Swift: Add abstraction for compilation mode

Add a `CMAKE_Swift_COMPILATION_MODE` variable and corresponding
`Swift_COMPILATION_MODE` target property to control the compilation
mode.  Select among `wholemodule`, `singlefile`, and `incremental`.

Add policy CMP0157 to remove the default `-wmo` flags in favor of the
abstract setting.

Issue: #25366

1 files changed, 33 insertions, 0 deletions

diff --git a/Help/prop_tgt/Swift_COMPILATION_MODE.rst b/Help/prop_tgt/Swift_COMPILATION_MODE.rst
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+Swift_COMPILATION_MODE
+----------------------
+
+.. versionadded:: 3.29
+
+Specify how Swift compiles a target.
+
+The allowed values are:
+
+.. include:: Swift_COMPILATION_MODE-VALUES.txt
+
+Use :manual:`generator expressions <cmake-generator-expressions(7)>` to support
+per-configuration specification. For example, the code:
+
+.. code-block:: cmake
+
+  add_library(foo foo.swift)
+  set_property(TARGET foo PROPERTY
+    Swift_COMPILATION_MODE "$<IF:$<CONFIG:Release>,wholemodule,incremental>")
+
+sets the Swift compilation mode to wholemodule mode in the release configuration
+and sets the property to incremental mode in other configurations.
+
+The property is initialized from the value of the
+:variable:`CMAKE_Swift_COMPILATION_MODE` variable, if it is set. If the property
+is not set or is empty, then CMake uses the default value ``incremental`` to
+specify the swift compilation mode.
+
+.. note::
+
+   This property only has effect when policy :policy:`CMP0157` is set to ``NEW``
+   prior to the first :command:`project` or :command:`enable_language` command
+   that enables the Swift language.