fix: Improve error message when no .prql files exist (#4424)

prqlc/prqlc/src/parser.rs

@@ -62,6 +62,15 @@ fn linearize_tree(tree: &SourceTree) -> Result<Vec<SourceFile>> {
     } else if let Some(root) = tree.sources.keys().find(path_starts_with_uppercase) {
         root_path = root;
     } else {
+        if tree.sources.is_empty() {
+            // TODO: should we allow non `.prql` files? We could require `.prql`
+            // for modules but then allow any file if a single file is passed
+            // (python allows this, for example)
+            return Err(Error::new_simple(
+                "No `.prql` files found in the source tree",
+            ));
+        }
+
         let file_names = tree
             .sources
             .keys()

prqlc/prqlc/tests/integration/cli.rs

@@ -458,3 +458,16 @@ fn normalize_prqlc(cmd: &mut Command) -> &mut Command {
         .env_remove("RUST_BACKTRACE")
         .env_remove("RUST_LOG")
 }
+
+#[test]
+fn compile_no_prql_files() {
+    assert_cmd_snapshot!(prqlc_command().args(["compile", "README.md"]), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+    Error: No `.prql` files found in the source tree
+
+    "###);
+}