[PATCH] D116518: [ast-matchers] Add hasSubstatementSequence matcher

[Richard via Phabricator via cfe-commits]

LegalizeAdulthood added inline comments.


================
Comment at: clang/include/clang/ASTMatchers/ASTMatchers.h:5435-5442
+/// Matches two consecutive statements within a compound statement.
+///
+/// Given
+/// \code
+///   { if (x > 0) return true; return false; }
+/// \endcode
+/// compoundStmt(hasSubstatementSequence(ifStmt(), returnStmt()))
----------------
aaron.ballman wrote:
> LegalizeAdulthood wrote:
> > aaron.ballman wrote:
> > > LegalizeAdulthood wrote:
> > > > aaron.ballman wrote:
> > > > > How do we extend this to support testing arbitrary sequences of statements? (If this supports two statements, someone will find a need for three, etc).
> > > > Yeah, I was wondering that too.  I didn't see (but didn't look extensively) any support for variadic matchers taking a parameter pack.
> > > > 
> > > > I stopped at 2 because this suits my immediate needs with `readability-simplify-boolean-expr` where I have to manually loop over `CompoundStmt` matches in order to verify that the `if (x) return true; return false;` constructs consist of two adjacent statements.
> > > I don't think we have any variadic matchers yet to model on, but I think if this is only needed for one check, we can add it in the current form as a local matcher for that check. Whenever we figure out how to give the better interface through the static and dynamic matchers, then we can figure out how to lift this to the general matcher interface.
> > > 
> > > WDYT?
> > I don't think it is harmful to make it visible to all and I think it is helpful.
> > Defining it in ASTMatchers, enables using it in `clang-query`, for instance.
> > 
> I contend this is not a generally useful matcher without supporting an arbitrary number of statements. Even then, to be honest, it's questionable whether there's sufficient need for this to be a public matcher. Typically, we don't expose a new public matcher unless there's a general need for it, and this one is already pretty borderline even if it's fully generalized. This file is *very* expensive to instantiate and it gets used in a lot of places, so that's one of the primary reasons we don't expose matchers from here unless they're generally useful.
> 
> Unless @klimek or another AST matcher code owner thinks this is useful in general (to multiple checks people are likely to want to write even if they're not contributing the checks to the community), I'm opposed to exposing this as-is. However, adding it as a private matcher for the check that needs the limited functionality would get no opposition from me (and this implementation looks correct as well).
My thoughts:

I'm OK with moving it as a private matcher as it would simplify a big chunk of code
in the simplify-boolean-expr check.

I agree that ASTMatchers.h is used all over the place and at a minimum causes a
huge amount of rebuild.

Regarding the general usefulness of the matcher, let me elaborate on my motivation
for adding this matcher.

I see it from the viewpoint of a developer of checks/refactorings.

I really want us to get to a world where a complete refactoring can be specified as
a script input to a refactoring tool.  Eliminating the need to write C++ that directly
manipulates the AST and the edits will lower the bar for entry for other people
writing automated changes to their codebases.

Since the last time I worked in the clang codebase, the transformer library has
been added.  With this new library, I think all we're missing is a parser that matches
the input script to the necessary calls to code in the transformer library.  Once we
do this, I think the need for more and higher-level matchers will become evident
and a matcher like this is the only way you can specify that a statement Y
immediately follows statement X purely through matchers.  Current matchers
don't even let you specify relative ordering of statements.  The best you can do
is assert that a block contains the statements of interest and then you must write
your own C++ code to walk the block and determine if they fit your actual
match criteria.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116518/new/

https://reviews.llvm.org/D116518