Merge #3994

3994: Fix synonyms with separators r=Kerollmops a=ManyTheFish

# Pull Request

## Related issue
Fixes #3977

## Available prototype
```
$ docker pull getmeili/meilisearch:prototype-fix-synonyms-with-separators-0
```

## What does this PR do?
- add a new test
- filter the empty synonyms after normalization


Co-authored-by: ManyTheFish <many@meilisearch.com>

meilisearch/tests/search/mod.rs

@@ -1104,3 +1104,59 @@ async fn camelcased_words() {
         })
         .await;
 }
+
+#[actix_rt::test]
+async fn simple_search_with_strange_synonyms() {
+    let server = Server::new().await;
+    let index = server.index("test");
+
+    index.update_settings(json!({ "synonyms": {"&": ["to"], "to": ["&"]} })).await;
+    let r = index.wait_task(0).await;
+    meili_snap::snapshot!(r["status"], @r###""succeeded""###);
+
+    let documents = DOCUMENTS.clone();
+    index.add_documents(documents, None).await;
+    index.wait_task(1).await;
+
+    index
+        .search(json!({"q": "How to train"}), |response, code| {
+            meili_snap::snapshot!(code, @"200 OK");
+            meili_snap::snapshot!(meili_snap::json_string!(response["hits"]), @r###"
+            [
+              {
+                "title": "How to Train Your Dragon: The Hidden World",
+                "id": "166428"
+              }
+            ]
+            "###);
+        })
+        .await;
+
+    index
+        .search(json!({"q": "How & train"}), |response, code| {
+            meili_snap::snapshot!(code, @"200 OK");
+            meili_snap::snapshot!(meili_snap::json_string!(response["hits"]), @r###"
+            [
+              {
+                "title": "How to Train Your Dragon: The Hidden World",
+                "id": "166428"
+              }
+            ]
+            "###);
+        })
+        .await;
+
+    index
+        .search(json!({"q": "to"}), |response, code| {
+            meili_snap::snapshot!(code, @"200 OK");
+            meili_snap::snapshot!(meili_snap::json_string!(response["hits"]), @r###"
+            [
+              {
+                "title": "How to Train Your Dragon: The Hidden World",
+                "id": "166428"
+              }
+            ]
+            "###);
+        })
+        .await;
+}

milli/src/update/settings.rs

@@ -608,13 +608,18 @@ impl<'a, 't, 'u, 'i> Settings<'a, 't, 'u, 'i> {
                 for (word, synonyms) in user_synonyms {
                     // Normalize both the word and associated synonyms.
                     let normalized_word = normalize(&tokenizer, word);
-                    let normalized_synonyms =
-                        synonyms.iter().map(|synonym| normalize(&tokenizer, synonym));
+                    let normalized_synonyms: Vec<_> = synonyms
+                        .iter()
+                        .map(|synonym| normalize(&tokenizer, synonym))
+                        .filter(|synonym| !synonym.is_empty())
+                        .collect();
 
                     // Store the normalized synonyms under the normalized word,
                     // merging the possible duplicate words.
+                    if !normalized_word.is_empty() && !normalized_synonyms.is_empty() {
                         let entry = new_synonyms.entry(normalized_word).or_insert_with(Vec::new);
-                    entry.extend(normalized_synonyms);
+                        entry.extend(normalized_synonyms.into_iter());
+                    }
                 }
 
                 // Make sure that we don't have duplicate synonyms.