Rust: Add isFieldless and isUnitOnly to Enum

rust/ql/lib/codeql/rust/elements/internal/EnumImpl.qll

@@ -31,5 +31,23 @@ module Impl {
       result = this.getVariantList().getAVariant() and
       result.getName().getText() = name
     }
+
+    /**
+     * Holds if this is a field-less enum, that is, an enum where no constructors contain fields.
+     *
+     * See: https://doc.rust-lang.org/reference/items/enumerations.html#r-items.enum.fieldless
+     */
+    predicate isFieldless() {
+      forall(Variant v | v = this.getVariantList().getAVariant() | v.getNumberOfFields() = 0)
+    }
+
+    /**
+     * Holds if this is a unit-only enum, that is, an enum where all constructors are unit variants.
+     *
+     * See: https://doc.rust-lang.org/reference/items/enumerations.html#r-items.enum.unit-only
+     */
+    predicate isUnitOnly() {
+      forall(Variant v | v = this.getVariantList().getAVariant() | v.isUnit())
+    }
   }
 }

rust/ql/lib/codeql/rust/elements/internal/VariantImpl.qll

@@ -36,6 +36,15 @@ module Impl {
     pragma[nomagic]
     TupleField getTupleField(int i) { result = this.getFieldList().(TupleFieldList).getField(i) }
 
+    int getNumberOfFields() {
+      not this.hasFieldList() and
+      result = 0
+      or
+      result = this.getFieldList().(StructFieldList).getNumberOfFields()
+      or
+      result = this.getFieldList().(TupleFieldList).getNumberOfFields()
+    }
+
     /** Holds if this variant uses tuple fields. */
     pragma[nomagic]
     predicate isTuple() { this.getFieldList() instanceof TupleFieldList }