Merge pull request #680 from ispyisail/feature/optional-pch

Add optional precompiled headers behind QET_ENABLE_PCH (default OFF)

CMakeLists.txt

@@ -145,6 +145,34 @@ else()
     )
 endif()
 
+# Optional precompiled headers -- see QET_ENABLE_PCH in
+# cmake/developer_options.cmake for what this trades away.
+#
+# target_precompile_headers() needs CMake 3.16; the project still declares a
+# 3.5 minimum, so guard rather than raise it for an opt-in developer feature.
+#
+# The generator expressions are load-bearing, not decoration: this target also
+# compiles the 18 C files of the bundled LZMA decoder
+# (sources/import/edz/lzma/*.c), and an unguarded list applies to every
+# language in the target, so the Qt headers would reach the C compiler and fail
+# with "unknown type name 'namespace'". $<ANGLE-R> is required because a
+# literal '>' would terminate the generator expression.
+if(QET_ENABLE_PCH)
+    if(CMAKE_VERSION VERSION_LESS 3.16)
+        message(WARNING
+            "QET_ENABLE_PCH needs CMake 3.16 or newer (found ${CMAKE_VERSION}); "
+            "building without precompiled headers.")
+    else()
+        target_precompile_headers(${PROJECT_NAME} PRIVATE
+            "$<$<COMPILE_LANGUAGE:CXX>:<QtCore/QtCore$<ANGLE-R>>"
+            "$<$<COMPILE_LANGUAGE:CXX>:<QtGui/QtGui$<ANGLE-R>>"
+            "$<$<COMPILE_LANGUAGE:CXX>:<QtWidgets/QtWidgets$<ANGLE-R>>"
+            "$<$<COMPILE_LANGUAGE:CXX>:<QtXml/QtXml$<ANGLE-R>>"
+        )
+        message(STATUS "QET_ENABLE_PCH: precompiled headers enabled")
+    endif()
+endif()
+
 target_link_libraries(
     ${PROJECT_NAME}
     PUBLIC

cmake/developer_options.cmake

@@ -33,3 +33,17 @@ add_definitions(-DQT_MESSAGELOGCONTEXT)
 
 # Build with KF5
 option(BUILD_WITH_KF5 "Build with KF5" ON)
+
+# Precompiled headers for the Qt umbrella headers.
+#
+# Off by default and intended for local development only. Building QET is
+# dominated by re-parsing Qt's headers: a 214-line .cpp expands to ~198,000
+# preprocessed lines, and compiling one translation unit costs ~4.1 s of which
+# only ~0.35 s is optimisation (-O0 instead of -O3 saves 8%). A PCH caches the
+# parsed header state and takes that ~4.1 s down to ~1.2 s.
+#
+# It is deliberately NOT on by default: a PCH satisfies includes that a source
+# file forgot to make itself, so code written with it enabled can fail to
+# compile for everyone else. Leaving it off keeps CI and contributors on the
+# strict behaviour, and only developers who opt in trade that for the speed.
+option(QET_ENABLE_PCH "Use precompiled headers (developer build speed; may mask missing #includes)" OFF)