Merge pull request #1259 from adamchalmers/supertraits

Fix #90: Add supertraits and Fully Qualified syntax

src/SUMMARY.md

@@ -139,6 +139,8 @@
     - [Iterators](trait/iter.md)
     - [`impl Trait`](trait/impl_trait.md)
     - [Clone](trait/clone.md)
+    - [Supertraits](traits/supertraits.md)
+    - [Disambiguating overlapping traits](traits/disambiguating.md)
 
 - [macro_rules!](macros.md)
     - [Syntax](macros/syntax.md)

src/traits/disambiguating.md

@@ -0,0 +1,62 @@
+# Disambiguating overlapping traits
+
+A type can implement many different traits. What if two traits both require the same name? For example, many traits might have a method named `get()`. They might even have different return types! 
+
+Good news: because each trait implementation gets its own `impl` block, it's 
+clear which trait's `get` method you're implementing. 
+
+What about when it comes time to _call_ those methods? To disambiguate between
+them, we have to use Fully Qualified Syntax.
+
+```rust,editable
+trait UsernameWidget {
+    // Get the selected username out of this widget
+    fn get(&self) -> String;
+}
+
+trait AgeWidget {
+    // Get the selected age out of this widget
+    fn get(&self) -> u8;
+}
+
+// A form with both a UsernameWidget and an AgeWidget
+struct Form {
+    username: String,
+    age: u8,
+}
+
+impl UsernameWidget for Form {
+    fn get(&self) -> String {
+        self.username.clone()
+    }
+}
+
+impl AgeWidget for Form {
+    fn get(&self) -> u8 {
+        self.age
+    }
+}
+
+fn main() {
+    let form = Form{
+        username: "rustacean".to_owned(),
+        age: 28,
+    };
+
+    // If you uncomment this line, you'll get an error saying 
+    // "multiple `get` found". Because, after all, there are multiple methods
+    // named `get`.
+    // println!("{}", form.get());
+
+    let username = <Form as UsernameWidget>::get(&form);
+    assert_eq!("rustacean".to_owned(), username);
+    let age = <Form as AgeWidget>::get(&form);
+    assert_eq!(28, age);
+}
+```
+
+### See also:
+
+[The Rust Programming Language chapter on Fully Qualified syntax][trpl_fqsyntax]
+
+[trpl_fqsyntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name

src/traits/supertraits.md

@@ -0,0 +1,43 @@
+# Supertraits
+
+Rust doesn't have "inheritance", but you can define a trait as being a superset
+of another trait. For example:
+
+```rust,editable
+trait Person {
+    fn name(&self) -> String;
+}
+
+// Student is a supertrait of Person.
+// Implementing Student requires you to also impl Person.
+trait Student: Person {
+    fn university(&self) -> String;
+}
+
+trait Programmer {
+    fn fav_language(&self) -> String;
+}
+
+// CompSciStudent (computer science student) is a supertrait of both Programmer 
+// and Student. Implementing CompSciStudent requires you to impl both subtraits.
+trait CompSciStudent: Programmer + Student {
+    fn git_username(&self) -> String;
+}
+
+fn comp_sci_student_greeting(student: &dyn CompSciStudent) -> String {
+    format!(
+        "My name is {} and I attend {}. My Git username is {}",
+        student.name(),
+        student.university(),
+        student.git_username()
+    )
+}
+
+fn main() {}
+```
+
+### See also:
+
+[The Rust Programming Language chapter on supertraits][trpl_supertraits]
+
+[trpl_supertraits]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#using-supertraits-to-require-one-traits-functionality-within-another-trait