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.
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.