error gracefully when @main is missing (#346)

src/ast.rs

@@ -505,18 +505,17 @@ impl Book {
     }
     let mut book = hvm::Book { defs: Vec::new() };
     for (fid, name) in &fid_to_name {
-      if let Some(ast_def) = self.defs.get(name) {
-        let mut def = hvm::Def {
-          name: name.clone(),
-          safe: true,
-          root: hvm::Port(0),
-          rbag: vec![],
-          node: vec![],
-          vars: 0,
-        };
-        ast_def.build(&mut def, &name_to_fid, &mut BTreeMap::new());
-        book.defs.push(def);
-      }
+      let ast_def = self.defs.get(name).expect("missing `@main` definition");
+      let mut def = hvm::Def {
+        name: name.clone(),
+        safe: true,
+        root: hvm::Port(0),
+        rbag: vec![],
+        node: vec![],
+        vars: 0,
+      };
+      ast_def.build(&mut def, &name_to_fid, &mut BTreeMap::new());
+      book.defs.push(def);
     }
     return book;
   }

tests/run.rs

@@ -60,7 +60,7 @@ fn execute_hvm(args: &[&OsStr]) -> Result<String, Box<dyn Error>> {
   stderr.read_to_string(&mut output)?;
 
   Ok(if !status.success() {
-    format!("exited with code {status}:\n{output}")
+    format!("{status}\n{output}")
   } else {
     parse_output(&output).unwrap_or_else(|err| {
       panic!("error parsing output:\n{err}\n\n{output}")

tests/snapshots/run__file@empty.hvm.snap

@@ -0,0 +1,9 @@
+---
+source: tests/run.rs
+expression: rust_output
+input_file: tests/programs/empty.hvm
+---
+exit status: 101
+thread 'main' panicked at src/ast.rs:508:41:
+missing `@main` definition
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace