[cfe-dev] SourceLocations of attribute list delimiters – [[ ... , ... ]] etc

[Aaron Ballman via cfe-dev]

On Thu, Sep 22, 2016 at 11:35 AM, Nico Weber <thakis at chromium.org> wrote:
> Hi,
>
> when parsing C++11 [[...]] attributes and Microsoft [...] attributes, we
> throw out the source locations of the commas separating the attributes, and
> of the surrounding bracket tokens.
>
> I'd like to write a fixit that suggests replacing [uuid(...)] with
> __declspec(uuid(...)), but that requires knowing if uuid() is the only thing
> in the [] list, and if so deleting the whole [...], and if it's not the only
> thing, deleting just the uuid() and either its trailing comma if it's not
> the last attribute, or the preceding comma if it is the last one. It's
> possible someone will want to do something similar with a C++11 attribute in
> the future.
>
> Since all C++11 attributes are in the AST, one approach would be to check if
> there's a preceding C++11 attribute and if so, start the removal after the
> last token of the predecessor, and do the same at the other end. But I think
> that can't differentiate between `[[good, remove_this, good]]` and `[[good]]
> [[remove_this]] [[good]]`. Also, in either approach and the locations of the
> [[ ]] delimiters aren't anywhere.

Correct, the AST does not track enough information to distinguish
between [[one, two]] and [[one]][[two]], for instance.

> For [] attributes, it's a bit harder still since we don't keep most of them
> in the AST.

We don't keep any ignored attributes in the AST, so you also cannot
tell between [[known, unknown]] and [[known]].

> The cleanest way to do this I can think of is to have AttributeClusters (or
> similar) in the AST and have them store the SourceLocs of the surrounding
> brackets, and the number of attributes therein.
>
> Does that sound useful? Doing this just for my fixit seems overkill :-)

It could be useful, but it does seem like possible overkill as well. I
certainly wouldn't complain if we tracked source information about
attributes better, though.

What kind of accuracy are you hoping for? For instance, do you want to
handle constructs like [[foo,,,bar]][[]][[baz]], or just things like
__declspec(foo) [[bar]] [baz, quux]?

~Aaron