skip FixUpBlockAttrs when we can detect types from a new SDK

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"
@@ -12,6 +14,10 @@ import (
 // type in the schema to be written with HCL block syntax as multiple nested
 // blocks with the attribute name as the block type.
 //
+// The fixup is only applied in the absence of structural attribute types. The
+// presence of these types indicate the use of a provider which does not
+// support mapping blocks to attributes.
+//
 // This partially restores some of the block/attribute confusion from HCL 1
 // so that existing patterns that depended on that confusion can continue to
 // be used in the short term while we settle on a longer-term strategy.
@@ -28,13 +34,62 @@ func FixUpBlockAttrs(body hcl.Body, schema *configschema.Block) hcl.Body {
 		schema = &configschema.Block{}
 	}
 
+	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
+	}
+
 	return &fixupBody{
 		original: body,
 		schema:   schema,
 		names:    ambiguousNames(schema),
 	}
 }
 
+// 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 {
+		if skipFixup(&block.Block) {
+			return true
+		}
+	}
+
+	return false
+}
+
 type fixupBody struct {
 	original hcl.Body
 	schema   *configschema.Block

internal/lang/blocktoattr/fixup_test.go

@@ -400,6 +400,67 @@ container {
 			}),
 			wantErrs: true,
 		},
+		"no fixup allowed with NestedType": {
+			src: `
+ container {
+   foo = "one"
+ }
+ `,
+			schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"container": {
+						NestedType: &configschema.Object{
+							Nesting: configschema.NestingList,
+							Attributes: map[string]*configschema.Attribute{
+								"foo": {
+									Type: 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,
+		},
+		"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{