Ensure that every item we constrain has a set of used template parameters

This is a follow up to c8a206a, and the support for blacklisting in the named
template parameter usage analysis. This ensures that ever item we ever call
`constrain` on has an entry in `used` for the set of template parameters it
uses. Additionally, it adds extra assertions to enforce the invariant.

We cannot completely avoid analyzing blacklisted items because we want to
consider all of a blacklisted template's parameters as used. This is why we
ensure that blacklisted items have a used template parameter set rather than
ensuring that blacklisted items never end up in the worklist.

Author: fitzgen

src/ir/named.rs

@@ -322,12 +322,14 @@ impl<'ctx, 'gen> MonotoneFramework for UsedTemplateParameters<'ctx, 'gen> {
 
         for item in whitelisted_items.iter().cloned() {
             dependencies.entry(item).or_insert(vec![]);
-            used.insert(item, Some(ItemSet::new()));
+            used.entry(item).or_insert(Some(ItemSet::new()));
 
             {
                 // We reverse our natural IR graph edges to find dependencies
                 // between nodes.
                 item.trace(ctx, &mut |sub_item, _| {
+                    used.entry(sub_item).or_insert(Some(ItemSet::new()));
+
                     // We won't be generating code for items that aren't
                     // whitelisted, so don't bother keeping track of their
                     // template parameters. But isn't whitelisting the
@@ -353,12 +355,17 @@ impl<'ctx, 'gen> MonotoneFramework for UsedTemplateParameters<'ctx, 'gen> {
                     &TypeKind::TemplateInstantiation(ref inst) => {
                         let decl = ctx.resolve_type(inst.template_definition());
                         let args = inst.template_arguments();
+
                         // Although template definitions should always have
                         // template parameters, there is a single exception:
                         // opaque templates. Hence the unwrap_or.
                         let params = decl.self_template_params(ctx)
                             .unwrap_or(vec![]);
+
                         for (arg, param) in args.iter().zip(params.iter()) {
+                            used.entry(*arg).or_insert(Some(ItemSet::new()));
+                            used.entry(*param).or_insert(Some(ItemSet::new()));
+
                             dependencies.entry(*arg)
                                 .or_insert(vec![])
                                 .push(*param);
@@ -368,6 +375,28 @@ impl<'ctx, 'gen> MonotoneFramework for UsedTemplateParameters<'ctx, 'gen> {
                 });
         }
 
+        if cfg!(feature = "testing_only_extra_assertions") {
+            // Invariant: The `used` map has an entry for every whitelisted
+            // item, as well as all explicitly blacklisted items that are
+            // reachable from whitelisted items.
+            //
+            // (This is so that every item we call `constrain` on is guaranteed
+            // to have a set of template parameters, and we can allow
+            // blacklisted templates to use all of their parameters).
+            for item in whitelisted_items.iter() {
+                extra_assert!(used.contains_key(item));
+                item.trace(ctx, &mut |sub_item, _| {
+                    extra_assert!(used.contains_key(&sub_item));
+                }, &())
+            }
+
+            // Invariant: the `dependencies` map has an entry for every
+            // whitelisted item.
+            for item in whitelisted_items.iter() {
+                extra_assert!(dependencies.contains_key(item));
+            }
+        }
+
         UsedTemplateParameters {
             ctx: ctx,
             used: used,