Turn missing body in a binding into a syntax error (#6107)

Test and fix for #5903.

engine/runtime/src/main/java/org/enso/compiler/TreeToIr.java

@@ -516,6 +516,9 @@ private IR.Expression translateFunction(Tree fun, IR.Name name, java.util.List<A
             block.copy$default$7()
           );
         }
+        if (body == null) {
+          body = translateSyntaxError(fun, IR$Error$Syntax$UnexpectedExpression$.MODULE$);
+        }
         return new IR$Expression$Binding(name, body,
           getIdentifiedLocation(fun), meta(), diag()
         );
@@ -842,6 +845,9 @@ yield switch (op.codeRepr()) {
       case Tree.Assignment assign -> {
         var name = buildNameOrQualifiedName(assign.getPattern());
         var expr = translateExpression(assign.getExpr(), false);
+        if (expr == null) {
+          expr = translateSyntaxError(assign, IR$Error$Syntax$UnexpectedExpression$.MODULE$);
+        }
         yield new IR$Expression$Binding(name, expr, getIdentifiedLocation(tree), meta(), diag());
       }
       case Tree.ArgumentBlockApplication body -> {

engine/runtime/src/test/java/org/enso/compiler/ExecCompilerTest.java

@@ -4,6 +4,7 @@
 import java.nio.file.Paths;
 import org.enso.polyglot.RuntimeOptions;
 import org.graalvm.polyglot.Context;
+import org.graalvm.polyglot.PolyglotException;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -46,4 +47,21 @@ public void testCaseOfWithNegativeConstant() throws Exception {
     var none = run.execute(33);
     assertEquals("none", none.asString());
   }
+
+  @Test
+  public void testHalfAssignment() throws Exception {
+    var module = ctx.eval("enso", """
+    run value =
+        x = 4
+        y =
+        z = 5
+    """);
+    var run = module.invokeMember("eval_expression", "run");
+    try {
+        var never = run.execute(-1);
+        fail("Unexpected result: " + never);
+    } catch (PolyglotException ex) {
+        assertEquals("Syntax error: Unexpected expression.", ex.getMessage());
+    }
+  }
 }