ConfigureLog: Add infrastructure for structured configure event logging

Add infrastructure for a "configure log".  Use YAML for a balance of
machine- and human-readability to records details of configure-time
events in a structured format.

Teach the RunCMake test framework to support matching the configure log.

Issue: #23200

3 files changed, 108 insertions, 0 deletions

diff --git a/Help/index.rst b/Help/index.rst
index fdbf847a9b..16c8f25cc5 100644
--- a/Help/index.rst
+++ b/Help/index.rst
@@ -57,6 +57,7 @@ Reference Manuals
    /manual/cmake-buildsystem.7
    /manual/cmake-commands.7
    /manual/cmake-compile-features.7
+   /manual/cmake-configure-log.7
    /manual/cmake-developer.7
    /manual/cmake-env-variables.7
    /manual/cmake-file-api.7
diff --git a/Help/manual/cmake-configure-log.7.rst b/Help/manual/cmake-configure-log.7.rst
new file mode 100644
index 0000000000..3ae7dcaf41
--- /dev/null
+++ b/Help/manual/cmake-configure-log.7.rst
@@ -0,0 +1,102 @@
+.. cmake-manual-description: CMake Configure Log
+
+cmake-configure-log(7)
+**********************
+
+.. versionadded:: 3.26
+
+.. only:: html
+
+   .. contents::
+
+Introduction
+============
+
+CMake writes a running log, known as the configure log,
+of certain events that occur during the "configure" step.
+The log file is located at::
+
+  ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeConfigureLog.yaml
+
+The configure log does *not* contain a log of all output, errors,
+or messages printed while configuring a project.  It is a log of
+detailed information about specific events, such as toolchain inspection
+by :command:`try_compile`, meant for use in debugging the configuration
+of a build tree.
+
+Log Structure
+=============
+
+The configure log is designed to be both machine- and human-readable.
+
+The log file is a YAML document stream containing zero or more YAML
+documents separated by document markers.  Each document begins
+with a ``---`` document marker line, contains a single YAML mapping
+that logs events from one CMake "configure" step, and, if the configure
+step finished normally, ends with a ``...`` document marker line:
+
+.. code-block:: yaml
+
+  ---
+  version:
+    major: 1
+    minor: 0
+  events:
+    -
+      kind: "..."
+      # (other fields omitted)
+    -
+      kind: "..."
+      # (other fields omitted)
+  ...
+
+A new document is appended to the log every time CMake configures
+the build tree and logs new events.
+
+The keys of the each document root mapping are:
+
+``version``
+  A YAML mapping that describes the schema version of the log document.
+  It has keys ``major`` and ``minor`` holding non-negative integer values.
+
+``events``
+  A YAML block sequence of nodes corresponding to events logged during
+  one CMake "configure" step.  Each event is a YAML node containing one
+  of the `Event Kinds`_ documented below.
+
+Text Block Encoding
+-------------------
+
+In order to make the log human-readable, text blocks are always
+represented using YAML literal block scalars (``|``).
+Since literal block scalars do not support escaping, backslashes
+and non-printable characters are encoded at the application layer:
+
+* ``\\`` encodes a backslash.
+* ``\xXX`` encodes a byte using two hexadecimal digits, ``XX``.
+
+.. _`configure-log event kinds`:
+
+Event Kinds
+===========
+
+Every event kind is represented by a YAML mapping of the form:
+
+.. code-block:: yaml
+
+  kind: "<kind>"
+  backtrace:
+    - "<file>:<line> (<function>)"
+  #...event-specific keys...
+
+The keys common to all events are:
+
+``kind``
+  A string identifying the event kind.
+
+``backtrace``
+  A YAML block sequence reporting the call stack of CMake source
+  locations at which the event occurred.  Each node is a string
+  specifying one location formatted as ``<file>:<line> (<function>)``.
+
+Additional mapping keys are specific to each event kind.
diff --git a/Help/release/dev/configure-log.rst b/Help/release/dev/configure-log.rst
new file mode 100644
index 0000000000..8518b21567
--- /dev/null
+++ b/Help/release/dev/configure-log.rst
@@ -0,0 +1,5 @@
+Configure Log
+-------------
+
+* CMake now writes a YAML log of configure-time checks.
+  See the :manual:`cmake-configure-log(7)` manual.