[book] document example of manual arbitrary impl (#514)

Co-authored-by: Cameron <[email protected]>

book/src/proptest/tutorial/arbitrary.md

@@ -10,5 +10,86 @@ In some cases, where it makes sense to define a canonical strategy, such as in
 the [JSON AST example](recursive.md), it is a good idea to implement
 `Arbitrary`.
 
+
+## Deriving `Arbitrary`
+
 The experimental [`proptest-derive` crate](../../proptest-derive/index.md) can
-be used to automate implementing `Arbitrary` in common cases.
+be used to automate implementing `Arbitrary` in common cases. For example, imagine we have a struct that represents a point in a 2-D coordinate space:
+```rust
+#[derive(Debug)]
+struct Point {
+    x: i32,
+    y: i32,
+}
+```
+This struct has the property that any pair of valid `i32`s can make a valid `Point`, so that is perfect for using `#[derive(Arbitrary)]`. 
+
+## Manual `Arbitrary` implementations
+
+Sometimes, however, there are extra constraints that your type has, which the derive macro can't understand. In these cases, you'll need to implement `Arbitrary` for your type manually.
+
+For example, consider this struct which represents a range (note, the derive API is can actually represent this case, it's just an example):
+```rust
+#[derive(Debug)]
+struct Range {
+    lower: i32,
+    upper: i32,
+}
+
+impl Range {
+    pub fn new(lower: i32, upper: i32) -> Option<Self> {
+        if lower <= upper {
+            Some(Self { lower, upper })
+        } else {
+            None
+        }
+    }
+}
+```
+This struct has an invariant: `lower <= upper`. However, if we derive an `Arbitrary` implementation naively, it might generate `Range { lower: 1, upper: 0 }`.
+
+Instead, we can write a manual implementation:
+```rust
+impl Arbitrary for Range {
+    type Parameters = ();
+    type Strategy = FilterMap<StrategyFor<(i32, i32)>, fn((i32, i32)) -> Option<Self>>;
+
+    fn arbitrary_with(_parameters: Self::Parameters) -> Self::Strategy {
+        any::<(i32, i32)>()  // generate 2 arbitrary i32s
+            .prop_map(|(a, b)| {
+                let (lower, upper) = if a < b {
+                    (a, b)
+                } else {
+                    (b, a)
+                };
+                Range::new(lower, upper).unwrap()
+            })
+    }
+}
+```
+Here, there are three items we need to define:
+ - `type Parameters` - the type of any parameters to `arbitrary_with`. Here (and in many cases), we don't need this, so `()` is used.
+ - `type Strategy` - the type of the strategy produced
+ - `fn arbitrary_with` - the code that creates the canonical `Strategy` for this type
+
+It's important to consider what type you want to use for `Strategy`. Here, we explicitly write the type out. This uses static dispatch, which is often faster and easier to optimize, but has a few downsides:
+ - you need to write out the type of the strategy. Even for this small function, it's a pretty lengthy function signature. In the worst case, it's impossible, since some types are unnameable (e.g. closures which capture their environment)
+ - it makes the implementation of `arbitrary_with` a part of your public API signature (if you expose `Arbitrary` impls in general from your crate). This means that changes to the implementation may require a breaking change.
+
+There are a couple of ways around this:
+ - heap-allocate the strategy by: 
+   - returning `BoxedStrategy<T>`
+   - calling `.boxed()` on the strategy before returning it
+ - use the nightly-only `#![feature(type_alias_impl_trait)]`:
+```rust
+type RangeStrategy = impl Strategy<Value = Range>;
+
+impl Arbitrary for Range {
+    type Parameters = ();
+    type Strategy = RangeStrategy;
+    // ...
+}
+```
+
+Using `BoxedStrategy` will incur some performance penalty relating to a heap allocation as well as dynamic dispatch, but it works on stable (as of November 2022).
+