[clang-tools-extra] [clang-tidy] Add support for determining constness of more expressions. (PR #82617)

[via cfe-commits]

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@@ -34,69 +36,185 @@ void extractNodesByIdTo(ArrayRef<BoundNodes> Matches, StringRef ID,
     Nodes.insert(Match.getNodeAs<Node>(ID));
 }
 
+// A matcher that matches DeclRefExprs that are used in ways such that the
+// underlying declaration is not modified.
+// If the declaration is of pointer type, `Indirections` specifies the level
+// of indirection of the object whose mutations we are tracking.
+//
+// For example, given:
+//   ```
+//   int i;
+//   int* p;
+//   p = &i;  // (A)
+//   *p = 3;  // (B)
+//   ```
+//
+//  `declRefExpr(to(varDecl(hasName("p"))), doesNotMutateObject(0))` matches
+//  (B), but `declRefExpr(to(varDecl(hasName("p"))), doesNotMutateObject(1))`
+//  matches (A).
+//
+AST_MATCHER_P(DeclRefExpr, doesNotMutateObject, int, Indirections) {
+  // We walk up the parents of the DeclRefExpr recursively until we end up on a
+  // parent that cannot modify the underlying object. There are a few kinds of
+  // expressions:
+  //  - Those that cannot be used to mutate the underlying object. We can stop
+  //    recursion there.
+  //  - Those that can be used to mutate the underlying object in analyzable
+  //    ways (such as taking the address or accessing a subobject). We have to
+  //    examine the parents.
+  //  - Those that we don't know how to analyze. In that case we stop there and
+  //    we assume that they can mutate the underlying expression.
+
+  struct StackEntry {
+    StackEntry(const Expr *E, int Indirections)
+        : E(E), Indirections(Indirections) {}
+    // The expression to analyze.
+    const Expr *E;
+    // The number of pointer indirections of the object being tracked (how
+    // many times an address was taken).
+    int Indirections;
+  };
+
+  llvm::SmallVector<StackEntry, 4> Stack;
+  Stack.emplace_back(&Node, Indirections);
+  auto &Ctx = Finder->getASTContext();
+
+  while (!Stack.empty()) {
+    const StackEntry Entry = Stack.back();
+    Stack.pop_back();
+
+    // If the expression type is const-qualified at the appropriate indirection
+    // level then we can not mutate the object.
+    QualType Ty = Entry.E->getType().getCanonicalType();
+    for (int I = 0; I < Entry.Indirections; ++I) {
+      assert(Ty->isPointerType());
+      Ty = Ty->getPointeeType().getCanonicalType();
+    }
+    if (Ty.isConstQualified()) {
+      continue;
+    }
+
+    // Otherwise we have to look at the parents to see how the expression is
+    // used.
+    const auto Parents = Ctx.getParents(*Entry.E);
+    // Note: most nodes have a single parents, but there exist nodes that have
+    // several parents, such as `InitListExpr` that have semantic and syntactic
+    // forms.
+    for (const auto &Parent : Parents) {
+      if (Parent.get<CompoundStmt>()) {
+        // Unused block-scope statement.
+        continue;
+      }
+      const Expr *const P = Parent.get<Expr>();
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EugeneZelenko wrote:

`auto` could be used here - type is explicitly stated.

https://github.com/llvm/llvm-project/pull/82617