Improve round trip error message

Fixes #1771

NEWS.md

@@ -1,5 +1,8 @@
 # testthat (development version)
 
+* `expect_snapshot_value()` has an improved error if the object can't be
+  safely seriazlied using the specified `style` (#1771).
+
 * `skip_if_offline()` now errors if you don't have curl installed (#1854).
 
 * All packages, regardless of whether or not they use rlang 1.0.0, now

R/snapshot-reporter.R

@@ -71,7 +71,6 @@ SnapshotReporter <- R6::R6Class("SnapshotReporter",
         comp
       } else {
         value_enc <- save(value)
-        check_roundtrip(value, load(value_enc), ..., tolerance = tolerance)
 
         self$cur_snaps$append(self$test, variant, value_enc)
 
@@ -161,12 +160,22 @@ SnapshotReporter <- R6::R6Class("SnapshotReporter",
 )
 
 
-check_roundtrip <- function(x, y, ..., tolerance = testthat_tolerance()) {
-  check <- waldo_compare(x, y, x_arg = "value", y_arg = "roundtrip", ..., tolerance = tolerance)
+check_roundtrip <- function(x,
+                            y,
+                            style,
+                            ...,
+                            tolerance = testthat_tolerance(),
+                            error_call = caller_env()) {
+  check <- waldo_compare(x, y, x_arg = "original", y_arg = "new", ..., tolerance = tolerance)
   if (length(check) > 0) {
     abort(c(
-      paste0("Serialization round-trip is not symmetric.\n\n", check, "\n"),
-      i = "You may need to consider serialization `style`")
+      paste0("Object could not be safely serialized with `style = \"", style, "\"`."),
+      " " = paste0(
+        "Serializing then deserializing the object returned something new:\n\n",
+        check, "\n"
+      ),
+      i = "You may need to try a different `style`."),
+      call = error_call
     )
   }
 }

R/snapshot.R

@@ -305,6 +305,8 @@ expect_snapshot_value <- function(x,
   )
 
   with_is_snapshotting(force(x))
+  check_roundtrip(x, load(save(x)), style = style, tolerance = tolerance)
+
   expect_snapshot_helper(lab, x,
     save = save,
     load = load,

tests/testthat/_snaps/snapshot-reporter.md

@@ -0,0 +1,14 @@
+# check_roundtrip() gives nice error
+
+    Code
+      wrapper(NULL, list(), style = "json")
+    Condition
+      Error in `wrapper()`:
+      ! Object could not be safely serialized with `style = "json"`.
+        Serializing then deserializing the object returned something new:
+      
+      `original` is NULL
+      `new` is a list
+      
+      i You may need to try a different `style`.
+

tests/testthat/test-snapshot-reporter.R

@@ -114,7 +114,15 @@ test_that("errors if can't roundtrip", {
   snapper <- local_snapshotter()
   snapper$start_file("snapshot-4", "test")
 
-  expect_error(expect_snapshot_value(NULL), "not symmetric")
+  expect_error(expect_snapshot_value(NULL), "safely serialized")
+})
+
+test_that("check_roundtrip() gives nice error", {
+  # disable crayon usage
+  local_bindings(crayon = FALSE, .env = get_reporter())
+
+  wrapper <- function(...) check_roundtrip(...)
+  expect_snapshot(wrapper(NULL, list(), style = "json"), error = TRUE)
 })
 
 test_that("errors in test doesn't change snapshot", {