EVIS: Add policy CMP0053

This policy switches between the old EVIS parser and the new, faster
parser.

Help/manual/cmake-policies.7.rst

@@ -104,3 +104,4 @@ All Policies
    /policy/CMP0050
    /policy/CMP0051
    /policy/CMP0052
+   /policy/CMP0053

Help/policy/CMP0010.rst

@@ -10,6 +10,9 @@ variable reference is an error.  The OLD behavior for this policy is
 to warn about the error, leave the string untouched, and continue.
 The NEW behavior for this policy is to report an error.
 
+If :policy:`CMP0053` is set to ``NEW``, this policy has no effect
+and is treated as always being ``NEW``.
+
 This policy was introduced in CMake version 2.6.3.  CMake version
 |release| warns when the policy is not set and uses OLD behavior.  Use
 the cmake_policy command to set it to OLD or NEW explicitly.

Help/policy/CMP0053.rst

@@ -0,0 +1,44 @@
+CMP0053
+-------
+
+Simplify variable reference and escape sequence evaluation.
+
+CMake 3.1 introduced a much faster implementation of evaluation of the
+:ref:`Variable References` and :ref:`Escape Sequences` documented in the
+:manual:`cmake-language(7)` manual.  While the behavior is identical
+to the legacy implementation in most cases, some corner cases were
+cleaned up to simplify the behavior.  Specifically:
+
+* Expansion of ``@VAR@`` reference syntax defined by the
+  :command:`configure_file` and :command:`string(CONFIGURE)`
+  commands is no longer performed in other contexts.
+
+* Literal ``${VAR}`` reference syntax may contain only
+  alphanumeric characters (``A-Z``, ``a-z``, ``0-9``) and
+  the characters ``_``, ``.``, ``/``, ``-``, and ``+``.
+  Variables with other characters in their name may still
+  be referenced indirectly, e.g.
+
+  .. code-block:: cmake
+
+    set(varname "otherwise & disallowed $ characters")
+    message("${${varname}}")
+
+* The setting of policy :policy:`CMP0010` is not considered,
+  so improper variable reference syntax is always an error.
+
+* More characters are allowed to be escaped in variable names.
+  Previously, only ``()#" \#@^`` were valid characters to
+  escape. Now any non-alphanumeric, non-semicolon, non-NUL
+  character may be escaped following the ``escape_identity``
+  production in the :ref:`Escape Sequences` section of the
+  :manual:`cmake-language(7)` manual.
+
+The ``OLD`` behavior for this policy is to honor the legacy behavior for
+variable references and escape sequences.  The ``NEW`` behavior is to
+use the simpler variable expansion and escape sequence evaluation rules.
+
+This policy was introduced in CMake version 3.1.
+CMake version |release| warns when the policy is not set and uses
+``OLD`` behavior.  Use the :command:`cmake_policy` command to set
+it to ``OLD`` or ``NEW`` explicitly.

Source/cmPolicies.cxx

@@ -354,6 +354,11 @@ cmPolicies::cmPolicies()
     "Reject source and build dirs in installed "
     "INTERFACE_INCLUDE_DIRECTORIES.",
     3,1,0, cmPolicies::WARN);
+
+  this->DefinePolicy(
+    CMP0053, "CMP0053",
+    "Simplify variable reference and escape sequence evaluation.",
+    3,1,0, cmPolicies::WARN);
 }
 
 cmPolicies::~cmPolicies()

Source/cmPolicies.h

@@ -108,6 +108,8 @@ public:
     CMP0052, ///< Reject source and build dirs in installed
     /// INTERFACE_INCLUDE_DIRECTORIES
 
+    CMP0053, ///< Simplify variable reference and escape sequence evaluation
+
     /** \brief Always the last entry.
      *
      * Useful mostly to avoid adding a comma the last policy when adding a new