Prevent module import inside @implementation

[Richard Smith]

On Wed, Feb 5, 2014 at 12:43 PM, Ben Langmuir <blangmuir at apple.com> wrote:

>
> On Feb 5, 2014, at 11:18 AM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> > I'm not especially fond of treating annot_module_begin and
> annot_module_include differently. That'll mean we get completely different
> diagnostics for a '#include' in a bad place depending on whether it's
> including something in the same top-level module or something in a
> different top-level module.
> >
>
> I approached this from the other side and didn't like that
> annot_module_include is handled so differently from @import.


I think they're fundamentally different, at least currently:

#include sometimes triggers a submodule build (in the same preprocessor +
parser + sema) so must only appear at the top level in such a case. We also
want the #include to behave (effectively) the same whether it textually
includes a file or imports a module, so that people don't accidentally
write code that means different things with compilers that support modules
and with compilers that do not (or depending on which module a particular
header is part of, or the order in which submodules of a module are built).

Conversely, @import merely makes declaration and macro names visible. It
never triggers an in-situ submodule build (sometimes a different module
will be built in a separate thread and context, but that's not a problem).
It can't (reliably) be used to import a submodule of the same module. So,
there's no *technical* reason not to allow @import anywhere, as far as I
can tell. That said, some of these properties are probably bugs in @import,
and maybe it should act more like #include.

> I'm also not sure that the diagnostic change here is an improvement.
> Previously:
> >
> > #include "not_module.h"
> > #include "module.h"
> >
> > ... where not_module.h contains:
> >
> > class X {
> >   // ...
> > // whoops, forgot the '};'
> >
> > would give a "missing '}'" diagnostic, pointing at the '{' that is left
> un-closed. It now gives a mysterious 'must be at the top level' diagnostic,
> not pointing at the '}' any more, followed by more errors (because
> 'module.h' didn't get imported and because the code after the #include is
> still within the class). The error here was almost certainly that a '}' was
> missing, so this seems like a regression.
>
> Ick, I'll take a look at this case.
>
> >
> > In what cases does this change allow us to recover from errors better?
>
>
> Whenever an implicit import is not at top level, we currently give errors
> like 'expected expression', or 'expected }', pointing at the location on
> #include/#import, and give no hint as to how a preprocessor directive
> (#include) could be the source of the error.


We can, and almost certainly should, customize the ExpectAndConsume
diagnostic for the case where the current token is one of the
annot_module_* tokens. I think that's probably the best way to attack this
issue.


> To make matters worse, once you break out of whatever parsing context you
> were in due to the module_include, any following code is likely to become
> errors.
>
> int foo() {
>   int x = 5;
>   #import "Bar.h" // expected }
>   if (x == 6) {// expected identifier or (
>     ++x; // expected identifier or (
>   }
>   return 1; // expected identifier of (
> } // extraneous closing brace


Do you think this is actually a likely error case? I find it implausible
that someone would deliberately include a modular header into the middle of
some scope. I think it's vastly more likely that they missed a close brace
before the include.

If this really is a likely scenario, then I think we should make the
diagnostic for an unexpected annot_module_* a FatalError, because there's
simply no way we can reasonably recover (we have two recovery options, and
either of them is disastrous if we're in the other situation).
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