Add product type to the Rule trait to simplify unit testing. (#10720)

### Problem

The `Rule` trait did not require that a `Rule` declare its own return type, which was ... strange. It also made unit testing more awkward, because rules needed to be declared in a type->rules map rather than a flat list.

### Solution

Add `Rule::product`, and simplify tests.

[ci skip-build-wheels]

src/rust/engine/rule_graph/src/builder.rs

@@ -171,29 +171,37 @@ type MonomorphizedGraph<R> = DiGraph<
 /// Given the set of Rules and Queries, produce a RuleGraph that allows dependency nodes
 /// to be found statically.
 ///
-pub struct Builder<'t, R: Rule> {
-  rules: &'t BTreeMap<R::TypeId, Vec<R>>,
+pub struct Builder<R: Rule> {
+  rules: BTreeMap<R::TypeId, Vec<R>>,
   queries: Vec<Query<R>>,
   params: ParamTypes<R::TypeId>,
 }
 
-impl<'t, R: Rule> Builder<'t, R> {
-  pub fn new(rules: &'t BTreeMap<R::TypeId, Vec<R>>, queries: Vec<Query<R>>) -> Builder<'t, R> {
+impl<R: Rule> Builder<R> {
+  pub fn new(rules: Vec<R>, queries: Vec<Query<R>>) -> Builder<R> {
+    // Group rules by product/return type.
+    let mut rules_by_type = BTreeMap::new();
+    for rule in rules {
+      rules_by_type
+        .entry(rule.product())
+        .or_insert_with(Vec::new)
+        .push(rule);
+    }
     // The set of all input Params in the graph: ie, those provided either via Queries, or via
     // a Rule with a DependencyKey that provides a Param.
     let params = queries
       .iter()
       .flat_map(|query| query.params.iter().cloned())
       .chain(
-        rules
+        rules_by_type
           .values()
           .flatten()
           .flat_map(|rule| rule.dependency_keys())
           .filter_map(|dk| dk.provided_param()),
       )
       .collect::<ParamTypes<_>>();
     Builder {
-      rules,
+      rules: rules_by_type,
       queries,
       params,
     }

src/rust/engine/rule_graph/src/lib.rs

@@ -30,7 +30,7 @@
 mod builder;
 mod rules;
 
-use std::collections::{BTreeMap, HashMap, HashSet};
+use std::collections::{HashMap, HashSet};
 use std::io;
 
 pub use crate::builder::Builder;
@@ -243,10 +243,7 @@ fn entry_with_deps_str<R: Rule>(entry: &EntryWithDeps<R>) -> String {
 }
 
 impl<R: Rule> RuleGraph<R> {
-  pub fn new(
-    rules: &BTreeMap<R::TypeId, Vec<R>>,
-    queries: Vec<Query<R>>,
-  ) -> Result<RuleGraph<R>, String> {
+  pub fn new(rules: Vec<R>, queries: Vec<Query<R>>) -> Result<RuleGraph<R>, String> {
     Builder::new(rules, queries).graph()
   }
 

src/rust/engine/rule_graph/src/rules.rs

@@ -77,6 +77,11 @@ pub trait Rule: Clone + Debug + Display + Hash + Eq + Sized + DisplayForGraph +
   type TypeId: TypeId;
   type DependencyKey: DependencyKey<TypeId = Self::TypeId>;
 
+  ///
+  /// Returns the product (output) type for this Rule.
+  ///
+  fn product(&self) -> Self::TypeId;
+
   ///
   /// Return keys for the dependencies of this Rule.
   ///