test(snap): Verify data formats

Cherry pick e7ef941 (assert-rs#277)

crates/snapbox/src/data/mod.rs

@@ -748,3 +748,89 @@ pub fn generate_snapshot_path(fn_path: &str, format: Option<DataFormat>) -> std:
     path.push_str(format.unwrap_or(DataFormat::Text).ext());
     path.into()
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[track_caller]
+    fn validate_cases(cases: &[(&str, bool)], input_format: DataFormat) {
+        for (input, valid) in cases.iter().copied() {
+            let (expected_is_format, expected_coerced_format) = if valid {
+                (input_format, input_format)
+            } else {
+                (DataFormat::Error, DataFormat::Text)
+            };
+
+            let actual_is = Data::text(input).is(input_format);
+            assert_eq!(
+                actual_is.format(),
+                expected_is_format,
+                "\n{input}\n{actual_is}"
+            );
+
+            let actual_coerced = Data::text(input).coerce_to(input_format);
+            assert_eq!(
+                actual_coerced.format(),
+                expected_coerced_format,
+                "\n{input}\n{actual_coerced}"
+            );
+
+            if valid {
+                assert_eq!(actual_is, actual_coerced);
+
+                let rendered = actual_is.render().unwrap();
+                let bytes = actual_is.to_bytes().unwrap();
+                assert_eq!(rendered, std::str::from_utf8(&bytes).unwrap());
+
+                assert_eq!(Data::text(&rendered).is(input_format), actual_is);
+            }
+        }
+    }
+
+    #[test]
+    fn text() {
+        let cases = [("", true), ("good", true), ("{}", true), ("\"\"", true)];
+        validate_cases(&cases, DataFormat::Text);
+    }
+
+    #[cfg(feature = "json")]
+    #[test]
+    fn json() {
+        let cases = [("", false), ("bad", false), ("{}", true), ("\"\"", true)];
+        validate_cases(&cases, DataFormat::Json);
+    }
+
+    #[cfg(feature = "json")]
+    #[test]
+    fn jsonlines() {
+        let cases = [
+            ("", true),
+            ("bad", false),
+            ("{}", true),
+            ("\"\"", true),
+            (
+                "
+{}
+{}
+", true,
+            ),
+            (
+                "
+{}
+
+{}
+", true,
+            ),
+            (
+                "
+{}
+bad
+{}
+",
+                false,
+            ),
+        ];
+        validate_cases(&cases, DataFormat::JsonLines);
+    }
+}