config: test that resources reference good providers

config/config.go

@@ -430,6 +430,15 @@ func (c *Config) Validate() error {
 			}
 		}
 
+		// Verify provider points to a provider that is configured
+		if r.Provider != "" {
+			if _, ok := providerSet[r.Provider]; !ok {
+				errs = append(errs, fmt.Errorf(
+					"%s: resource depends on non-configured provider '%s'",
+					n, r.Provider))
+			}
+		}
+
 		// Verify provisioners don't contain any splats
 		for _, p := range r.Provisioners {
 			// This validation checks that there are now splat variables

config/config_test.go

@@ -217,6 +217,20 @@ func TestConfigValidate_providerMultiGood(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_providerMultiRefGood(t *testing.T) {
+	c := testConfig(t, "validate-provider-multi-ref-good")
+	if err := c.Validate(); err != nil {
+		t.Fatalf("should be valid: %s", err)
+	}
+}
+
+func TestConfigValidate_providerMultiRefBad(t *testing.T) {
+	c := testConfig(t, "validate-provider-multi-ref-bad")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_provConnSplatOther(t *testing.T) {
 	c := testConfig(t, "validate-prov-conn-splat-other")
 	if err := c.Validate(); err != nil {

config/test-fixtures/validate-provider-multi-ref-bad/main.tf

@@ -0,0 +1,7 @@
+provider "aws" {
+    alias = "bar"
+}
+
+resource "aws_instance" "foo" {
+    provider = "aws.baz"
+}

config/test-fixtures/validate-provider-multi-ref-good/main.tf

@@ -0,0 +1,7 @@
+provider "aws" {
+    alias = "bar"
+}
+
+resource "aws_instance" "foo" {
+    provider = "aws.bar"
+}