don't assert set block length with unknowns

If a planned NestingList block value looks like it may represent a
dynamic block, we don't check the length since it may be unknown. This
check was missing in the NestingSet case, but it applies for the same
reason.

<

plans/objchange/compatible.go

@@ -169,6 +169,14 @@ func assertObjectCompatible(schema *configschema.Block, planned, actual cty.Valu
 			})
 			errs = append(errs, setErrs...)
 
+			if maybeUnknownBlocks {
+				// When unknown blocks are present the final blocks may be
+				// at different indices than the planned blocks, so unfortunately
+				// we can't do our usual checks in this case without generating
+				// false negatives.
+				continue
+			}
+
 			// There can be fewer elements in a set after its elements are all
 			// known (values that turn out to be equal will coalesce) but the
 			// number of elements must never get larger.

plans/objchange/compatible_test.go

@@ -1068,9 +1068,9 @@ func TestAssertObjectCompatible(t *testing.T) {
 					}),
 				}),
 			}),
-			[]string{
-				`.block: block set length changed from 2 to 3`,
-			},
+			// there is no error here, because the presence of unknowns
+			// indicates this may be a dynamic block, and the length is unknown
+			nil,
 		},
 		{
 			&configschema.Block{
@@ -1135,6 +1135,34 @@ func TestAssertObjectCompatible(t *testing.T) {
 			}),
 			nil,
 		},
+		{
+			&configschema.Block{
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"block": {
+						Nesting: configschema.NestingSet,
+						Block:   schemaWithFoo,
+					},
+				},
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"block": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"foo": cty.UnknownVal(cty.String),
+					}),
+				}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"block": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"foo": cty.StringVal("a"),
+					}),
+					cty.ObjectVal(map[string]cty.Value{
+						"foo": cty.StringVal("b"),
+					}),
+				}),
+			}),
+			nil,
+		},
 		{
 			&configschema.Block{
 				BlockTypes: map[string]*configschema.NestedBlock{