Rust: Tweak existing isStruct predicates

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

@@ -46,12 +46,11 @@ module Impl {
     pragma[nomagic]
     predicate isTuple() { this.getFieldList() instanceof TupleFieldList }
 
-    /**
-     * Holds if this struct uses record fields.
-     *
-     * Empty structs are considered to use record fields.
-     */
+    /** Holds if this struct uses struct fields. */
     pragma[nomagic]
-    predicate isStruct() { not this.isTuple() }
+    predicate isStruct() { this.getFieldList() instanceof StructFieldList }
+
+    /** Holds if this struct does not have a field list. */
+    predicate isUnit() { not this.hasFieldList() }
   }
 }

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

@@ -40,13 +40,13 @@ module Impl {
     pragma[nomagic]
     predicate isTuple() { this.getFieldList() instanceof TupleFieldList }
 
-    /**
-     * Holds if this variant uses struct fields.
-     *
-     * Empty variants are considered to use struct fields.
-     */
+    /** Holds if this variant uses struct fields. */
     pragma[nomagic]
-    predicate isStruct() { not this.isTuple() }
+    predicate isStruct() { this.getFieldList() instanceof StructFieldList }
+
+    /** Holds if this variant does not have a field list. */
+    pragma[nomagic]
+    predicate isUnit() { not this.hasFieldList() }
 
     /** Gets the enum that this variant belongs to. */
     Enum getEnum() { this = result.getVariantList().getAVariant() }

rust/ql/lib/codeql/rust/internal/PathResolution.qll

@@ -659,7 +659,7 @@ private class VariantItemNode extends ParameterizableItemNode instanceof Variant
   override string getName() { result = Variant.super.getName().getText() }
 
   override Namespace getNamespace() {
-    if super.getFieldList() instanceof StructFieldList then result.isType() else result.isValue()
+    if super.isStruct() then result.isType() else result.isValue()
   }
 
   override TypeParam getTypeParam(int i) {
@@ -969,7 +969,7 @@ private class StructItemNode extends TypeItemNode, ParameterizableItemNode insta
   override Namespace getNamespace() {
     result.isType() // the struct itself
     or
-    not super.getFieldList() instanceof StructFieldList and
+    not super.isStruct() and
     result.isValue() // the constructor
   }
 

rust/ql/lib/codeql/rust/internal/TypeInference.qll

@@ -787,7 +787,7 @@ private module StructExprMatchingInput implements MatchingInputSig {
   }
 
   private class StructDecl extends Declaration, Struct {
-    StructDecl() { this.isStruct() }
+    StructDecl() { this.isStruct() or this.isUnit() }
 
     override TypeParam getATypeParam() { result = this.getGenericParamList().getATypeParam() }
 
@@ -804,7 +804,7 @@ private module StructExprMatchingInput implements MatchingInputSig {
   }
 
   private class StructVariantDecl extends Declaration, Variant {
-    StructVariantDecl() { this.isStruct() }
+    StructVariantDecl() { this.isStruct() or this.isUnit() }
 
     Enum getEnum() { result.getVariantList().getAVariant() = this }