terraform: user friendly error when using old map overrides

Related to #8036

We have had this behavior for a _long_ time now (since 0.7.0) but it
seems people are still periodically getting bit by it. This adds an
explicit error message that explains that this kind of override isn't
allowed anymore.

terraform/context_validate_test.go

@@ -44,6 +44,28 @@ func TestContext2Validate_badVar(t *testing.T) {
 	}
 }
 
+func TestContext2Validate_varMapOverrideOld(t *testing.T) {
+	m := testModule(t, "validate-module-pc-vars")
+	p := testProvider("aws")
+	c := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		Variables: map[string]interface{}{
+			"foo.foo": "bar",
+		},
+	})
+
+	w, e := c.Validate()
+	if len(w) > 0 {
+		t.Fatalf("bad: %#v", w)
+	}
+	if len(e) == 0 {
+		t.Fatalf("bad: %s", e)
+	}
+}
+
 func TestContext2Validate_varNoDefaultExplicitType(t *testing.T) {
 	m := testModule(t, "validate-var-no-default-explicit-type")
 	c := testContext2(t, &ContextOpts{

terraform/semantics.go

@@ -2,6 +2,7 @@ package terraform
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/config"
@@ -96,6 +97,21 @@ func smcUserVariables(c *config.Config, vs map[string]interface{}) []error {
 
 	// Check that types match up
 	for name, proposedValue := range vs {
+		// Check for "map.key" fields. These stopped working with Terraform
+		// 0.7 but we do this to surface a better error message informing
+		// the user what happened.
+		if idx := strings.Index(name, "."); idx > 0 {
+			key := name[:idx]
+			if _, ok := cvs[key]; ok {
+				errs = append(errs, fmt.Errorf(
+					"%s: Overriding map keys with the format `name.key` is no "+
+						"longer allowed. You may still override keys by setting "+
+						"`name = { key = value }`. The maps will be merged. This "+
+						"behavior appeared in 0.7.0.", name))
+				continue
+			}
+		}
+
 		schema, ok := cvs[name]
 		if !ok {
 			continue

terraform/semantics_test.go

@@ -24,7 +24,7 @@ func TestSMCUserVariables(t *testing.T) {
 		"foo":     "bar",
 		"map.foo": "baz",
 	})
-	if len(errs) != 0 {
+	if len(errs) == 0 {
 		t.Fatalf("err: %#v", errs)
 	}
 

terraform/test-fixtures/validate-var-map-override-old/main.tf

@@ -0,0 +1 @@
+variable "foo" { default = { foo = "bar" } }