Add and use exprRefersToNil predicate

Author: owen-mc

go/ql/lib/semmle/go/Expr.qll

@@ -2043,6 +2043,8 @@ class ConstantName extends ValueName {
   override string getAPrimaryQlClass() { result = "ConstantName" }
 }
 
+predicate exprRefersToNil(Expr e) { e.(ConstantName).getTarget() = Builtin::nil() }
+
 /**
  * A name referring to a variable.
  *
@@ -2183,7 +2185,7 @@ private predicate isTypeExprTopDown(Expr e) {
   or
   e = any(TypeSwitchStmt s).getACase().getExpr(_) and
   // special case: `nil` is allowed in a type case but isn't a type
-  not e = Builtin::nil().getAReference()
+  not exprRefersToNil(e)
   or
   e = any(SelectorExpr sel | isTypeExprTopDown(sel)).getBase()
   or

go/ql/lib/semmle/go/dataflow/Properties.qll

@@ -22,7 +22,7 @@ class Property extends TProperty {
       isTrue = eq.getPolarity().booleanXor(e.getBoolValue().booleanXor(outcome))
       or
       this = IsNil(isTrue) and
-      e = Builtin::nil().getAReference() and
+      exprRefersToNil(e) and
       isTrue = eq.getPolarity().booleanXor(outcome).booleanNot()
     )
     or

go/ql/lib/semmle/go/dataflow/internal/DataFlowUtil.qll

@@ -560,7 +560,7 @@ private predicate onlyPossibleReturnOfBool(FuncDecl fd, FunctionOutput res, Node
  */
 predicate possiblyReturnsNonNil(FuncDecl fd, FunctionOutput res, Node ret) {
   ret = res.getEntryNode(fd) and
-  not ret.asExpr() = Builtin::nil().getAReference()
+  not exprRefersToNil(ret.asExpr())
 }
 
 /**
@@ -570,7 +570,7 @@ predicate possiblyReturnsNonNil(FuncDecl fd, FunctionOutput res, Node ret) {
 private predicate onlyPossibleReturnOfNonNil(FuncDecl fd, FunctionOutput res, Node ret) {
   possiblyReturnsNonNil(fd, res, ret) and
   forall(Node otherRet | otherRet = res.getEntryNode(fd) and otherRet != ret |
-    otherRet.asExpr() = Builtin::nil().getAReference()
+    exprRefersToNil(otherRet.asExpr())
   )
 }
 
@@ -609,7 +609,7 @@ private predicate isCertainlyNotNil(DataFlow::Node node) {
  */
 private predicate onlyPossibleReturnOfNil(FuncDecl fd, FunctionOutput res, DataFlow::Node ret) {
   ret = res.getEntryNode(fd) and
-  ret.asExpr() = Builtin::nil().getAReference() and
+  exprRefersToNil(ret.asExpr()) and
   forall(DataFlow::Node otherRet | otherRet = res.getEntryNode(fd) and otherRet != ret |
     isCertainlyNotNil(otherRet)
   )

go/ql/lib/semmle/go/dataflow/internal/TaintTrackingUtil.qll

@@ -418,7 +418,7 @@ predicate functionEnsuresInputIsConstant(
     forex(DataFlow::Node ret, IR::ReturnInstruction ri |
       ret = outp.getEntryNode(fd) and
       ri.getReturnStmt().getAnExpr() = ret.asExpr() and
-      ret.asExpr() = Builtin::nil().getAReference()
+      exprRefersToNil(ret.asExpr())
     |
       DataFlow::localFlow(inp.getExitNode(fd), _) and
       mustPassConstantCaseTestToReach(ri, inp.getExitNode(fd))

go/ql/src/RedundantCode/UnreachableStatement.ql

@@ -26,7 +26,7 @@ ControlFlow::Node nonGuardPredecessor(ControlFlow::Node nd) {
  * Matches if `retval` is a constant or a struct composed wholly of constants.
  */
 predicate isAllowedReturnValue(Expr retval) {
-  retval = Builtin::nil().getAReference()
+  exprRefersToNil(retval)
   or
   retval = Builtin::true_().getAReference()
   or

go/ql/src/experimental/CWE-203/Timing.ql

@@ -36,7 +36,7 @@ private class SensitiveStringCompareSink extends Sink {
       not op1 = nonSensitiveOperand and
       not (
         // Comparisons with `nil` should be excluded.
-        nonSensitiveOperand = Builtin::nil().getAReference()
+        exprRefersToNil(nonSensitiveOperand)
         or
         // Comparisons with empty string should also be excluded.
         nonSensitiveOperand.getStringValue().length() = 0
@@ -60,7 +60,7 @@ private class SensitiveCompareSink extends Sink {
       not op1 = op2 and
       not (
         // Comparisons with `nil` should be excluded.
-        op2 = Builtin::nil().getAReference()
+        exprRefersToNil(op2)
         or
         // Comparisons with empty string should also be excluded.
         op2.getStringValue().length() = 0
@@ -85,7 +85,7 @@ private class SensitiveStringSink extends Sink {
       not op1 = op2 and
       not (
         // Comparisons with `nil` should be excluded.
-        op2 = Builtin::nil().getAReference()
+        exprRefersToNil(op2)
         or
         // Comparisons with empty string should also be excluded.
         op2.getStringValue().length() = 0