Question 14: auto-ref in trait method call

questions/014-trait-autoref.md

@@ -0,0 +1,28 @@
+Answer: 10
+Difficulty: 1
+
+# Hint
+
+Trait method auto-ref is covered in [this excellent Stack Overflow answer][SO].
+
+[SO]: https://stackoverflow.com/a/28552082/6086311
+
+# Explanation
+
+Trait impls anywhere in a program are always in scope, so there is no
+significance to the `impl Trait for char` being written inside of a block of
+code. In particular, that impl is visible throughout the whole program, not just
+within the block containing the impl.
+
+This question relates to the behavior of trait method auto-ref which is covered
+in [this Stack Overflow answer][SO].
+
+[SO]: https://stackoverflow.com/a/28552082/6086311
+
+The call to `0.is_reference()` observes that there is no implementation of
+`Trait` for an integer type that we could call directly. Method resolution
+inserts an auto-ref, effectively evaluating `(&0).is_reference()`. This time the
+call matches `impl<'a, T> Trait for &'a T` and prints `1`.
+
+The call to `'?'.is_reference()` instead finds `impl Trait for char`, printing
+`0`.

questions/014-trait-autoref.rs

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+trait Trait: Sized {
+    fn is_reference(self) -> bool;
+}
+
+impl<'a, T> Trait for &'a T {
+    fn is_reference(self) -> bool {
+        true
+    }
+}
+
+fn main() {
+    match 0.is_reference() {
+        true => print!("1"),
+        false => print!("0"),
+    }
+
+    match '?'.is_reference() {
+        true => print!("1"),
+        false => {
+            impl Trait for char {
+                fn is_reference(self) -> bool {
+                    false
+                }
+            }
+            print!("0")
+        }
+    }
+}