() {
-    println!("Hello, world!");
+    let re = Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+    println!("Did our date match? {}", re.is_match("2014-01-01"));
 }
 ```
 
 The `cargo` section is called a [***manifest***][def-manifest], and it contains all of the
 metadata that Cargo needs to compile your package. This is written in the
 [TOML] format (pronounced /tɑməl/).
 
-`time = "0.1.12"` is the name of the [crate][def-crate] and a [SemVer] version
+`regex = "1.8.0"` is the name of the [crate][def-crate] and a [SemVer] version
 requirement. The [specifying
 dependencies](https://doc.rust-lang.org/cargo/guide/../reference/specifying-dependencies.html) docs have more
 information about the options you have here.
 
-If we also wanted to add a dependency on the `regex` crate, we would not need
-to add `[dependencies]` for each crate listed. Here's what your whole
-`hello_world.rs` file would look like with dependencies on the `time` and `regex`
-crates:
-
-```rust
-#!/usr/bin/env cargo
-
-//! ```cargo
-//! [dependencies]
-//! time = "0.1.12"
-//! regex = "0.1.41"
-//! ```
-
-fn main() {
-    let re = Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
-    println!("Did our date match? {}", re.is_match("2014-01-01"));
-}
-```
-
-You can then re-run this and Cargo will fetch the new dependencies and all of their dependencies.  You can see this by passing in `--verbose`:
+You can then re-run this and Cargo will fetch the new dependencies and all of
+their dependencies.  You can see this by passing in `--verbose`:
 ```console
 $ cargo --verbose ./hello_world.rs
       Updating crates.io index
@@ -194,6 +176,11 @@ $ cargo --verbose ./hello_world.rs
 Did our date match? true
 ```
 
+Cargo will cache the exact information about which revision of all of these dependencies we used.
+
+Now, if `regex` gets updated, we will still build with the same revision until
+we choose to `cargo update --manifest-path hello_world.rs`.
+
 ## Package Layout
 
 *(Adapted from [the cargo book](https://doc.rust-lang.org/cargo/guide/project-layout.html))*