Minor adjustments to SSA library for self variables

ruby/ql/lib/codeql/ruby/ast/Variable.qll

@@ -71,8 +71,11 @@ class ClassVariable extends Variable instanceof ClassVariableImpl {
   final override ClassVariableAccess getAnAccess() { result.getVariable() = this }
 }
 
-/** The `self` variable */
-class SelfVariable extends Variable instanceof SelfVariableImpl { }
+/** A `self` variable. */
+class SelfVariable extends LocalVariable instanceof SelfVariableImpl {
+  /** Gets the method that this `self` variable belongs to. */
+  MethodBase getMethod() { result = this.getDeclaringScope() }
+}
 
 /** An access to a variable. */
 class VariableAccess extends Expr instanceof VariableAccessImpl {

ruby/ql/lib/codeql/ruby/dataflow/SSA.qll

@@ -220,6 +220,19 @@ module Ssa {
     final override Location getLocation() { result = this.getControlFlowNode().getLocation() }
   }
 
+  /**
+   * An SSA definition that corresponds to the value of `self` upon method entry.
+   */
+  class SelfDefinition extends Definition, SsaImplCommon::WriteDefinition {
+    private SelfVariable v;
+
+    SelfDefinition() { this.definesAt(v, _, _) }
+
+    final override string toString() { result = "self (" + v.getMethod() + ")" }
+
+    final override Location getLocation() { result = this.getControlFlowNode().getLocation() }
+  }
+
   /**
    * An SSA definition inserted at the beginning of a scope to represent an
    * uninitialized local variable. For example, in

ruby/ql/lib/codeql/ruby/dataflow/internal/SsaImpl.qll

@@ -7,7 +7,8 @@ private import CfgNodes::ExprNodes
 /** Holds if `v` is uninitialized at index `i` in entry block `bb`. */
 predicate uninitializedWrite(EntryBasicBlock bb, int i, LocalVariable v) {
   v.getDeclaringScope() = bb.getScope() and
-  i = -1
+  i = -1 and
+  not v instanceof SelfVariable
 }
 
 /** Holds if `bb` contains a caputured read of variable `v`. */

ruby/ql/lib/codeql/ruby/dataflow/internal/SsaImplSpecific.qll

@@ -24,11 +24,8 @@ class SourceVariable = LocalVariable;
 predicate variableWrite(BasicBlock bb, int i, SourceVariable v, boolean certain) {
   (
     // We consider the `self` variable to have a single write at the entry to a method block.
-    exists(SelfVariableAccess access |
-      access.getCfgScope() = bb.getScope() and
-      access.getVariable() = v and
-      i = 0
-    )
+    v.(SelfVariable).getDeclaringScope() = bb.(BasicBlocks::EntryBasicBlock).getScope() and
+    i = 0
     or
     SsaImpl::uninitializedWrite(bb, i, v)
     or