Rust: Minor refactor and add comment

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

@@ -147,7 +147,9 @@ module Impl {
     predicate isCaptured() { this.getAnAccess().isCapture() }
 
     /** Gets the parameter that introduces this variable, if any. */
-    Param getParameter() { parameterDeclInScope(result, this, _) }
+    AstNode getParameter() {
+      result = this.getSelfParam() or result = getAVariablePatAncestor(this).getParentNode()
+    }
 
     /** Hold is this variable is mutable. */
     predicate isMutable() { this.getPat().isMut() }
@@ -156,7 +158,11 @@ module Impl {
     predicate isImmutable() { not this.isMutable() }
   }
 
-  /** A path expression that may access a local variable. */
+  /**
+   * A path expression that may access a local variable. These are paths that
+   * only consists of a simple name (i.e., without generic arguments,
+   * qualifiers, etc.).
+   */
   private class VariableAccessCand extends PathExprBase {
     string name_;
 
@@ -202,10 +208,7 @@ module Impl {
   private VariableScope getEnclosingScope(AstNode n) { result = getAnAncestorInVariableScope(n) }
 
   private Pat getAVariablePatAncestor(Variable v) {
-    exists(AstNode definingNode, string name |
-      v = MkVariable(definingNode, name) and
-      variableDecl(definingNode, result, name)
-    )
+    result = v.getPat()
     or
     exists(Pat mid |
       mid = getAVariablePatAncestor(v) and
@@ -217,13 +220,9 @@ module Impl {
    * Holds if parameter `p` introduces the variable `v` inside variable scope
    * `scope`.
    */
-  private predicate parameterDeclInScope(AstNode p, Variable v, VariableScope scope) {
+  private predicate parameterDeclInScope(Variable v, VariableScope scope) {
     exists(Callable f |
-      (
-        p = f.getParamList().getSelfParam() and p = v.getSelfParam()
-        or
-        p = f.getParamList().getAParam() and p.(Param).getPat() = getAVariablePatAncestor(v)
-      ) and
+      v.getParameter() = [f.getParamList().getSelfParam().(AstNode), f.getParamList().getAParam()] and
       scope = [f.(Function).getBody(), f.(ClosureExpr).getBody()]
     )
   }
@@ -237,7 +236,7 @@ module Impl {
   ) {
     name = v.getName() and
     (
-      parameterDeclInScope(_, v, scope) and
+      parameterDeclInScope(v, scope) and
       scope.getLocation().hasLocationFileInfo(_, line, column, _, _)
       or
       exists(Pat pat | pat = getAVariablePatAncestor(v) |