meson: Add option for controlling PCH

In practice it turns out that turning off PCH is pretty
useful when debugging incorrect includes. I didn't want
to add this option initially because of the sheer amount
of copy-pasta that meson requires. But considering that
this is useful and the cost of copying the option a single
time is not too high this seems acceptable.

meson.options

@@ -27,3 +27,13 @@ option(
   value : false,
   description : 'Build benchmarks (requires gbenchmark)',
 )
+
+# Emulating 'auto' options, since we don't want auto_features to affect this
+# and there's no other way of conditionally defaulting the value.
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+)

nix-meson-build-support/common/meson.build

@@ -29,7 +29,11 @@ add_project_arguments(
 )
 
 # GCC doesn't benefit much from precompiled headers.
-do_pch = cxx.get_id() == 'clang'
+if get_option('pch') == 'auto'
+  do_pch = cxx.get_id() == 'clang'
+else
+  do_pch = get_option('pch') == 'enabled'
+endif
 
 # This is a clang-only option for improving build times.
 # It forces the instantiation of templates in the PCH itself and
@@ -38,7 +42,7 @@ do_pch = cxx.get_id() == 'clang'
 # bother checking the version.
 # This feature helps in particular with the expensive nlohmann::json template
 # instantiations in libutil and libstore.
-if cxx.get_id() == 'clang'
+if cxx.get_id() == 'clang' and do_pch
   add_project_arguments('-fpch-instantiate-templates', language : 'cpp')
 endif
 

src/libcmd/meson.options

@@ -13,3 +13,13 @@ option(
   value : 'editline',
   description : 'Which library to use for nice line editing with the Nix language REPL',
 )
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)
+

src/libexpr-tests/meson.options

@@ -0,0 +1,10 @@
+# vim: filetype=meson
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)

src/libexpr/meson.options

@@ -3,3 +3,13 @@ option(
   type : 'feature',
   description : 'enable garbage collection in the Nix expression evaluator (requires Boehm GC)',
 )
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)
+

src/libfetchers/meson.options

@@ -0,0 +1,10 @@
+# vim: filetype=meson
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)

src/libstore-tests/meson.options

@@ -7,3 +7,13 @@ option(
   description : 'Build benchmarks (requires gbenchmark)',
   yield : true,
 )
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)
+

src/libstore/meson.options

@@ -40,3 +40,13 @@ option(
   value : 'disabled',
   description : 'Enable curl-based S3 binary cache store support (requires aws-crt-cpp and curl >= 7.75.0)',
 )
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)
+

src/libutil-tests/meson.options

@@ -0,0 +1,10 @@
+# vim: filetype=meson
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)

src/libutil/meson.options

@@ -5,3 +5,13 @@ option(
   type : 'feature',
   description : 'determine microarchitecture levels with libcpuid (only relevant on x86_64)',
 )
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)
+

src/nix/meson.options

@@ -7,3 +7,13 @@ option(
   value : 'etc/profile.d',
   description : 'the path to install shell profile files',
 )
+
+option(
+  'pch',
+  type : 'combo',
+  choices : [ 'auto', 'enabled', 'disabled' ],
+  value : 'auto',
+  description : 'Use C++ Precompiled Headers to speed up compilation',
+  yield : true,
+)
+