refine the skipFixup heuristic

We can also rule out some attribute types as indicating something other
than the legacy SDK.

- Tuple types were not generated at all.
- There were no single objects types, the convention was to use a block
  list or set of length 1.
- Maps of objects were not possible to generate, since named blocks were
  not implemented.
- Nested collections were not supported, but when they were generated they
  would have primitive types.

internal/lang/blocktoattr/fixup.go

@@ -1,6 +1,8 @@
 package blocktoattr
 
 import (
+	"log"
+
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/hcldec"
 	"github.com/hashicorp/terraform/internal/configs/configschema"
@@ -33,6 +35,10 @@ func FixUpBlockAttrs(body hcl.Body, schema *configschema.Block) hcl.Body {
 	}
 
 	if skipFixup(schema) {
+		// we don't have any context for the resource name or type, but
+		// hopefully this could help locate the evaluation in the logs if there
+		// were a problem
+		log.Println("[DEBUG] skipping FixUpBlockAttrs")
 		return body
 	}
 
@@ -43,14 +49,36 @@ func FixUpBlockAttrs(body hcl.Body, schema *configschema.Block) hcl.Body {
 	}
 }
 
-// skipFixup detects any use of Attribute.NestedType. Because the fixup was
-// only supported for the legacy SDK, there is no situation where structural
-// attributes are used where the fixup is expected.
+// skipFixup detects any use of Attribute.NestedType, or Types which could not
+// be generate by the legacy SDK when taking SchemaConfigModeAttr into account.
 func skipFixup(schema *configschema.Block) bool {
 	for _, attr := range schema.Attributes {
 		if attr.NestedType != nil {
 			return true
 		}
+		ty := attr.Type
+
+		// Lists and sets of objects could be generated by
+		// SchemaConfigModeAttr, but some other combinations can be ruled out.
+
+		// Tuples and objects could not be generated at all.
+		if ty.IsTupleType() || ty.IsObjectType() {
+			return true
+		}
+
+		// A map of objects was not possible.
+		if ty.IsMapType() && ty.ElementType().IsObjectType() {
+			return true
+		}
+
+		// Nested collections were not really supported, but could be generated
+		// with string types (though we conservatively limit this to primitive types)
+		if ty.IsCollectionType() {
+			ety := ty.ElementType()
+			if ety.IsCollectionType() && !ety.ElementType().IsPrimitiveType() {
+				return true
+			}
+		}
 	}
 
 	for _, block := range schema.BlockTypes {

internal/lang/blocktoattr/fixup_test.go

@@ -400,7 +400,7 @@ container {
 			}),
 			wantErrs: true,
 		},
-		"no fixup allowed": {
+		"no fixup allowed with NestedType": {
 			src: `
  container {
    foo = "one"
@@ -429,6 +429,38 @@ container {
 			}),
 			wantErrs: true,
 		},
+		"no fixup allowed new types": {
+			src: `
+ container {
+   foo = "one"
+ }
+ `,
+			schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					// This could be a ConfigModeAttr fixup
+					"container": {
+						Type: cty.List(cty.Object(map[string]cty.Type{
+							"foo": cty.String,
+						})),
+					},
+					// But the presence of this type means it must have been
+					// declared by a new SDK
+					"new_type": {
+						Type: cty.Object(map[string]cty.Type{
+							"boo": cty.String,
+						}),
+					},
+				},
+			},
+			want: cty.ObjectVal(map[string]cty.Value{
+				"container": cty.NullVal(cty.List(
+					cty.Object(map[string]cty.Type{
+						"foo": cty.String,
+					}),
+				)),
+			}),
+			wantErrs: true,
+		},
 	}
 
 	ctx := &hcl.EvalContext{