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  requires ! feature

The purpose of this is to allow (for instance) the module map for /usr/include
to exclude <tgmath.h> and <complex.h> when building in C++ (these headers are
instead provided by the C++ standard library in this case, and the glibc C
<tgmath.h> header would otherwise try to include <complex.h>, resulting in a
module cycle).


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With this option, arbitrarily named module map files can be specified
to be loaded as required for headers in the respective (sub)directories.

This, together with the extern module declaration allows for specifying
module maps in a modular fashion without the need for files called
"module.map".

Among other things, this allows a directory to contain two modules that
are completely independent of one another.

Review: http://llvm-reviews.chandlerc.com/D1697.

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Review: http://llvm-reviews.chandlerc.com/D1546.

I have picked up this patch form Lawrence
(http://llvm-reviews.chandlerc.com/D1063) and did a few changes.

From the original change description (updated as appropriate):
This patch adds a check that ensures that modules only use modules they
have so declared. To this end, it adds a statement on intended module
use to the module.map grammar:

  use module-id

A module can then only use headers from other modules if it 'uses' them.
This enforcement is off by default, but may be turned on with the new
option -fmodules-decluse.

When enforcing the module semantics, we also need to consider a source
file part of a module. This is achieved with a compiler option

-fmodule-name=<module-id>.

The compiler at present only applies restrictions to the module directly
being built.

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This patch is the first step to make module-map-files modular (instead
of requiring a single "module.map"-file per include directory). This
step adds a new "extern module" declaration that enables
module-map-files to reference one another along with a very basic
implementation.

The next steps are:

* Combine this with the use-declaration (from
  http://llvm-reviews.chandlerc.com/D1546) in order to only load module
  map files required for a specific compilation.
* Add an additional flag to start with a specific module-map-file (instead
  of requiring there to be at least one "module.map").

Review: http://llvm-reviews.chandlerc.com/D1637

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It was really hard to tell that modules are experimental. This makes it
really clear by

* Noting the experimental status in the title.
* Moving the "warning" above the table of contents.

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This patch was created by Lawrence Crowl and reviewed in:
http://llvm-reviews.chandlerc.com/D963

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headers may be included from within the module, but not from outside
the module.



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 - There is no reason to have a modules specific flag for disabling
   autolinking. Instead, convert the existing flag into -fno-autolink (which
   should cover other autolinking code generation paths like #pragmas if and
   when we support them).

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<rdar://problem/13509689> Introduce -module-file-info option that provides information about a particular module file.

This option can be useful for end users who want to know why they
ended up with a ton of different variants of the "std" module in their
module cache. This problem should go away over time, as we reduce the
need for module variants, but it will never go away entirely.



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More modules documentation, including the straw-man import declaration syntax and "how to modularize a platform".


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And ask for people to try it out and send us bug reports!

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