core: even more nil checks to catch missing objects

These are all things that ought to be present in normal use but can end up
being nil in incorrect tests. Test debugging is simpler if these things
return errors gracefully, rather than crashing.

terraform/eval_diff.go

@@ -97,6 +97,10 @@ func (n *EvalDiff) Eval(ctx EvalContext) (interface{}, error) {
 	provider := *n.Provider
 	providerSchema := *n.ProviderSchema
 
+	if providerSchema == nil {
+		return nil, fmt.Errorf("provider schema is unavailable for %s", n.Addr)
+	}
+
 	var diags tfdiags.Diagnostics
 
 	// The provider and hook APIs still expect our legacy InstanceInfo type.

terraform/eval_provider.go

@@ -59,6 +59,9 @@ func (n *EvalConfigProvider) Eval(ctx EvalContext) (interface{}, error) {
 		diags = diags.Append(err)
 		return nil, diags.NonFatalErr()
 	}
+	if schema == nil {
+		return nil, fmt.Errorf("schema not available for %s", n.Addr)
+	}
 
 	configSchema := schema.Provider
 	configBody := buildProviderConfig(ctx, n.Addr, config.Config)

terraform/eval_read_data.go

@@ -30,6 +30,10 @@ type EvalReadDataDiff struct {
 func (n *EvalReadDataDiff) Eval(ctx EvalContext) (interface{}, error) {
 	// TODO: test
 
+	if n.ProviderSchema == nil || *n.ProviderSchema == nil {
+		return nil, fmt.Errorf("provider schema not available for %s", n.Addr)
+	}
+
 	var diags tfdiags.Diagnostics
 
 	// The provider and hook APIs still expect our legacy InstanceInfo type.

terraform/eval_validate.go

@@ -83,6 +83,9 @@ func (n *EvalValidateProvider) Eval(ctx EvalContext) (interface{}, error) {
 		diags = diags.Append(err)
 		return nil, diags.NonFatalErr()
 	}
+	if schema == nil {
+		return nil, fmt.Errorf("no schema is available for %s", n.Addr)
+	}
 
 	configSchema := schema.Provider
 	configBody := buildProviderConfig(ctx, n.Addr, config.Config)
@@ -319,7 +322,7 @@ type EvalValidateResource struct {
 }
 
 func (n *EvalValidateResource) Eval(ctx EvalContext) (interface{}, error) {
-	if n.ProviderSchema == nil {
+	if n.ProviderSchema == nil || *n.ProviderSchema == nil {
 		return nil, fmt.Errorf("EvalValidateResource has nil schema for %s", n.Addr)
 	}
 

terraform/node_resource_abstract.go

@@ -193,8 +193,10 @@ func (n *NodeAbstractResource) References() []*addrs.Reference {
 				if p.When != configs.ProvisionerWhenCreate {
 					continue
 				}
-				refs, _ = lang.ReferencesInBlock(p.Connection.Config, connectionBlockSupersetSchema)
-				result = append(result, refs...)
+				if p.Connection != nil {
+					refs, _ = lang.ReferencesInBlock(p.Connection.Config, connectionBlockSupersetSchema)
+					result = append(result, refs...)
+				}
 
 				schema := n.ProvisionerSchemas[p.Type]
 				refs, _ = lang.ReferencesInBlock(p.Config, schema)

terraform/node_resource_destroy.go

@@ -83,7 +83,9 @@ func (n *NodeDestroyResourceInstance) References() []*addrs.Reference {
 			schema := n.ProvisionerSchemas[p.Type]
 
 			if p.When == configs.ProvisionerWhenDestroy {
-				result = append(result, ReferencesFromConfig(p.Connection.Config, connectionBlockSupersetSchema)...)
+				if p.Connection != nil {
+					result = append(result, ReferencesFromConfig(p.Connection.Config, connectionBlockSupersetSchema)...)
+				}
 				result = append(result, ReferencesFromConfig(p.Config, schema)...)
 			}
 		}

terraform/terraform_test.go

@@ -93,7 +93,9 @@ func testModule(t *testing.T, name string) *configs.Config {
 	// sources only this ultimately just records all of the module paths
 	// in a JSON file so that we can load them below.
 	diags := loader.InstallModules(dir, true, configload.InstallHooksImpl{})
-	t.Fatal(diags.Error())
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
 
 	config, diags := loader.LoadConfig(dir)
 	if diags.HasErrors() {