Merge #461

461: Add a new error message when the `valid_fields` is empty r=curquiza a=brunoocasali

I've created a test case to handle the new error formatting behavior, but I'm not sure if:

- this is the right place to add the test?
- this is the best way to test this behavior?

And I'm not sure also regarding the `match` implementation, is this something required? Or maybe just an `if` statement is ok as well?

I left the two messages literally without "reusing the prefix" in the implementation because I think this could help the "searchability" of the error in the future.

# Pull Request

## What does this PR do?
Fixes https://github.com/meilisearch/meilisearch/issues/2140

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue?
- [ ] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!


Co-authored-by: Bruno Casali <brunoocasali@gmail.com>

milli/src/error.rs

@@ -256,11 +256,20 @@ only composed of alphanumeric characters (a-z A-Z 0-9), hyphens (-) and undersco
             Self::InvalidSortableAttribute { field, valid_fields } => {
                 let valid_names =
                     valid_fields.iter().map(AsRef::as_ref).collect::<Vec<_>>().join(", ");
-                write!(
-                    f,
-                    "Attribute `{}` is not sortable. Available sortable attributes are: `{}`.",
-                    field, valid_names
-                )
+
+                    if valid_names.is_empty() {
+                        write!(
+                            f,
+                            "Attribute `{}` is not sortable. This index does not have configured sortable attributes.",
+                            field
+                        )
+                    } else {
+                        write!(
+                            f,
+                            "Attribute `{}` is not sortable. Available sortable attributes are: `{}`.",
+                            field, valid_names
+                        )
+                    }
             }
             Self::SortRankingRuleMissing => f.write_str(
                 "The sort ranking rule must be specified in the \
@@ -320,3 +329,19 @@ impl fmt::Display for SerializationError {
 }
 
 impl StdError for SerializationError {}
+
+#[test]
+fn conditionally_lookup_for_error_message() {
+    let prefix = "Attribute `name` is not sortable.";
+    let messages = vec![
+        (BTreeSet::new(), "This index does not have configured sortable attributes."),
+        (BTreeSet::from(["age".to_string()]), "Available sortable attributes are: `age`."),
+    ];
+
+    for (list, suffix) in messages {
+        let err =
+            UserError::InvalidSortableAttribute { field: "name".to_string(), valid_fields: list };
+
+        assert_eq!(err.to_string(), format!("{} {}", prefix, suffix));
+    }
+}