Step 1.9: Phantom types

1_concepts/1_9_phantom/Cargo.toml

@@ -3,3 +3,6 @@ name = "step_1_9"
 version = "0.1.0"
 edition = "2024"
 publish = false
+
+[dependencies]
+rand = "0.8"

1_concepts/1_9_phantom/src/main.rs

@@ -1,3 +1,33 @@
+use std::marker::PhantomData;
+use rand;
+
+trait HasFacts {
+    const FACTS: &'static [&'static str];
+}
+
+impl<T> HasFacts for Vec<T> {
+    const FACTS: &'static [&'static str] = &[
+        "Vec is heap-allocated.",
+        "Vec may re-allocate on growing.",
+    ];
+}
+
+struct Fact<T>(PhantomData<T>);
+
+impl<T: HasFacts> Fact<T> {
+    fn new() -> Self {
+        Fact(PhantomData)
+    }
+
+    fn fact(&self) -> &str {
+        let facts = T::FACTS;
+        let idx = rand::random::<usize>() % facts.len();
+        facts[idx]
+    }
+}
+
 fn main() {
-    println!("Implement me!");
+    let f: Fact<Vec<i32>> = Fact::new();
+    println!("Fact about Vec: {}", f.fact());
+    println!("Fact about Vec: {}", f.fact());
 }

README.md

@@ -79,7 +79,7 @@ Each step must be performed as a separate [PR (pull request)][PR] with an approp
     - [ ] [1.6. Static and dynamic dispatch][Step 1.6] (1 day)
     - [ ] [1.7. `Sized` and `?Sized` types][Step 1.7] (1 day)
     - [ ] [1.8. Thread safety][Step 1.8] (1 day)
-    - [ ] [1.9. Phantom types][Step 1.9] (1 day)
+    - [x] [1.9. Phantom types][Step 1.9] (1 day)
 - [ ] [2. Idioms][Step 2] (2 days, after all sub-steps)
     - [ ] [2.1. Rich types ensure correctness][Step 2.1] (1 day)
     - [ ] [2.2. Swapping values with `mem::replace`][Step 2.2] (1 day)