[PATCH] D106394: [clang][pp] adds '#pragma include_instead'

[Arthur O'Dwyer via Phabricator via cfe-commits]

Quuxplusone added inline comments.


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Comment at: clang/lib/Lex/Pragma.cpp:1996
   AddPragmaHandler("clang", new PragmaSystemHeaderHandler());
+  AddPragmaHandler("clang", new PragmaIncludeInsteadHandler());
   AddPragmaHandler("clang", new PragmaDebugHandler());
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@rsmith wrote:
> I'm not in love with the design of this feature. Basing the accept / warn decision on whether the inclusion is in a system header seems fragile and doesn't seem to enforce the intended semantics of the warning (that you can only reach the implementation detail header through one of the named headers), and limits the utility of the feature to system headers. As an alternative, have you considered checking whether any of the headers named in the pragma are in the include stack, and warning if not?

@rsmith, I think you misunderstand part of the design goal here. You said "you can only reach the implementation detail header through one of the named headers" — this is true only for a maybe-surprising definition of "you." For example, if the user-programmer's `"MyThing.hpp"` tries to include `<__utility/move.h>` directly (or if IWYU wants to suggest how to include it), we expect the tool to suggest that you must include `<utility>`, not `<__utility/move.h>`. **However**, if the library's `<algorithm>` tries to include `<__utility/move.h>` directly, **that's** totally fine and expected. The entire point of these detail headers is to make sure that library headers don't have to `#include <utility>` when all they need is one little piece of it.

So we do need **some** way to distinguish user-programmer headers like `"MyThing.hpp"` from library headers like `<algorithm>`. At the moment this is done by limiting the usefulness of the feature to just system headers, and making every system header a "friend" of every other system header.

But one could imagine allowing a detail header to specify not only "here's how my non-friends should get to me" (e.g. by including `<utility>`) but also "here's a list of headers and/or directories which should be considered friends of mine" (and which could then include me directly, e.g. `<algorithm>` or `<__iterator/iter_move.h>`). We'd need a reliable syntax for that, though.


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  https://reviews.llvm.org/D106394/new/

https://reviews.llvm.org/D106394