[cfe-dev] RFC: AST Matcher Sub-expression references implementation straw man

[Manuel Klimek]

+djasper, alexfh

On Tue, Apr 16, 2013 at 7:15 PM, Vane, Edwin <edwin.vane at intel.com> wrote:

> Hi all,
>
> I've been looking at implementing sub-expression references in AST
> Matchers. I'd like to propose an implementation and get feedback,
> especially if I've overlooked something.
>
> ======
> Quick motivation first:
> The goal of sub-expression references is to write matchers that can refer
> to nodes tagged by the matcher and use these nodes in more tests. Consider
> the following simple example that wants to find variable declarations whose
> type exactly matches the initializer type (for simplicity, lets ignore all
> the implicit cast stuff that will exist around the initializer):
>
> varDecl(hasType(qualType(...).bind("declType")),
>                hasInitializer(hasType(sameAs("declType")))
> )
>
> There's potential for other sub-expression reference matchers beyond the
> "sameAs()" matcher proposed by this example. Name comparisons for example.
> ======
>
> Proposed changes to implement sub-expression references:
> - In a sub-ex matcher, look-up in the provided BoundNodesTreeBuilder for
> the given id.
>   - If the id exists perform matcher logic, return truth value.
>   - If an id doesn't exist, record an unresolved sub-expression reference
> in the Builder and return true.
>     - Info recorded: id we're looking for and some sort of matcher
> "future": a callback or functor for executing matcher logic at a later time
> along with all necessary data matcher needs.
> - When control returns to the top level finder logic
> (MatchASTVisitor::match()) we tend to unresolved sub-ex refs. For each
> recorded unresolved sub-ex ref, do a search through the BoundNodesTree for
> a matching id. If one is found, execute matcher future. If the future
> returns false, the match fails and the Callback that would normally get
> called will not be. If no id is found, match also fails. If the matcher
> future returns true, proceed to calling Callback for the match. The
> handling of unresolved sub-ex refs could be handled by modifying
> MatchVisitor.
>
> Outstanding questions:
> - Where should unresolved sub-ex refs be stored? Top-most level of the
> BoundNodesTree or just in the level at which the ref happens. Does it
> matter? Sub-ex refs should be able to refer to nodes anywhere else in the
> matcher and not have specific scope. Even if refs are stored at the current
> level, they should be collected and handled all at once when matching is
> complete.
>
> --
> Edwin Vane
>   Software Developer
>   Intel of Canada, Inc.
>
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