meson/docs/markdown/snippets/feature_if_methods.md

Add a FeatureOption.enable_if and .disable_if

These are useful when features need to be constrained to pass to dependency, as the behavior of an auto and disabled or enabled feature is markedly different. consider the following case:

opt = get_option('feature').disable_auto_if(not foo)
if opt.enabled() and not foo
  error('Cannot enable feat when foo is not also enabled')
endif
dep = dependency('foo', required : opt)

This could be simplified to

opt = get_option('feature').disable_if(not foo, error_message : 'Cannot enable feature when foo is not also enabled')
dep = dependency('foo', required : opt)

For a real life example, here is some code in mesa:

_llvm = get_option('llvm')
dep_llvm = null_dep
with_llvm = false
if _llvm.allowed()
  dep_llvm = dependency(
    'llvm',
    version : _llvm_version,
    modules : llvm_modules,
    optional_modules : llvm_optional_modules,
    required : (
      with_amd_vk or with_gallium_radeonsi or with_gallium_opencl or with_clc
      or _llvm.enabled()
    ),
    static : not _shared_llvm,
    fallback : ['llvm', 'dep_llvm'],
    include_type : 'system',
  )
  with_llvm = dep_llvm.found()
endif
if with_llvm
  ...
elif with_amd_vk and with_aco_tests
  error('ACO tests require LLVM, but LLVM is disabled.')
elif with_gallium_radeonsi or with_swrast_vk
  error('The following drivers require LLVM: RadeonSI, SWR, Lavapipe. One of these is enabled, but LLVM is disabled.')
elif with_gallium_opencl
  error('The OpenCL "Clover" state tracker requires LLVM, but LLVM is disabled.')
elif with_clc
  error('The CLC compiler requires LLVM, but LLVM is disabled.')
else
  draw_with_llvm = false
endif

simplified to:

_llvm = get_option('llvm') \
  .enable_if(with_amd_vk and with_aco_tests, error_message : 'ACO tests requires LLVM') \
  .enable_if(with_gallium_radeonsi, error_message : 'RadeonSI requires LLVM') \
  .enable_if(with_swrast_vk, error_message : 'Vulkan SWRAST requires LLVM') \
  .enable_if(with_gallium_opencl, error_message : 'The OpenCL Clover state trackers requires LLVM') \
  .enable_if(with_clc, error_message : 'CLC library requires LLVM')

dep_llvm = dependency(
  'llvm',
  version : _llvm_version,
  modules : llvm_modules,
  optional_modules : llvm_optional_modules,
  required : _llvm,
  static : not _shared_llvm,
  fallback : ['llvm', 'dep_llvm'],
  include_type : 'system',
)
with_llvm = dep_llvm.found()