Add example for underscore syntax with generics (type-inference.md)

src/basic-syntax/type-inference.md

@@ -20,3 +20,23 @@ fn main() {
     // takes_u32(y);
 }
 ```
+
+<details>
+
+This slide demonstrates how the Rust compiler infers types based on constraints given by variable declarations and usages.
+
+The following code tells the compiler to copy into a certain generic container without the code ever explicitly specifying the contained type, using `_` as a placeholder:
+
+```rust,editable
+fn main() {
+    let mut v = Vec::new();
+    v.push((10, false));
+    v.push((20, true));
+    println!("v: {v:?}");
+
+    let vv = v.iter().collect::<std::collections::HashSet<_>>();
+    println!("vv: {vv:?}");
+}
+```
+
+</details>