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, 40 insertions, 0 deletions

diff --git a/Tests/RunCMake/CMP0019/CMP0019-WARN-stderr.txt b/Tests/RunCMake/CMP0019/CMP0019-WARN-stderr.txt
new file mode 100644
index 0000000..03faef9
--- /dev/null
+++ b/Tests/RunCMake/CMP0019/CMP0019-WARN-stderr.txt
@@ -0,0 +1,40 @@
+CMake Warning \(dev\) in CMakeLists.txt:
+  Policy CMP0019 is not set: Do not re-expand variables in include and link
+  information.  Run "cmake --help-policy CMP0019" for policy details.  Use
+  the cmake_policy command to set the policy and suppress this warning.
+
+  The following variable evaluations were encountered:
+
+  Evaluated directory INCLUDE_DIRECTORIES
+
+    /usr/include/\${VAR_INCLUDE};/usr/include/normal
+
+  as
+
+    /usr/include/VAL_INCLUDE;/usr/include/normal
+
+  Evaluated target some_target INCLUDE_DIRECTORIES
+
+    /usr/include/\${VAR_INCLUDE};/usr/include/normal
+
+  as
+
+    /usr/include/VAL_INCLUDE;/usr/include/normal
+
+  Evaluated link directory
+
+    /usr/lib/\${VAR_LINK_DIRS}
+
+  as
+
+    /usr/lib/VAL_LINK_DIRS
+
+  Evaluated link library
+
+    \${VAR_LINK_LIBS}
+
+  as
+
+    VAL_LINK_LIBS
+
+This warning is for project developers.  Use -Wno-dev to suppress it.$