Cover Supertraits, Generic Traits (#1854)

src/generics/generic-traits.md

@@ -0,0 +1,52 @@
+---
+Minutes: 5
+---
+
+# Generic Traits
+
+Traits can also be generic, just like types and functions. A trait's parameters
+get concrete types when it is used.
+
+```rust,editable
+#[derive(Debug)]
+struct Foo(String);
+
+impl From<u32> for Foo {
+    fn from(from: u32) -> Foo {
+        Foo(format!("Converted from integer: {from}"))
+    }
+}
+
+impl From<bool> for Foo {
+    fn from(from: bool) -> Foo {
+        Foo(format!("Converted from bool: {from}"))
+    }
+}
+
+fn main() {
+    let from_int = Foo::from(123);
+    let from_bool = Foo::from(true);
+    println!("{from_int:?}, {from_bool:?}");
+}
+```
+
+<details>
+
+- The `From` trait will be covered later in the course, but its
+  [definition in the `std` docs](https://doc.rust-lang.org/std/convert/trait.From.html)
+  is simple.
+
+- Implementations of the trait do not need to cover all possible type
+  parameters. Here, `Foo::From("hello")` would not compile because there is no
+  `From<&str>` implementation for `Foo`.
+
+- Generic traits take types as "input", while associated types are a kind of
+  "output type. A trait can have multiple implementations for different input
+  types.
+
+- In fact, Rust requires that at most one implementation of a trait match for
+  any type T. Unlike some other languages, Rust has no heuristic for choosing
+  the "most specific" match. There is work on adding this support, called
+  [specialization](https://rust-lang.github.io/rfcs/1210-impl-specialization.html).
+
+</details>