Working MILP version of predicate splitter

src/lang/error.rs

@@ -399,6 +399,16 @@ pub enum SplittingError {
         max_allowed: usize,
         suggestion: Option<Box<RefactorSuggestion>>,
     },
+
+    #[error("Could not split predicate '{predicate}' into a chain: no feasible partition exists with up to {max_links} links. \
+             The predicate's wildcard structure may be too dense for any chain to fit within max_statement_args ({max_statement_args}) \
+             and max_custom_predicate_wildcards ({max_wildcards}) per link.")]
+    Infeasible {
+        predicate: String,
+        max_links: usize,
+        max_statement_args: usize,
+        max_wildcards: usize,
+    },
 }
 
 impl From<ParseError> for LangError {

src/lang/frontend_ast.rs

@@ -134,6 +134,18 @@ pub struct StatementTmpl {
     pub span: Option<Span>,
 }
 
+impl StatementTmpl {
+    /// Names of all wildcards referenced by this statement's arguments,
+    /// in argument order with duplicates included.
+    pub fn wildcard_names(&self) -> impl Iterator<Item = &str> {
+        self.args.iter().filter_map(|arg| match arg {
+            StatementTmplArg::Wildcard(id) => Some(id.name.as_str()),
+            StatementTmplArg::AnchoredKey(ak) => Some(ak.root.name.as_str()),
+            StatementTmplArg::Literal(_) | StatementTmplArg::SelfPredicateHash(_) => None,
+        })
+    }
+}
+
 /// Reference to a predicate (local or qualified with module name)
 #[derive(Debug, Clone, PartialEq)]
 pub enum PredicateRef {

src/lang/frontend_ast_lower.rs

@@ -528,21 +528,9 @@ impl<'a> Lowerer<'a> {
         names: &mut Vec<String>,
         seen: &mut HashSet<String>,
     ) {
-        for arg in &stmt.args {
-            match arg {
-                StatementTmplArg::Wildcard(id) => {
-                    if !seen.contains(&id.name) {
-                        seen.insert(id.name.clone());
-                        names.push(id.name.clone());
-                    }
-                }
-                StatementTmplArg::AnchoredKey(ak) => {
-                    if !seen.contains(&ak.root.name) {
-                        seen.insert(ak.root.name.clone());
-                        names.push(ak.root.name.clone());
-                    }
-                }
-                StatementTmplArg::Literal(_) | StatementTmplArg::SelfPredicateHash(_) => {}
+        for name in stmt.wildcard_names() {
+            if seen.insert(name.to_string()) {
+                names.push(name.to_string());
             }
         }
     }