fix: Disallow else blocks for set rules (#403)

else blocks following contains and old-style sets will raise
a parse error. Consistent with OPA.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/parser.rs

@@ -1617,6 +1617,12 @@ impl<'source> Parser<'source> {
         let bodies = self.parse_rule_bodies()?;
         span.end = self.end;
 
+        if let RuleHead::Set { .. } = &head {
+            if let Some(RuleBody { span, .. }) = bodies.get(1) {
+                bail!(span.error("else cannot be used with set rules"));
+            }
+        }
+
         if self.rego_v1 && bodies.is_empty() {
             match &head {
                 RuleHead::Compr { assign, .. } | RuleHead::Func { assign, .. }

tests/parser/cases/rules/else.yaml

@@ -41,7 +41,7 @@ cases:
                   - literal:
                       expr:
                         true
-    
+
   - note: no-else
     rego: |
       package test
@@ -77,7 +77,7 @@ cases:
     rego: |
       package test
       import future.keywords.if
-      
+
       x = 10 if {
         false
       } {
@@ -108,7 +108,7 @@ cases:
   - note: rule-named-if
     rego: |
       package test
-      
+
       x = 10 if {
         false
       } {
@@ -141,11 +141,11 @@ cases:
                   - literal:
                       expr:
                          true
-                        
+
   - note: query-else
     rego: |
       package test
-      
+
       x = 10 {
         false
       } else {
@@ -172,12 +172,12 @@ cases:
                   - literal:
                       expr:
                         true
- 
+
   - note: if-literal-else
     rego: |
       package test
       import future.keywords.if
-      
+
       x = 10 if 1 < 0 else {
         true
       }
@@ -212,7 +212,7 @@ cases:
     rego: |
       package test
       import future.keywords.if
-      
+
       # This will evaluate to 10
       x = 10 if 1 < 0 else := 20 {
         true
@@ -247,4 +247,26 @@ cases:
                   - literal:
                       expr:
                         true
-                        
+
+  - note: contains-else-error
+    rego: |
+      package test
+      import rego.v1
+
+      a contains "b" if {
+        false
+      } else if {
+        true
+      }
+    error: else cannot be used with set rules
+
+  - note: old-style-set-else-error
+    rego: |
+      package test
+
+      a["b"] {
+        false
+      } else {
+        true
+      }
+    error: else cannot be used with set rules