refactor crate loading

src/context.ts

@@ -0,0 +1,49 @@
+import { Crate, DepCrate, Final, Item, ItemId, Phase } from "./ast";
+import { DUMMY_SPAN, Span } from "./error";
+import { Ids, unwrap } from "./utils";
+
+export type CrateLoader = (
+  gcx: GlobalContext,
+  name: string,
+  span: Span
+) => DepCrate;
+
+/**
+ * The global context containing information about the _global compilation session_,
+ * like loaded crates.
+ * Notably, the global context is _not_ supposed to have information specific to the "local crate",
+ * because with the current compilation model, there is no "local crate" in a session.
+ *
+ * There is a "downstream"/"binary"/"final" crate with crateId=0, where `function main()` lives, but
+ * dependencies (which also use the same context) do not care about that.
+ */
+export class GlobalContext {
+  public depCrates: Crate<Final>[] = [];
+  public crateId: Ids = new Ids();
+
+  constructor(public crateLoader: CrateLoader) {}
+
+  public findItem<P extends Phase>(
+    id: ItemId,
+    localCrate: Crate<P>
+  ): Item<P | Final> {
+    const crate = unwrap(
+      [localCrate, ...this.depCrates].find((crate) => crate.id === id.crateId)
+    );
+
+    if (id.itemIdx === 0) {
+      // Return a synthetic module representing the crate root.
+      return {
+        kind: "mod",
+        node: {
+          contents: crate.rootItems,
+          name: crate.packageName,
+        },
+        span: DUMMY_SPAN,
+        id,
+      };
+    }
+
+    return unwrap(crate.itemsById.get(id));
+  }
+}