[AST] Allow limiting the scope of common AST traversals (getParents, RAV).

Summary:
The goal is to allow analyses such as clang-tidy checks to run on a
subset of the AST, e.g. "only on main-file decls" for interactive tools.

Today, these become "problematically global" by running RecursiveASTVisitors
rooted at the TUDecl, or by navigating up via ASTContext::getParent().

The scope is restricted using a set of top-level-decls that RecursiveASTVisitors
should be rooted at. This also applies to the visitor that populates the
parent map, and so the top-level-decls are considered to have no parents.

This patch makes the traversal scope a mutable property of ASTContext.
The more obvious way to do this is to pass the top-level decls to
relevant functions directly, but this has some problems:
 - it's error-prone: accidentally mixing restricted and unrestricted
   scopes is a performance trap. Interleaving multiple analyses is
   common (many clang-tidy checks run matchers or RAVs from matcher callbacks)
 - it doesn't map well to the actual use cases, where we really do want
   *all* traversals to be restricted.
 - it involves a lot of plumbing in parts of the code that don't care
   about traversals.
This approach was tried out in D54259 and D54261, I wanted to like it
but it feels pretty awful in practice.

Caveats: to get scope-limiting behavior of RecursiveASTVisitors, callers
have to call the new TraverseAST(Ctx) function instead of TraverseDecl(TU).
I think this is an improvement to the API regardless.

Reviewers: klimek, ioeric

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D54309

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346847 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 17 insertions, 6 deletions

diff --git a/lib/ASTMatchers/ASTMatchFinder.cpp b/lib/ASTMatchers/ASTMatchFinder.cpp
index 63f8395b82..8d65714317 100644
--- a/lib/ASTMatchers/ASTMatchFinder.cpp
+++ b/lib/ASTMatchers/ASTMatchFinder.cpp
@@ -635,10 +635,6 @@ private:
   bool memoizedMatchesAncestorOfRecursively(
       const ast_type_traits::DynTypedNode &Node, const DynTypedMatcher &Matcher,
       BoundNodesTreeBuilder *Builder, AncestorMatchMode MatchMode) {
-    if (Node.get<TranslationUnitDecl>() ==
-        ActiveASTContext->getTranslationUnitDecl())
-      return false;
-
     // For AST-nodes that don't have an identity, we can't memoize.
     if (!Builder->isComparable())
       return matchesAncestorOfRecursively(Node, Matcher, Builder, MatchMode);
@@ -673,7 +669,22 @@ private:
                                     BoundNodesTreeBuilder *Builder,
                                     AncestorMatchMode MatchMode) {
     const auto &Parents = ActiveASTContext->getParents(Node);
-    assert(!Parents.empty() && "Found node that is not in the parent map.");
+    if (Parents.empty()) {
+      // Nodes may have no parents if:
+      //  a) the node is the TranslationUnitDecl
+      //  b) we have a limited traversal scope that excludes the parent edges
+      //  c) there is a bug in the AST, and the node is not reachable
+      // Usually the traversal scope is the whole AST, which precludes b.
+      // Bugs are common enough that it's worthwhile asserting when we can.
+      assert(Node.get<TranslationUnitDecl>() ||
+             /* Traversal scope is limited if none of the bounds are the TU */
+             llvm::none_of(ActiveASTContext->getTraversalScope(),
+                           [](Decl *D) {
+                             return D->getKind() == Decl::TranslationUnit;
+                           }) &&
+                 "Found node that is not in the complete parent map!");
+      return false;
+    }
     if (Parents.size() == 1) {
       // Only one parent - do recursive memoization.
       const ast_type_traits::DynTypedNode Parent = Parents[0];
@@ -1019,7 +1030,7 @@ void MatchFinder::matchAST(ASTContext &Context) {
   internal::MatchASTVisitor Visitor(&Matchers, Options);
   Visitor.set_active_ast_context(&Context);
   Visitor.onStartOfTranslationUnit();
-  Visitor.TraverseDecl(Context.getTranslationUnitDecl());
+  Visitor.TraverseAST(Context);
   Visitor.onEndOfTranslationUnit();
 }