[fine] Test for function return type mismatch

fine/build.rs

@@ -87,6 +87,11 @@ fn generate_test_for_file(path: PathBuf) -> String {
             assertions.push(quote! {
                 crate::assert_eval_ok(&_tree, &_lines, #expected);
             });
+        } else if let Some(line) = line.strip_prefix("@check-error:") {
+            let expected = line.trim();
+            assertions.push(quote! {
+                crate::assert_check_error(&_tree, &_lines, #expected);
+            });
         } else if line.starts_with("@") {
             panic!("Test file {display_path} has unknown directive: {line}");
         }

fine/src/semantics.rs

@@ -664,11 +664,6 @@ impl<'a> Semantics<'a> {
 
         // NOTE: These return `None` if they encounter some problem.
         let result = result.unwrap_or(Type::Error);
-
-        if result.is_error() {
-            eprintln!("OH NO AN ERROR AT {}: {:?}", t.index(), tree);
-        }
-
         self.types.borrow_mut()[t.index()] = Incremental::Complete(result.clone());
         result
     }

fine/tests/example_tests.rs

@@ -259,7 +259,6 @@ fn assert_no_errors(tree: &SyntaxTree, lines: &Lines) {
     );
 }
 
-#[allow(dead_code)]
 fn assert_eval_ok(tree: &SyntaxTree, lines: &Lines, expected: &str) {
     let semantics = Semantics::new(tree, lines);
 
@@ -284,4 +283,17 @@ fn assert_eval_ok(tree: &SyntaxTree, lines: &Lines, expected: &str) {
     }
 }
 
+fn assert_check_error(tree: &SyntaxTree, lines: &Lines, expected: &str) {
+    let semantics = Semantics::new(tree, lines);
+    check(&semantics);
+
+    let errors = semantics.snapshot_errors();
+    semantic_assert!(
+        &semantics,
+        None,
+        errors.iter().any(|e| e.message == expected),
+        "Unable to find the expected error message '{expected}'"
+    );
+}
+
 include!(concat!(env!("OUT_DIR"), "/generated_tests.rs"));

fine/tests/expression/errors/return_type_mismatch.fine

@@ -0,0 +1,5 @@
+fun test() -> bool {
+  32
+}
+
+// @check-error: the body of this function yields a value of type `f64`, but callers expect this function to produce a `bool`