bug fixes and performance improvements

src/pipeline/import_resolution/alias_map.rs

@@ -0,0 +1,53 @@
+use hashbrown::{HashMap, HashSet};
+
+use std::hash::Hash;
+
+use crate::interner::Token;
+
+#[derive(Clone, Debug, Default)]
+pub struct AliasMap{
+  pub targets: HashMap<Token<Vec<Token<String>>>, Token<Vec<Token<String>>>>,
+  pub aliases: HashMap<Token<Vec<Token<String>>>, HashSet<Token<Vec<Token<String>>>>>,
+}
+impl AliasMap {
+  pub fn new() -> Self {Self::default()}
+
+  pub fn link(&mut self, alias: Token<Vec<Token<String>>>, target: Token<Vec<Token<String>>>) {
+    let prev = self.targets.insert(alias, target);
+    debug_assert!(prev.is_none(), "Alias already has a target");
+    multimap_entry(&mut self.aliases, &target).insert(alias);
+    // Remove aliases of the alias
+    if let Some(alts) = self.aliases.remove(&alias) {
+      for alt in alts {
+        // Assert that this step has always been done in the past
+        debug_assert!(
+          self.aliases.get(&alt)
+            .map(HashSet::is_empty)
+            .unwrap_or(true),
+          "Alias set of alias not empty"
+        );
+        debug_assert!(
+          self.targets.insert(alt, target) == Some(alias),
+          "Name not target of its own alias"
+        );
+        multimap_entry(&mut self.aliases, &target).insert(alt);
+      }
+    }
+  }
+
+  pub fn resolve(&self, alias: Token<Vec<Token<String>>>) -> Option<Token<Vec<Token<String>>>> {
+    self.targets.get(&alias).copied()
+  }
+}
+
+/// find or create the set belonging to the given key in the given
+/// map-to-set (aka. multimap)
+fn multimap_entry<'a, K: Eq + Hash + Clone, V>(
+  map: &'a mut HashMap<K, HashSet<V>>,
+  key: &'_ K
+) -> &'a mut HashSet<V> {
+  map.raw_entry_mut()
+    .from_key(key)
+    .or_insert_with(|| (key.clone(), HashSet::new()))
+    .1
+}