helper/schema: Force field names to be alphanum lowercase + underscores

helper/schema/schema.go

@@ -15,6 +15,7 @@ import (
 	"fmt"
 	"os"
 	"reflect"
+	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -661,14 +662,25 @@ func (m schemaMap) InternalValidate(topSchemaMap schemaMap) error {
 		if v.ValidateFunc != nil {
 			switch v.Type {
 			case TypeList, TypeSet:
-				return fmt.Errorf("ValidateFunc is not yet supported on lists or sets.")
+				return fmt.Errorf("%s: ValidateFunc is not yet supported on lists or sets.", k)
+			}
+		}
+
+		if v.Deprecated == "" && v.Removed == "" {
+			if !isValidFieldName(k) {
+				return fmt.Errorf("%s: Field name may only contain lowercase alphanumeric characters & underscores.", k)
 			}
 		}
 	}
 
 	return nil
 }
 
+func isValidFieldName(name string) bool {
+	re := regexp.MustCompile("^[a-z0-9_]+$")
+	return re.MatchString(name)
+}
+
 func (m schemaMap) diff(
 	k string,
 	schema *Schema,

helper/schema/schema_test.go

@@ -3353,6 +3353,48 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			},
 			true,
 		},
+
+		"invalid field name format #1": {
+			map[string]*Schema{
+				"with space": &Schema{
+					Type:     TypeString,
+					Optional: true,
+				},
+			},
+			true,
+		},
+
+		"invalid field name format #2": {
+			map[string]*Schema{
+				"WithCapitals": &Schema{
+					Type:     TypeString,
+					Optional: true,
+				},
+			},
+			true,
+		},
+
+		"invalid field name format of a Deprecated field": {
+			map[string]*Schema{
+				"WithCapitals": &Schema{
+					Type:       TypeString,
+					Optional:   true,
+					Deprecated: "Use with_underscores instead",
+				},
+			},
+			false,
+		},
+
+		"invalid field name format of a Removed field": {
+			map[string]*Schema{
+				"WithCapitals": &Schema{
+					Type:     TypeString,
+					Optional: true,
+					Removed:  "Use with_underscores instead",
+				},
+			},
+			false,
+		},
 	}
 
 	for tn, tc := range cases {