Disallow language keyword for unsupported deps.

Also, document it in the manual.

docs/markdown/Reference-manual.md

@@ -193,6 +193,7 @@ Finds an external dependency with the given name with `pkg-config` if possible a
 - `fallback` specifies a subproject fallback to use in case the dependency is not found in the system. The value is an array `['subproj_name', 'subproj_dep']` where the first value is the name of the subproject and the second is the variable name in that subproject that contains the value of [`declare_dependency`](#declare_dependency).
 - `default_options` *(added 0.37.0)* an array of option values that override those set in the project's `default_options` invocation (like `default_options` in [`project()`](#project), they only have effect when Meson is run for the first time, and command line arguments override any default options in build files)
 - `method` defines the way the dependency is detected, the default is `auto` but can be overridden to be e.g. `qmake` for Qt development, and different dependencies support different values for this (though `auto` will work on all of them)
+- `language` *(added 0.42.0)* defines what language-specific dependency to find if it's available for multiple languages.
 
 The returned object also has methods that are documented in the [object methods section](#dependency-object) below.
 

mesonbuild/dependencies/__init__.py

@@ -15,7 +15,7 @@
 from .base import (  # noqa: F401
     Dependency, DependencyException, DependencyMethods, ExternalProgram,
     ExternalDependency, ExternalLibrary, ExtraFrameworkDependency, InternalDependency,
-    PkgConfigDependency, find_external_dependency, get_dep_identifier, packages)
+    PkgConfigDependency, find_external_dependency, get_dep_identifier, packages, _packages_accept_language)
 from .dev import GMockDependency, GTestDependency, LLVMDependency, ValgrindDependency
 from .misc import (BoostDependency, MPIDependency, Python3Dependency, ThreadDependency)
 from .platform import AppleFrameworks
@@ -47,3 +47,6 @@ packages.update({
     'wxwidgets': WxDependency,
     'vulkan': VulkanDependency,
 })
+_packages_accept_language.update({
+    'mpi',
+})

mesonbuild/dependencies/base.py

@@ -26,8 +26,9 @@ from .. import mesonlib
 from ..mesonlib import MesonException, Popen_safe, flatten, version_compare_many
 
 
-# This must be defined in this file to avoid cyclical references.
+# These must be defined in this file to avoid cyclical references.
 packages = {}
+_packages_accept_language = set()
 
 
 class DependencyException(MesonException):
@@ -610,10 +611,15 @@ def find_external_dependency(name, env, kwargs):
         raise DependencyException('Keyword "method" must be a string.')
     lname = name.lower()
     if lname in packages:
+        if lname not in _packages_accept_language and 'language' in kwargs:
+            raise DependencyException('%s dependency does not accept "language" keyword argument' % (lname, ))
         dep = packages[lname](env, kwargs)
         if required and not dep.found():
             raise DependencyException('Dependency "%s" not found' % name)
         return dep
+    if 'language' in kwargs:
+        # Remove check when PkgConfigDependency supports language.
+        raise DependencyException('%s dependency does not accept "language" keyword argument' % (lname, ))
     pkg_exc = None
     pkgdep = None
     try: