[ES|QL] Prevent using unnamed fields in the RERANK command (#127416)

Author: svilen-mihaylov-elastic

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

@@ -87,6 +87,14 @@ field
     : (qualifiedName ASSIGN)? booleanExpression
     ;
 
+rerankFields
+    : rerankField (COMMA rerankField)*
+    ;
+
+rerankField
+    : qualifiedName (ASSIGN booleanExpression)?
+    ;
+
 fromCommand
     : FROM indexPatternAndMetadataFields
     ;
@@ -296,7 +304,7 @@ rrfCommand
    ;
 
 rerankCommand
-    : DEV_RERANK queryText=constant ON fields WITH inferenceId=identifierOrParameter
+    : DEV_RERANK queryText=constant ON rerankFields WITH inferenceId=identifierOrParameter
     ;
 
 completionCommand

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/EsqlBaseParser.interp

@@ -826,6 +826,25 @@ public List<Alias> visitFields(EsqlBaseParser.FieldsContext ctx) {
         return ctx != null ? visitList(this, ctx.field(), Alias.class) : new ArrayList<>();
     }
 
+    @Override
+    public Alias visitRerankField(EsqlBaseParser.RerankFieldContext ctx) {
+        return visitRerankField(ctx, source(ctx));
+    }
+
+    private Alias visitRerankField(EsqlBaseParser.RerankFieldContext ctx, Source source) {
+        UnresolvedAttribute id = visitQualifiedName(ctx.qualifiedName());
+        assert id != null;
+
+        var boolExprCtx = ctx.booleanExpression();
+        Expression value = boolExprCtx == null ? id : expression(boolExprCtx);
+        return new Alias(source, id.name(), value);
+    }
+
+    @Override
+    public List<Alias> visitRerankFields(EsqlBaseParser.RerankFieldsContext ctx) {
+        return ctx != null ? visitList(this, ctx.rerankField(), Alias.class) : new ArrayList<>();
+    }
+
     @Override
     public NamedExpression visitAggField(EsqlBaseParser.AggFieldContext ctx) {
         Source source = source(ctx);

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/EsqlBaseParser.java

@@ -731,7 +731,7 @@ public PlanFactory visitRerankCommand(EsqlBaseParser.RerankCommandContext ctx) {
             );
         }
 
-        return p -> new Rerank(source, p, inferenceId(ctx.inferenceId), queryText, visitFields(ctx.fields()));
+        return p -> new Rerank(source, p, inferenceId(ctx.inferenceId), queryText, visitRerankFields(ctx.rerankFields()));
     }
 
     @Override

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/EsqlBaseParserBaseListener.java

@@ -3613,6 +3613,19 @@ public void testResolveRerankFields() {
             assertThat(rerank.scoreAttribute(), equalTo(getAttributeByName(relation.output(), MetadataAttribute.SCORE)));
         }
 
+        {
+            // Unnamed field.
+            try {
+                LogicalPlan plan = analyze("""
+                    FROM books METADATA _score
+                    | WHERE title:"food"
+                    | RERANK "food" ON title, SUBSTRING(description, 0, 100), yearRenamed=year WITH `reranking-inference-id`
+                    """, "mapping-books.json");
+            } catch (ParsingException ex) {
+                assertThat(ex.getMessage(), containsString("line 3:36: mismatched input '(' expecting {'=', ',', '.', 'with'}"));
+            }
+        }
+
         {
             VerificationException ve = expectThrows(
                 VerificationException.class,