meson: fix warnings about comparing unlike types

In the old days (0.42.x), when mesa's meson system was written the
recommendation for handling conditional dependencies was to define them
as empty lists. When meson would evaluate the dependencies of a target
it would recursively flatten all of the arguments, and empty lists would
be removed. There are some problems with this, among them that lists and
dependencies have different methods (namely .found()), so the
recommendation changed to use `dependency('', required : false)` for
such cases.  This has the advantage of providing a .found() method, so
there is no need to do things like `dep_foo != [] and dep_foo.found()`,
such a dependency should never exist.

I've tested this with 0.42 (the minimum we claim to support) and 0.45.
On 0.45 this removes warnings about comparing unlike types, such as:

meson.build:1337: WARNING: Trying to compare values of different types
(DependencyHolder, list) using !=.

v2: - Use dependency('', required : false) instead of
      declare_dependency(), the later will always report that it is
      found, which is not what we want.

Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

meson.build

@@ -29,6 +29,8 @@ project(
   default_options : ['buildtype=debugoptimized', 'c_std=c99', 'cpp_std=c++11']
 )
 
+null_dep = dependency('', required : false)
+
 system_has_kms_drm = ['openbsd', 'netbsd', 'freebsd', 'dragonfly', 'linux'].contains(host_machine.system())
 
 # Arguments for the preprocessor, put these in a separate array from the C and
@@ -422,7 +424,7 @@ elif _vdpau == 'auto'
   _vdpau = 'true'
 endif
 with_gallium_vdpau = _vdpau == 'true'
-dep_vdpau = []
+dep_vdpau = null_dep
 if with_gallium_vdpau
   dep_vdpau = dependency('vdpau', version : '>= 1.1')
   dep_vdpau = declare_dependency(
@@ -461,7 +463,7 @@ elif _xvmc == 'auto'
   _xvmc = 'true'
 endif
 with_gallium_xvmc = _xvmc == 'true'
-dep_xvmc = []
+dep_xvmc = null_dep
 if with_gallium_xvmc
   dep_xvmc = dependency('xvmc', version : '>= 1.0.6')
 endif
@@ -491,7 +493,8 @@ elif not (with_gallium_r600 or with_gallium_radeonsi or with_gallium_nouveau)
     error('OMX state tracker requires at least one of the following gallium drivers: r600, radeonsi, nouveau.')
   endif
 endif
-dep_omx = []
+with_gallium_omx = _omx
+dep_omx = null_dep
 dep_omx_other = []
 if ['auto', 'bellagio'].contains(_omx)
   dep_omx = dependency(
@@ -579,7 +582,7 @@ elif _va == 'auto'
   _va = 'true'
 endif
 with_gallium_va = _va == 'true'
-dep_va = []
+dep_va = null_dep
 if with_gallium_va
   dep_va = dependency('libva', version : '>= 0.38.0')
   dep_va_headers = declare_dependency(
@@ -638,7 +641,7 @@ if _opencl != 'disabled'
   with_gallium_opencl = true
   with_opencl_icd = _opencl == 'icd'
 else
-  dep_clc = []
+  dep_clc = null_dep
   with_gallium_opencl = false
   with_gallium_icd = false
 endif
@@ -831,7 +834,7 @@ else
 endif
 
 # Check for GCC style atomics
-dep_atomic = declare_dependency()
+dep_atomic = null_dep
 
 if cc.compiles('int main() { int n; return __atomic_load_n(&n, __ATOMIC_ACQUIRE); }',
                name : 'GCC atomic builtins')
@@ -976,7 +979,7 @@ endif
 
 # check for dl support
 if cc.has_function('dlopen')
-  dep_dl = []
+  dep_dl = null_dep
 else
   dep_dl = cc.find_library('dl')
 endif
@@ -995,7 +998,7 @@ endif
 
 # Determine whether or not the rt library is needed for time functions
 if cc.has_function('clock_gettime')
-  dep_clock = []
+  dep_clock = null_dep
 else
   dep_clock = cc.find_library('rt')
 endif
@@ -1013,7 +1016,7 @@ if with_amd_vk or with_gallium_radeonsi or with_gallium_r600 or with_gallium_ope
     dep_elf = cc.find_library('elf')
   endif
 else
-  dep_elf = []
+  dep_elf = null_dep
 endif
 dep_expat = dependency('expat')
 # this only exists on linux so either this is linux and it will be found, or
@@ -1024,12 +1027,12 @@ dep_m = cc.find_library('m', required : false)
 # but we always want to use the same version for all libdrm modules. That means
 # even if driver foo requires 2.4.0 and driver bar requires 2.4.3, if foo and
 # bar are both on use 2.4.3 for both of them
-dep_libdrm_amdgpu = []
+dep_libdrm_amdgpu = null_dep
-dep_libdrm_radeon = []
+dep_libdrm_radeon = null_dep
-dep_libdrm_nouveau = []
+dep_libdrm_nouveau = null_dep
-dep_libdrm_etnaviv = []
+dep_libdrm_etnaviv = null_dep
-dep_libdrm_freedreno = []
+dep_libdrm_freedreno = null_dep
-dep_libdrm_intel = []
+dep_libdrm_intel = null_dep
 
 _drm_amdgpu_ver = '2.4.91'
 _drm_radeon_ver = '2.4.71'
@@ -1114,7 +1117,7 @@ elif _llvm == 'true'
   dep_llvm = dependency('llvm', version : _llvm_version, modules : llvm_modules)
   with_llvm = true
 else
-  dep_llvm = []
+  dep_llvm = null_dep
   with_llvm = false
 endif
 if with_llvm
@@ -1144,7 +1147,7 @@ elif with_amd_vk or with_gallium_radeonsi or with_gallium_swr
   error('The following drivers require LLVM: Radv, RadeonSI, SWR. One of these is enabled, but LLVM is disabled.')
 endif
 
-dep_glvnd = []
+dep_glvnd = null_dep
 if with_glvnd
   dep_glvnd = dependency('libglvnd', version : '>= 0.2.0')
   pre_args += '-DUSE_LIBGLVND=1'
@@ -1156,7 +1159,7 @@ if with_valgrind != 'false'
     pre_args += '-DHAVE_VALGRIND'
   endif
 else
-  dep_valgrind = []
+  dep_valgrind = null_dep
 endif
 
 # pthread stubs. Lets not and say we didn't
@@ -1164,7 +1167,7 @@ endif
 prog_bison = find_program('bison', required : with_any_opengl)
 prog_flex = find_program('flex', required : with_any_opengl)
 
-dep_selinux = []
+dep_selinux = null_dep
 if get_option('selinux')
   dep_selinux = dependency('libselinux')
   pre_args += '-DMESA_SELINUX'
@@ -1178,7 +1181,7 @@ if with_libunwind != 'false'
     pre_args += '-DHAVE_LIBUNWIND'
   endif
 else
-  dep_unwind = []
+  dep_unwind = null_dep
 endif
 
 # TODO: gallium-hud
@@ -1217,29 +1220,29 @@ if with_platform_wayland
   pre_args += ['-DHAVE_WAYLAND_PLATFORM', '-DWL_HIDE_DEPRECATED']
 else
   prog_wl_scanner = []
-  dep_wl_protocols = []
+  dep_wl_protocols = null_dep
-  dep_wayland_client = []
+  dep_wayland_client = null_dep
-  dep_wayland_server = []
+  dep_wayland_server = null_dep
   wayland_dmabuf_xml = ''
 endif
 
-dep_x11 = []
+dep_x11 = null_dep
-dep_xext = []
+dep_xext = null_dep
-dep_xdamage = []
+dep_xdamage = null_dep
-dep_xfixes = []
+dep_xfixes = null_dep
-dep_x11_xcb = []
+dep_x11_xcb = null_dep
-dep_xcb = []
+dep_xcb = null_dep
-dep_xcb_glx = []
+dep_xcb_glx = null_dep
-dep_xcb_dri2 = []
+dep_xcb_dri2 = null_dep
-dep_xcb_dri3 = []
+dep_xcb_dri3 = null_dep
-dep_dri2proto = []
+dep_dri2proto = null_dep
-dep_glproto = []
+dep_glproto = null_dep
-dep_xxf86vm = []
+dep_xxf86vm = null_dep
-dep_xcb_dri3 = []
+dep_xcb_dri3 = null_dep
-dep_xcb_present = []
+dep_xcb_present = null_dep
-dep_xcb_sync = []
+dep_xcb_sync = null_dep
-dep_xcb_xfixes = []
+dep_xcb_xfixes = null_dep
-dep_xshmfence = []
+dep_xshmfence = null_dep
 if with_platform_x11
   if with_glx == 'xlib' or with_glx == 'gallium-xlib'
     dep_x11 = dependency('x11')
@@ -1299,7 +1302,7 @@ if _sensors != 'false'
     pre_args += '-DHAVE_LIBSENSORS=1'
   endif
 else
-  dep_lmsensors = []
+  dep_lmsensors = null_dep
 endif
 
 # TODO: gallium tests
@@ -1334,7 +1337,7 @@ gl_priv_reqs = [
 if dep_libdrm.found()
   gl_priv_reqs += 'libdrm >= 2.4.75'
 endif
-if dep_xxf86vm != [] and dep_xxf86vm.found()
+if dep_xxf86vm.found()
   gl_priv_reqs += 'xxf86vm'
 endif
 if with_dri_platform == 'drm'
@@ -1348,7 +1351,7 @@ endif
 if dep_m.found()
   gl_priv_libs += '-lm'
 endif
-if dep_dl != [] and dep_dl.found()
+if dep_dl.found()
   gl_priv_libs += '-ldl'
 endif
 

src/gallium/auxiliary/meson.build

@@ -339,7 +339,7 @@ files_libgallium = files(
   'nir/tgsi_to_nir.h',
 )
 
-if dep_libdrm != [] and dep_libdrm.found()
+if dep_libdrm.found()
   files_libgallium += files(
     'renderonly/renderonly.c',
     'renderonly/renderonly.h',

src/glx/apple/meson.build

@@ -46,7 +46,7 @@ files_libappleglx = files(
   'glx_empty.c',
 )
 
-dep_xplugin = []
+dep_xplugin = null_dep
 if with_dri_platform == 'apple'
   dep_xplugin = meson.get_compiler('c').find_library('Xplugin')
 endif

src/glx/meson.build

@@ -137,7 +137,7 @@ gl_lib_cargs = [
   '-DDEFAULT_DRIVER_DIR="@0@"'.format(dri_search_path),
 ]
 
-if dep_xxf86vm != [] and dep_xxf86vm.found()
+if dep_xxf86vm.found()
   gl_lib_cargs += '-DHAVE_XF86VIDMODE'
 endif