Rearrange internal scope documentation, add note in target import

2 files changed, 150 insertions, 148 deletions

diff --git a/NEWS b/NEWS
index 01a3ff1..48664e2 100644
--- a/NEWS
+++ b/NEWS
@@ -101,7 +101,7 @@ Version 0.14.0
     side-building BMIs for external module interfaces and header units.
 
     Note that this functionality is not without limitations and drawbacks and,
-    if needed, should be enabled explicitly. See the "C++ Compilation Internal
+    if needed, should be enabled explicitly. See the "Compilation Internal
     Scope" section in the manual for details.
 
   * C++20 modules support in GCC 11 using the module mapper.
diff --git a/doc/manual.cli b/doc/manual.cli
index bc7a68e..d63e887 100644
--- a/doc/manual.cli
+++ b/doc/manual.cli
@@ -2580,6 +2580,13 @@ also try to locate its \c{pkg-config(1)} file and, if present, extract
 additional compile/link flags from it. The link rule also automatically
 produces \c{pkg-config(1)} files for libraries that it installs.|
 
+\N|A common problem with importing and using third-party C/C++ libraries is
+compiler warnings. Specifically, we are likely to include their headers into
+our project's source files which means we may see warnings in such headers
+(which we cannot always fix) mixed with warnings in our code (which we should
+normally be able to fix). See \l{#cc-internal-scope Compilation Internal
+Scope} for a mechanism to deal with this problem.|
+
 Let's now examine the exporting side of the importation mechanism. While a
 project doesn't need to do anything special to be found by \c{import}, it does
 need to handle locating the exported target (or targets; there could be
@@ -6491,6 +6498,148 @@ $ b config.cxx=\"g++ -m32\"
 $ b config.cxx=\"clang++ -stdlib=libc++\"
 \
 
+\h#cc-internal-scope|Compilation Internal Scope|
+
+\N|While this section uses the \c{cxx} module and C++ compilation as an
+example, the same functionality is available for C compilation \- simply
+replace \c{cxx} with \c{c} in the module and variable names.|
+
+The \c{cxx} module has a notion of a project's internal scope. During
+compilation of a project's C/C++ translation units a header search path
+(\c{-I}) exported by a library that is outside of the internal scope is
+considered external and, if supported by the compiler, the corresponding
+\c{-I} option is translated to an appropriate \"external header search path\"
+option (\c{-isystem} for GCC/Clang, \c{/external:I} for MSVC 16.10 and
+later). In particular, this suppresses compiler warnings in such external
+headers (\c{/external:W0} is automatically added unless a custom
+\c{/external:Wn} is specified).
+
+\N|While the aim of this functionality is to control warnings in external
+libraries, the underlying mechanisms currently provided by compilers have
+limitations and undesirable side effects. In particular, \c{-isystem} paths
+are searched after \c{-I} so translating \c{-I} to \c{-isystem} alters the
+search order. This should normally be harmless when using a development build
+of a library but may result in a change of semantics for installed
+libraries. Also, marking the search path as system has additional (to warning
+suppression) effects, see
+\l{https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html System Headers} in
+the GCC documentation for details. On the MSVC side, \c{/external:W0}
+currently does not suppress some warnings (refer to the MSVC documentation for
+details).
+
+Another issue is warnings in template instantiations. Each such warning could
+be either due to a (potential) issue in the template itself or due to the
+template arguments we are instantiating it with. By default, all such warnings
+are suppressed and there is currently no way to change this with GCC/Clang
+\c{-isystem}. While MSVC provides \c{/external:templates-}, it cannot be
+applied on the library by library basis, only globally for the entire
+compilation. See MSVC \c{/external:templates-} documentation for more
+background on this issue.|
+
+\N|In the future this functionality will be extended to side-building BMIs for
+external module interfaces and header units.|
+
+The internal scope can be specified by the project with the
+\c{cxx.internal.scope} variable and overridden by the user with the
+\c{config.cxx.internal.scope} variable. Note that \c{cxx.internal.scope} must
+be specified before loading the \c{cxx} module (\c{cxx.config}, more
+precisely) and after which it contains the effective value (see below). For
+example:
+
+\
+# root.build
+
+cxx.std = latest
+cxx.internal.scope = current
+
+using cxx
+\
+
+Valid values for \c{cxx.internal.scope} are:
+
+\
+current  -- current root scope (where variable is assigned)
+base     -- target's base scope
+root     -- target's root scope
+bundle   -- target's bundle amalgamation
+strong   -- target's strong amalgamation
+weak     -- target's weak amalgamation
+global   -- global scope (everything is internal)
+\
+
+Valid values for \c{config.cxx.internal.scope} are the same except for
+\c{current}.
+
+Note also that there are \c{[config.]cc.internal.scope} variables that can be
+used to specify the internal scope for all the \c{cc}-based modules.
+
+The project's effective internal scope is chosen based on the following
+priority list:
+
+\ol|
+
+\li|\c{config.cxx.internal.scope}|
+
+\li|\c{config.cc.internal.scope}|
+
+\li|effective scope from bundle amalgamation|
+
+\li|\c{cxx.internal.scope}|
+
+\li|\c{cc.internal.scope}||
+
+In particular, item #3 allows an amalgamation that bundles a project to
+override its internal scope.
+
+\N|If no \c{*.internal.scope} is specified by the project, user, or bundle,
+then this functionality is disabled and all libraries are treated as internal
+regardless of their location.
+
+While it may seem natural to have this enabled by default, the limitations and
+side effects of the underlying mechanisms as well as cases where it would be
+undesirable (such as in separate \c{*-tests} projects, see below) all suggest
+that explicit opt-in is probably the correct choice.|
+
+The recommended value for a typical project is \c{current}, meaning that only
+headers inside the project will be considered internal. The \c{tests}
+subproject, if present, will inherit its value from the project (which acts as
+a bundle amalgamation), unless it is being built out of source (for example,
+to test an installed library).
+
+A project can also whitelist specific libraries using the
+\c{cxx.internal.libs} variable. If a library target name (that is, the name
+inside \c{lib{\}}) matches any of the wildcard patterns listed in this
+variable, then the library is considered internal regardless of its
+location. For example (notice that the pattern is quoted):
+
+\
+# root.build
+
+cxx.std = latest
+cxx.internal.scope = current
+cxx.internal.libs = foo 'bar-*'
+
+using cxx
+\
+
+Note that this variable should also be set before loading the \c{cxx} module
+and there is the common \c{cc.internal.libs} equivalent. However, there are
+no \c{config.*} versions nor the override by the bundle amalgamation
+semantics.
+
+Typically you would want to whitelist libraries that are developed together
+but reside in separate build system projects. In particular, a separate
+\c{*-tests} project for a library should whitelist the library being tested if
+the internal scope functionality is in use. Another reason to whitelist is to
+catch warnings in instantiations of templates that belong to a library that is
+otherwise warning-free (see the MSVC \c{/external:templates-} option for
+background).
+
+Note also that if multiple libraries are installed into the same location (or
+otherwise share the same header search paths, for example, as a family of
+libraries), then the whitelist may not be effective.
+
+
 \h#cc-gcc|GCC Compiler Toolchain|
 
 The GCC compiler id is \c{gcc}.
@@ -6719,16 +6868,6 @@ config.c.internal.scope
   c.internal.scope
 \
 
-\h#c-internal-scope|C Compilation Internal Scope|
-
-The \c{c} module has a notion of a project's internal scope. During
-compilation of a project's C translation units a header search path (\c{-I})
-exported by a library that is outside of the internal scope is considered
-external and, if supported by the compiler, the corresponding \c{-I} option is
-translated to an appropriate \"external header search path\" option. See
-\l{#cxx-internal-scope C++ Compilation Internal Scope} for details on this
-functionality.
-
 
 \h1#module-cxx|\c{cxx} Module|
 
@@ -6796,143 +6935,6 @@ config.cxx.translate_include
   cxx.translate_include
 \
 
-\h#cxx-internal-scope|C++ Compilation Internal Scope|
-
-The \c{cxx} module has a notion of a project's internal scope. During
-compilation of a project's C++ translation units a header search path (\c{-I})
-exported by a library that is outside of the internal scope is considered
-external and, if supported by the compiler, the corresponding \c{-I} option is
-translated to an appropriate \"external header search path\" option
-(\c{-isystem} for GCC/Clang, \c{/external:I} for MSVC 16.10 and later). In
-particular, this suppresses compiler warnings in such external headers
-(\c{/external:W0} is automatically added unless a custom \c{/external:Wn} is
-specified).
-
-\N|While the aim of this functionality is to control warnings in external
-libraries, the underlying mechanisms currently provided by compilers have
-limitations and undesirable side effects. In particular, \c{-isystem} paths
-are searched after \c{-I} so translating \c{-I} to \c{-isystem} alters the
-search order. This should normally be harmless when using a development build
-of a library but may result in a change of semantics for installed
-libraries. Also, marking the search path as system has additional (to warning
-suppression) effects, see
-\l{https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html System Headers} in
-the GCC documentation for details. On the MSVC side, \c{/external:W0}
-currently does not suppress some warnings (refer to the MSVC documentation for
-details).
-
-Another issue is warnings in template instantiations. Each such warning could
-be either due to a (potential) issue in the template itself or due to the
-template arguments we are instantiating it with. By default, all such warnings
-are suppressed and there is currently no way to change this with GCC/Clang
-\c{-isystem}. While MSVC provides \c{/external:templates-}, it cannot be
-applied on the library by library basis, only globally for the entire
-compilation. See MSVC \c{/external:templates-} documentation for more
-background on this issue.|
-
-\N|In the future this functionality will be extended to side-building BMIs for
-external module interfaces and header units.|
-
-The internal scope can be specified by the project with the
-\c{cxx.internal.scope} variable and overridden by the user with the
-\c{config.cxx.internal.scope} variable. Note that \c{cxx.internal.scope} must
-be specified before loading the \c{cxx} module (\c{cxx.config}, more
-precisely) and after which it contains the effective value (see below). For
-example:
-
-\
-# root.build
-
-cxx.std = latest
-cxx.internal.scope = current
-
-using cxx
-\
-
-Valid values for \c{cxx.internal.scope} are:
-
-\
-current  -- current root scope (where variable is assigned)
-base     -- target's base scope
-root     -- target's root scope
-bundle   -- target's bundle amalgamation
-strong   -- target's strong amalgamation
-weak     -- target's weak amalgamation
-global   -- global scope (everything is internal)
-\
-
-Valid values for \c{config.cxx.internal.scope} are the same except for
-\c{current}.
-
-Note also that there are \c{[config.]cc.internal.scope} variables that can be
-used to specify the internal scope for all the \c{cc}-based modules.
-
-The project's effective internal scope is chosen based on the following
-priority list:
-
-\ol|
-
-\li|\c{config.cxx.internal.scope}|
-
-\li|\c{config.cc.internal.scope}|
-
-\li|effective scope from bundle amalgamation|
-
-\li|\c{cxx.internal.scope}|
-
-\li|\c{cc.internal.scope}||
-
-In particular, item #3 allows an amalgamation that bundles a project to
-override its internal scope.
-
-\N|If no \c{*.internal.scope} is specified by the project, user, or bundle,
-then this functionality is disabled and all libraries are treated as internal
-regardless of their location.
-
-While it may seem natural to have this enabled by default, the limitations and
-side effects of the underlying mechanisms as well as cases where it would be
-undesirable (such as in separate \c{*-tests} projects, see below) all suggest
-that explicit opt-in is probably the correct choice.|
-
-The recommended value for a typical project is \c{current}, meaning that only
-headers inside the project will be considered internal. The \c{tests}
-subproject, if present, will inherit its value from the project (which acts as
-a bundle amalgamation), unless it is being built out of source (for example,
-to test an installed library).
-
-A project can also whitelist specific libraries using the
-\c{cxx.internal.libs} variable. If a library target name (that is, the name
-inside \c{lib{\}}) matches any of the wildcard patterns listed in this
-variable, then the library is considered internal regardless of its
-location. For example (notice that the pattern is quoted):
-
-\
-# root.build
-
-cxx.std = latest
-cxx.internal.scope = current
-cxx.internal.libs = foo 'bar-*'
-
-using cxx
-\
-
-Note that this variable should also be set before loading the \c{cxx} module
-and there is the common \c{cc.internal.libs} equivalent. However, there are
-no \c{config.*} versions nor the override by the bundle amalgamation
-semantics.
-
-Typically you would want to whitelist libraries that are developed together
-but reside in separate build system projects. In particular, a separate
-\c{*-tests} project for a library should whitelist the library being tested if
-the internal scope functionality is in use. Another reason to whitelist is to
-catch warnings in instantiations of templates that belong to a library that is
-otherwise warning-free (see the MSVC \c{/external:templates-} option for
-background).
-
-Note also that if multiple libraries are installed into the same location (or
-otherwise share the same header search paths, for example, as a family of
-libraries), then the whitelist may not be effective.
-
 
 \h#cxx-modules|C++ Modules Support|