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, 9 insertions, 0 deletions

diff --git a/Tests/RunCMake/CMP0019/CMP0019-code.cmake b/Tests/RunCMake/CMP0019/CMP0019-code.cmake
new file mode 100644
index 0000000..26c0e5b
--- /dev/null
+++ b/Tests/RunCMake/CMP0019/CMP0019-code.cmake
@@ -0,0 +1,9 @@
+set(VAR_INCLUDE "VAL_INCLUDE")
+set(VAR_LINK_DIRS "VAL_LINK_DIRS")
+set(VAR_LINK_LIBS "VAL_LINK_LIBS")
+add_custom_target(some_target)
+include_directories("/usr/include/\${VAR_INCLUDE}" /usr/include/normal)
+link_directories("/usr/lib/\${VAR_LINK_DIRS}"  /usr/lib/normal)
+link_libraries("\${VAR_LINK_LIBS}" normal)
+add_custom_target(other_target)
+set_property(TARGET other_target PROPERTY INCLUDE_DIRECTORIES "")