Merge #3834

3834: Define searchable fields at runtime r=Kerollmops a=ManyTheFish

## Summary
This feature allows the end-user to search in one or multiple attributes using the search parameter `attributesToSearchOn`:

```json
{
  "q": "Captain Marvel",
  "attributesToSearchOn": ["title"]
}
```

This feature act like a filter, forcing Meilisearch to only return the documents containing the requested words in the attributes-to-search-on. Note that, with the matching strategy `last`, Meilisearch will only ensure that the first word is in the attributes-to-search-on, but, the retrieved documents will be ordered taking into account the word contained in the attributes-to-search-on. 

## Trying the prototype

A dedicated docker image has been released for this feature:

#### last prototype version:

```bash
docker pull getmeili/meilisearch:prototype-define-searchable-fields-at-search-time-1
```

#### others prototype versions:

```bash
docker pull getmeili/meilisearch:prototype-define-searchable-fields-at-search-time-0
```

## Technical Detail

The attributes-to-search-on list is given to the search context, then, the search context uses the `fid_word_docids`database using only the allowed field ids instead of the global `word_docids` database. This is the same for the prefix databases.
The database cache is updated with the merged values, meaning that the union of the field-id-database values is only made if the requested key is missing from the cache.

### Relevancy limits

Almost all ranking rules behave as expected when ordering the documents.
Only `proximity` could miss-order documents if all the searched words are in the restricted attribute but a better proximity is found in an ignored attribute in a document that should be ranked lower. I put below a failing test showing it:
```rust
#[actix_rt::test]
async fn proximity_ranking_rule_order() {
    let server = Server::new().await;
    let index = index_with_documents(
        &server,
        &json!([
        {
            "title": "Captain super mega cool. A Marvel story",
            // Perfect distance between words in an ignored attribute
            "desc": "Captain Marvel",
            "id": "1",
        },
        {
            "title": "Captain America from Marvel",
            "desc": "a Shazam ersatz",
            "id": "2",
        }]),
    )
    .await;

    // Document 2 should appear before document 1.
    index
        .search(json!({"q": "Captain Marvel", "attributesToSearchOn": ["title"], "attributesToRetrieve": ["id"]}), |response, code| {
            assert_eq!(code, 200, "{}", response);
            assert_eq!(
                response["hits"],
                json!([
                    {"id": "2"},
                    {"id": "1"},
                ])
            );
        })
        .await;
}
```

Fixing this would force us to create a `fid_word_pair_proximity_docids` and a `fid_word_prefix_pair_proximity_docids` databases which may multiply the keys of `word_pair_proximity_docids` and `word_prefix_pair_proximity_docids` by the number of attributes in the searchable_attributes list. If we think we should fix this test, I'll suggest doing it in another PR.

## Related

Fixes #3772

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

milli/src/search/new/mod.rs

@@ -20,7 +20,7 @@ mod sort;
 #[cfg(test)]
 mod tests;
 
-use std::collections::HashSet;
+use std::collections::{BTreeSet, HashSet};
 
 use bucket_sort::{bucket_sort, BucketSortOutput};
 use charabia::TokenizerBuilder;
@@ -46,6 +46,7 @@ use self::geo_sort::GeoSort;
 pub use self::geo_sort::Strategy as GeoSortStrategy;
 use self::graph_based_ranking_rule::Words;
 use self::interner::Interned;
+use crate::error::FieldIdMapMissingEntry;
 use crate::score_details::{ScoreDetails, ScoringStrategy};
 use crate::search::new::distinct::apply_distinct_rule;
 use crate::{
@@ -62,6 +63,7 @@ pub struct SearchContext<'ctx> {
     pub phrase_interner: DedupInterner<Phrase>,
     pub term_interner: Interner<QueryTerm>,
     pub phrase_docids: PhraseDocIdsCache,
+    pub restricted_fids: Option<Vec<u16>>,
 }
 
 impl<'ctx> SearchContext<'ctx> {
@@ -74,8 +76,66 @@ impl<'ctx> SearchContext<'ctx> {
             phrase_interner: <_>::default(),
             term_interner: <_>::default(),
             phrase_docids: <_>::default(),
+            restricted_fids: None,
         }
     }
+
+    pub fn searchable_attributes(&mut self, searchable_attributes: &'ctx [String]) -> Result<()> {
+        let fids_map = self.index.fields_ids_map(self.txn)?;
+        let searchable_names = self.index.searchable_fields(self.txn)?;
+
+        let mut restricted_fids = Vec::new();
+        for field_name in searchable_attributes {
+            let searchable_contains_name =
+                searchable_names.as_ref().map(|sn| sn.iter().any(|name| name == field_name));
+            let fid = match (fids_map.id(field_name), searchable_contains_name) {
+                // The Field id exist and the field is searchable
+                (Some(fid), Some(true)) | (Some(fid), None) => fid,
+                // The field is searchable but the Field id doesn't exist => Internal Error
+                (None, Some(true)) => {
+                    return Err(FieldIdMapMissingEntry::FieldName {
+                        field_name: field_name.to_string(),
+                        process: "search",
+                    }
+                    .into())
+                }
+                // The field is not searchable => User error
+                _otherwise => {
+                    let mut valid_fields: BTreeSet<_> =
+                        fids_map.names().map(String::from).collect();
+
+                    // Filter by the searchable names
+                    if let Some(sn) = searchable_names {
+                        let searchable_names = sn.iter().map(|s| s.to_string()).collect();
+                        valid_fields = &valid_fields & &searchable_names;
+                    }
+
+                    let searchable_count = valid_fields.len();
+
+                    // Remove hidden fields
+                    if let Some(dn) = self.index.displayed_fields(self.txn)? {
+                        let displayable_names = dn.iter().map(|s| s.to_string()).collect();
+                        valid_fields = &valid_fields & &displayable_names;
+                    }
+
+                    let hidden_fields = searchable_count > valid_fields.len();
+                    let field = field_name.to_string();
+                    return Err(UserError::InvalidSearchableAttribute {
+                        field,
+                        valid_fields,
+                        hidden_fields,
+                    }
+                    .into());
+                }
+            };
+
+            restricted_fids.push(fid);
+        }
+
+        self.restricted_fids = Some(restricted_fids);
+
+        Ok(())
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, PartialOrd, Ord, Eq)]