TRPL: UFCS

AKA, two four-letter acronyms 😉

Author: steveklabnik

src/doc/trpl/ufcs.md

@@ -1,3 +1,128 @@
 % Universal Function Call Syntax
 
-Coming soon
+Sometimes, functions can have the same names. Consider this code:
+
+```rust
+trait Foo {
+    fn f(&self);
+}
+
+trait Bar {
+    fn f(&self);
+}
+
+struct Baz;
+
+impl Foo for Baz {
+    fn f(&self) { println!("Baz’s impl of Foo"); }
+}
+
+impl Bar for Baz {
+    fn f(&self) { println!("Baz’s impl of Bar"); }
+}
+
+let b = Baz;
+```
+
+If we were to try to call `b.f()`, we’d get an error:
+
+```text
+error: multiple applicable methods in scope [E0034]
+b.f();
+  ^~~
+note: candidate #1 is defined in an impl of the trait `main::Foo` for the type
+`main::Baz`
+    fn f(&self) { println!("Baz’s impl of Foo"); }
+    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+note: candidate #2 is defined in an impl of the trait `main::Bar` for the type
+`main::Baz`
+    fn f(&self) { println!("Baz’s impl of Bar"); }
+    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+```
+
+We need a way to disambiguate which method we need. This feature is called
+‘universal function call syntax’, and it looks like this:
+
+```rust
+# trait Foo {
+#     fn f(&self);
+# }
+# trait Bar {
+#     fn f(&self);
+# }
+# struct Baz;
+# impl Foo for Baz {
+#     fn f(&self) { println!("Baz’s impl of Foo"); }
+# }
+# impl Bar for Baz {
+#     fn f(&self) { println!("Baz’s impl of Bar"); }
+# }
+# let b = Baz;
+Foo::f(&b);
+Bar::f(&b);
+```
+
+Let’s break it down.
+
+```rust,ignore
+Foo::
+Bar::
+```
+
+These halves of the invocation are the types of the two traits: `Foo` and
+`Bar`. This is what ends up actually doing the disambiguation between the two:
+Rust calls the one from the trait name you use.
+
+```rust,ignore
+f(&b)
+```
+
+When we call a method like `b.f()` using [method syntax][methodsyntax], Rust
+will automatically borrow `b` if `f()` takes `&self`. In this case, Rust will
+not, and so we need to pass an explicit `&b`.
+
+[methodsyntax]: method-syntax.html
+
+# Angle-bracket Form
+
+The form of UFCS we just talked about:
+
+```rust,ignore
+Type::method(args);
+```
+
+Is a short-hand. There’s an expanded form of this that’s needed in some
+situations:
+
+```rust,ignore
+<Type as Trait>::method(args);
+```
+
+The `<>::` syntax is a means of providing a type hint. The type goes inside the
+`<>`s. In this case, the type is `Type as Trait`, indicating that we want
+`Trait`’s version of `method` to be called here. The `as Trait` part is
+optional if it’s not ambiguous, and if you’ve left it off, the angle brackets
+are too. Hence the shorter form.
+
+You need this version if you’re doing something like this:
+
+```rust
+trait Foo {
+    fn clone(&self);
+}
+
+#[derive(Clone)]
+struct Bar;
+
+impl Foo for Bar {
+    fn clone(&self) {
+        println!("Making a clone of Bar");
+        <Bar as Clone>::clone(self);
+    }
+}
+```
+
+The only way to choose `Clone::clone()` over `Foo::clone()` is to use this
+syntax.
+