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By removing the INTERFACE_ prefix, we can use this in more contexts.

Stephen Kelly authored Jul 02, 2013 and Brad King committed Jul 16, 2013

Unlike other target properties, this does not have a corresponding
non-INTERFACE variant.

This allows propagation of system attribute on include directories
from link dependents.

This is an internal expression for use by target_link_libraries
for static libraries.

This will allow testing whether we are evaluating the link libraries
of a particular target.

Hopefully this will prevent regressions when adding further transitive
properties in the future.

This method reads generator expressions from the COMPILE_OPTIONS
target property, as well as INTERFACE_COMPILE_OPTIONS from linked
dependents.

While porting boost to use these features, the generation step took
too long (several minutes before I stopped it). The reason was that
the boost libraries form a large interdependent mesh. The libraries
list their dependencies in their INTERFACE such as:

 $<LINKED:boost::core>;$<LINKED:boost::config>;$<LINKED:boost::mpl>

As boost::core already depends on the boost::config libraries, that
expression has no impact on the end-content, as it is removed after
the generation step. There is no DAG issue though, so the generator
expression evaluation would fully evaluate them. In the case of the
config library, it also depends on the core library, so all depends
are followed through that again, despite the fact that they've just
been evaluated. After this patch, the evaluation skips libraries if
they have already been seen via depends or directly in the content.

This patch keeps track of targets whose INTERFACE has been consumed
already. The INCLUDE_DIRECTORIES and COMPILE_DEFINITIONS properties
are whitelisted because repeated content will be stripped out later
during generation. For other properties now and in the future, that
may not be the case.

This will allow the implementation of the LINKED generator expression.

The properties are evaluated as link-dependent interface properties when
evaluating the generator expressions.

Constructs such as these are an error as they are direct self-references:

 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>)
 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:INCLUDE_DIRECTORIES>)

However, this is an indirect self-reference in a cycle, and not an error:

 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:bar,INCLUDE_DIRECTORIES>)
 set_property(TARGET bar APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>)

Stephen Kelly authored Sep 18, 2012 and Brad King committed Sep 28, 2012

Initially this will only be used to check for self-references, but
can be extended to check for cycles when chaining properties of other
targets.