terraform: improve error messages to assist REPL

repl/session_test.go

@@ -25,6 +25,21 @@ func TestSession_basicState(t *testing.T) {
 					},
 				},
 			},
+
+			&terraform.ModuleState{
+				Path: []string{"root", "module"},
+				Resources: map[string]*terraform.ResourceState{
+					"test_instance.foo": &terraform.ResourceState{
+						Type: "test_instance",
+						Primary: &terraform.InstanceState{
+							ID: "bar",
+							Attributes: map[string]string{
+								"id": "bar",
+							},
+						},
+					},
+				},
+			},
 		},
 	}
 
@@ -64,6 +79,32 @@ func TestSession_basicState(t *testing.T) {
 			},
 		})
 	})
+
+	t.Run("missing module", func(t *testing.T) {
+		testSession(t, testSessionTest{
+			State: state,
+			Inputs: []testSessionInput{
+				{
+					Input:         "module.child.foo",
+					Error:         true,
+					ErrorContains: "Couldn't find module \"child\"",
+				},
+			},
+		})
+	})
+
+	t.Run("missing module output", func(t *testing.T) {
+		testSession(t, testSessionTest{
+			State: state,
+			Inputs: []testSessionInput{
+				{
+					Input:         "module.module.foo",
+					Error:         true,
+					ErrorContains: "Couldn't find output \"foo\"",
+				},
+			},
+		})
+	})
 }
 
 func TestSession_stateless(t *testing.T) {

terraform/interpolate.go

@@ -160,6 +160,13 @@ func (i *Interpolater) valueModuleVar(
 		// ensure that the module is in the state, so if we reach this
 		// point otherwise it really is a panic.
 		result[n] = unknownVariable()
+
+		// During apply this is always an error
+		if i.Operation == walkApply {
+			return fmt.Errorf(
+				"Couldn't find module %q for var: %s",
+				v.Name, v.FullKey())
+		}
 	} else {
 		// Get the value from the outputs
 		if outputState, ok := mod.Outputs[v.Field]; ok {
@@ -171,6 +178,13 @@ func (i *Interpolater) valueModuleVar(
 		} else {
 			// Same reasons as the comment above.
 			result[n] = unknownVariable()
+
+			// During apply this is always an error
+			if i.Operation == walkApply {
+				return fmt.Errorf(
+					"Couldn't find output %q for module var: %s",
+					v.Field, v.FullKey())
+			}
 		}
 	}