target_link_libraries: Allow use with targets in other directories

Previously the command did not allow naming targets on the LHS that
were not created in the calling directory.  Lift this restriction to
enable more flexible use by projects.

Targets named on the RHS will need to be looked up during generation in
the scope of the call site rather than the scope of the LHS target.
Introduce an internal syntax in `[INTERFACE_]LINK_LIBRARIES` properties
to specify target names that need to be looked up in a directory other
than that containing the target on which the property is set.  Add
minimal documentation of the syntax to help users that encounter it.

Unfortunately CMake previously did allow such calls in the case that
only `INTERFACE` libraries are specified, but those libraries would be
looked up in the target's directory rather than the caller's.  Add
policy `CMP0079` to enable the new behavior with new lookup scope in a
compatible way.

Fixes: #17943

1 files changed, 11 insertions, 0 deletions

diff --git a/Source/cmGlobalGenerator.h b/Source/cmGlobalGenerator.h
index 6b972eb..f240f1d 100644
--- a/Source/cmGlobalGenerator.h
+++ b/Source/cmGlobalGenerator.h
@@ -26,6 +26,9 @@
 #  include "cmFileLockPool.h"
 #endif
 
+#define CMAKE_DIRECTORY_ID_SEP "::@"
+
+class cmDirectoryId;
 class cmExportBuildFileGenerator;
 class cmExternalMakefileProjectGenerator;
 class cmGeneratorTarget;
@@ -284,6 +287,7 @@ public:
   bool NameResolvesToFramework(const std::string& libname) const;
 
   cmMakefile* FindMakefile(const std::string& start_dir) const;
+  cmLocalGenerator* FindLocalGenerator(cmDirectoryId const& id) const;
 
   /** Append the subdirectory for the given configuration.  If anything is
       appended the given prefix and suffix will be appended around it, which
@@ -516,6 +520,7 @@ private:
   typedef std::unordered_map<std::string, cmGeneratorTarget*>
     GeneratorTargetMap;
   typedef std::unordered_map<std::string, cmMakefile*> MakefileMap;
+  typedef std::unordered_map<std::string, cmLocalGenerator*> LocalGeneratorMap;
   // Map efficiently from target name to cmTarget instance.
   // Do not use this structure for looping over all targets.
   // It contains both normal and globally visible imported targets.
@@ -527,6 +532,11 @@ private:
   // It may not contain all of them (see note in IndexMakefile method).
   MakefileMap MakefileSearchIndex;
 
+  // Map efficiently from source directory path to cmLocalGenerator instance.
+  // Do not use this structure for looping over all directories.
+  // Its order is not deterministic.
+  LocalGeneratorMap LocalGeneratorSearchIndex;
+
   cmMakefile* TryCompileOuterMakefile;
   // If you add a new map here, make sure it is copied
   // in EnableLanguagesFromGenerator
@@ -585,6 +595,7 @@ private:
                     std::string const& reason) const;
 
   void IndexMakefile(cmMakefile* mf);
+  void IndexLocalGenerator(cmLocalGenerator* lg);
 
   virtual const char* GetBuildIgnoreErrorsFlag() const { return nullptr; }