Add if_found kwarg to subdir().

docs/markdown/Reference-manual.md

@@ -1019,7 +1019,7 @@ has one argument the others don't have:
 ### subdir()
 
 ``` meson
-    void subdir(dir_name)
+    void subdir(dir_name, ...)
 ```
 
 Enters the specified subdirectory and executes the `meson.build` file
@@ -1032,6 +1032,12 @@ current build file and in all subsequent build files executed with
 Note that this means that each `meson.build` file in a source tree can
 and must only be executed once.
 
+This function has one keyword argument.
+
+ - `if_found` takes one or several dependency objects and will only
+   recurse in the subdir if they all return `true` when queried with
+   `.found()`
+
 ### subproject()
 
 ``` meson

docs/markdown/snippets/if-found.md

@@ -0,0 +1,13 @@
+# Added `if_found` to subdir
+
+Added a new keyword argument to the `subdir` command. It is given a
+list of dependency objects and the function will only recurse in the
+subdirectory if they are all found. Typical usage goes like this.
+
+    d1 = dependency('foo') # This is found
+    d2 = dependency('bar') # This is not found
+
+    subdir('somedir', if_found : [d1, d2])
+
+In this case the subdirectory would not be entered since `d2` could
+not be found.

mesonbuild/interpreter.py

@@ -1374,6 +1374,7 @@ permitted_kwargs = {'add_global_arguments': {'language'},
                     'shared_library': shlib_kwargs,
                     'shared_module': shmod_kwargs,
                     'static_library': stlib_kwargs,
+                    'subdir': {'if_found'},
                     'subproject': {'version', 'default_options'},
                     'test': {'args', 'env', 'is_parallel', 'should_fail', 'timeout', 'workdir', 'suite'},
                     'vcs_tag': {'input', 'output', 'fallback', 'command', 'replace_string'},
@@ -2477,7 +2478,7 @@ to directly access options of other subprojects.''')
         self.build.man.append(m)
         return m
 
-    @noKwargs
+    @permittedKwargs(permitted_kwargs['subdir'])
     def func_subdir(self, node, args, kwargs):
         self.validate_arguments(args, 1, [str])
         if '..' in args[0]:
@@ -2486,6 +2487,11 @@ to directly access options of other subprojects.''')
             raise InvalidArguments('Must not go into subprojects dir with subdir(), use subproject() instead.')
         if self.subdir == '' and args[0].startswith('meson-'):
             raise InvalidArguments('The "meson-" prefix is reserved and cannot be used for top-level subdir().')
+        for i in mesonlib.extract_as_list(kwargs, 'if_found'):
+            if not hasattr(i, 'found_method'):
+                raise InterpreterException('Object used in if_found does not have a found method.')
+            if not i.found_method([], {}):
+                return
         prev_subdir = self.subdir
         subdir = os.path.join(prev_subdir, args[0])
         if os.path.isabs(subdir):

test cases/common/165 subdir if_found/meson.build

@@ -0,0 +1,11 @@
+project('subdir if found', 'c')
+
+found_dep = declare_dependency()
+not_found_dep = dependency('nonexisting', required : false)
+
+subdir('nonexisting_dir', if_found : not_found_dep)
+
+variable = 3
+
+subdir('subdir', if_found : found_dep)
+assert(variable == 5, 'Subdir was not properly entered.')

test cases/common/165 subdir if_found/subdir/meson.build

@@ -0,0 +1 @@
+variable = 5