[cfe-dev] clang-tidy attribute enforcement

[Steve Sudit via cfe-dev]

You're right: my example <https://godbolt.org/z/5j949m> just doesn't work
because standard attributes (as opposed to compiler-specific ones) are not
allowed on the call site. Thanks for pointing that out.

An alternative would be to replace [[out]] with OUT, a precompiler macro
which is defined but empty. Given that this is visible to clang-tidy, the
semantically-neutral token could perhaps be used as an indicator. (This is
similar to what was done with MIDL headers, fwiw.)

Does this seem more plausible? I wonder if clang-tidy is even the right
place or if the compiler would have to be involved because it's in a
position to match the call to the definition, which clang-tidy might not be.

(Ironically, clang already supports
<https://clang.llvm.org/docs/AttributeReference.html#read-only-write-only-read-write-read-only-write-only-read-write>
compiler-specific per-parameter attributes whose names are similar to what
I'd want: read_only, write_only, read_write.)

On Thu, Sep 12, 2019 at 3:26 PM Aaron Ballman <aaron at aaronballman.com>
wrote:

> On Thu, Sep 12, 2019 at 12:33 PM Steve Sudit via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > I have a question about a clang-tidy feature that would be useful to me.
> >
> > The feature would be to enforce the presence of an attribute on
> arguments when it is present in the parameter.
> >
> > So, for example, if a function is defined like:
> >   bool tryGet(const std::string& key, [[out]] std::string& value);
> >
> > Then calls to it would need to have a matching `[[out]]` attribute, like:
> >   if (!directory.tryGet(k, [[out]] v)) {
> >
> > If the attribute is omitted at the call site, a warning would be
> generated. Likewise, if the attribute appeared at the call site, but not at
> the function declaration, that would generate a warning.
> >
> > No deeper enforcement is necessary. Presumably, which attributes would
> get this enforcement would be configurable.
> >
> > Is this or anything similar already supported? If not, could someone
> please point me towards the steps I'd need to take get it added? This could
> either be as a feature request for someone else to fulfill, or if
> necessary, my own contribution to the project.
>
> Clang does not have a plugin approach for introducing new attributes,
> so there is currently not a way for a clang-tidy check to introduce a
> new attribute. Instead, you'd have to modify clang itself to support
> the attribute. Making a new parameter attribute isn't too difficult,
> there are examples in Attr.td you could use. However, the attribute
> you'd like to use at the call site is not possible -- neither C nor
> C++ allow attributes at the expression level currently so there's no
> syntax for what you want to do. That said, you may not require the
> attribute there anyway. For instance, at the call site you can look to
> the FunctionDecl for the CallExpr (if any) and inspect its parameters
> to see if any have the [[out]] attribute and use that information to
> perform your action within the clang tidy check (perhaps).
>
> ~Aaron
>
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