[boost] <link>header?

Subject: [boost] <link>header?
From: Peter Dimov (lists_at_[hidden])
Date: 2018-01-22 03:17:00

While refactoring the test Jamfile for Boost.System, I wrote this rule:

rule system-run- ( sources + )
{
   local result ;

   result += [ run $(sources) : : : <link>static : $(sources[1]:B)_static ] 
;
   result += [ run $(sources) : : : <link>shared : $(sources[1]:B)_shared ] 
;
   result += [ run $(sources) : : : -<library>/boost/system//boost_system 
<define>BOOST_ERROR_CODE_HEADER_ONLY : $(sources[1]:B)_header ] ;

   return $(result) ;
}

to avoid repeating each test three times.

The inconsistency is somewhat annoying. We could normalize the third line by 
introducing a third link type, <link>header, in response to which the 
library can switch to header-only mode.

This is more or less the same suggestion as Niall's _hl targets from the 
CMake discussion, but it fits Boost.Build better, because the <link> 
property is automatically propagated to dependencies.

Furthermore, after thinking about it a bit, I think that we might even have 
room for a fourth link type, <link>source, in response to which the library 
target would pass an appropriate <source> usage-requirement upwards. 
(<source>error_code.cpp in Boost.System's case.)

<link>source is kind of best of both worlds, as it doesn't reparse the 
source code each time the library is used, and at the same time is not prone 
to the ODR violations introduced by inadvertently compiling the library and 
the executable with different, ABI-incompatible options.

Comments?