Error if wit file is found for package

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

Author: rylev

crates/wac-parser/src/ast/import.rs

@@ -118,7 +118,7 @@ impl<'a> Parse<'a> for ImportType<'a> {
     }
 }
 
-/// Represents a package path in the AST.
+/// AST representation of a path to an item such as a world in a package (e.g. `foo:bar/qux`).
 #[derive(Debug, Clone, Serialize)]
 #[serde(rename_all = "camelCase")]
 pub struct PackagePath<'a> {

crates/wac-parser/src/resolution/ast.rs

@@ -184,6 +184,7 @@ impl<'a> AstResolver<'a> {
         // If there's a target world in the directive, validate the composition
         // conforms to the target
         if let Some(path) = &self.document.directive.targets {
+            log::debug!("validating composition targets world `{}`", path.string);
             let item = self.resolve_package_export(&mut state, path)?;
             match item {
                 ItemKind::Type(Type::World(world)) => {
@@ -1563,6 +1564,7 @@ impl<'a> AstResolver<'a> {
         state: &mut State<'a>,
         path: &'a ast::PackagePath<'a>,
     ) -> ResolutionResult<ItemKind> {
+        log::debug!("resolving package export `{}`", path.string);
         // Check for reference to local item
         if path.name == self.document.directive.package.name {
             return self.resolve_local_export(state, path);

crates/wac-resolver/src/fs.rs

@@ -114,6 +114,15 @@ impl FileSystemPackageResolver {
                 );
 
                 continue;
+            } else if path.extension().and_then(std::ffi::OsStr::to_str) == Some("wit") {
+                return Err(Error::PackageResolutionFailure {
+                    name: key.name.to_string(),
+                    span: *span,
+                    source: anyhow!(
+                        "WIT packages must be directories, not files: `{path}`",
+                        path = path.display()
+                    ),
+                });
             }
 
             if !path.is_file() {