Add policy CMP0019 to skip include/link variable re-expansion

Historically CMake has always expanded ${} variable references in the
values given to include_directories(), link_directories(), and
link_libraries().  This has been unnecessary since general ${}
evaluation syntax was added to the language a LONG time ago, but has
remained for compatibility with VERY early CMake versions.

For a long time the re-expansion was a lightweight operation because it
was only processed once at the directory level and the fast-path of
cmMakefile::ExpandVariablesInString was usually taken because values did
not have any '$' in them.  Then commit d899eb71 (Call
ExpandVariablesInString for each target's INCLUDE_DIRECTORIES,
2012-02-22) made the operation a bit heavier because the expansion is
now needed on a per-target basis.  In the future we will support
generator expressions in INCLUDE_DIRECTORIES with $<> syntax, so the
fast-path in cmMakefile::ExpandVariablesInString will no longer be taken
and re-expansion will be very expensive.

Add policy CMP0019 to skip the re-expansion altogether in NEW behavior.
In OLD behavior perform the expansion but improve the fast-path
heuristic to match ${} but not $<>.  If the policy is not set then warn
if expansion actually does anything.  We expect this to be encountered
very rarely in practice.

1 files changed, 17 insertions, 0 deletions

diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index 6aef502..eb7d666 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -491,6 +491,23 @@ cmPolicies::cmPolicies()
     "CMAKE_SHARED_LIBRARY_<Lang>_FLAGS whether it is modified or not and "
     "honor the POSITION_INDEPENDENT_CODE target property.",
     2,8,9,0, cmPolicies::WARN);
+
+  this->DefinePolicy(
+    CMP0019, "CMP0019",
+    "Do not re-expand variables in include and link information.",
+    "CMake 2.8.10 and lower re-evaluated values given to the "
+    "include_directories, link_directories, and link_libraries "
+    "commands to expand any leftover variable references at the "
+    "end of the configuration step.  "
+    "This was for strict compatibility with VERY early CMake versions "
+    "because all variable references are now normally evaluated during "
+    "CMake language processing.  "
+    "CMake 2.8.11 and higher prefer to skip the extra evaluation."
+    "\n"
+    "The OLD behavior for this policy is to re-evaluate the values "
+    "for strict compatibility.  "
+    "The NEW behavior for this policy is to leave the values untouched.",
+    2,8,11,0, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()