Allow CustomTarget's to be indexed

This allows a CustomTarget to be indexed, and the resulting indexed
value (a CustomTargetIndex type), to be used as a source in other
targets. This will confer a dependency on the original target, but only
inserts the source file returning by index the original target's
outputs. This can allow a CustomTarget that creates both a header and a
code file to have it's outputs split, for example.

Fixes #1470

7 files changed, 69 insertions, 17 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index 97959b6..12fb3eb 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -174,7 +174,7 @@ class Backend:
         Returns the full path of the generated source relative to the build root
         """
         # CustomTarget generators output to the build dir of the CustomTarget
-        if isinstance(gensrc, build.CustomTarget):
+        if isinstance(gensrc, (build.CustomTarget, build.CustomTargetIndex)):
             return os.path.join(self.get_target_dir(gensrc), src)
         # GeneratedList generators output to the private build directory of the
         # target that the GeneratedList is used in
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 41b93cb..d2ba49f 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -245,7 +245,7 @@ int dummy;
         header_deps = []
         # XXX: Why don't we add deps to CustomTarget headers here?
         for genlist in target.get_generated_sources():
-            if isinstance(genlist, build.CustomTarget):
+            if isinstance(genlist, (build.CustomTarget, build.CustomTargetIndex)):
                 continue
             for src in genlist.get_outputs():
                 if self.environment.is_header(src):
@@ -1761,10 +1761,11 @@ rule FORTRAN_DEP_HACK
         outfile.write('\n')
 
     def generate_generator_list_rules(self, target, outfile):
-        # CustomTargets have already written their rules,
-        # so write rules for GeneratedLists here
+        # CustomTargets have already written their rules and
+        # CustomTargetIndexes don't actually get generated, so write rules for
+        # GeneratedLists here
         for genlist in target.get_generated_sources():
-            if isinstance(genlist, build.CustomTarget):
+            if isinstance(genlist, (build.CustomTarget, build.CustomTargetIndex)):
                 continue
             self.generate_genlist_for_target(genlist, target, outfile)
 
@@ -2013,7 +2014,7 @@ rule FORTRAN_DEP_HACK
             # Generator output goes into the target private dir which is
             # already in the include paths list. Only custom targets have their
             # own target build dir.
-            if not isinstance(i, build.CustomTarget):
+            if not isinstance(i, (build.CustomTarget, build.CustomTargetIndex)):
                 continue
             idir = self.get_target_dir(i)
             if idir not in custom_target_include_dirs:
diff --git a/mesonbuild/backend/vs2010backend.py b/mesonbuild/backend/vs2010backend.py
index 22c1779..cb8dad6 100644
--- a/mesonbuild/backend/vs2010backend.py
+++ b/mesonbuild/backend/vs2010backend.py
@@ -91,7 +91,7 @@ class Vs2010Backend(backends.Backend):
         source_target_dir = self.get_target_source_dir(target)
         down = self.target_to_build_root(target)
         for genlist in target.get_generated_sources():
-            if isinstance(genlist, build.CustomTarget):
+            if isinstance(genlist, (build.CustomTarget, build.CustomTargetIndex)):
                 for i in genlist.get_outputs():
                     # Path to the generated source from the current vcxproj dir via the build root
                     ipath = os.path.join(down, self.get_target_dir(genlist), i)
@@ -201,6 +201,8 @@ class Vs2010Backend(backends.Backend):
                 for gendep in target.get_generated_sources():
                     if isinstance(gendep, build.CustomTarget):
                         all_deps[gendep.get_id()] = gendep
+                    elif isinstance(gendep, build.CustomTargetIndex):
+                        all_deps[gendep.target.get_id()] = gendep.target
                     else:
                         gen_exe = gendep.generator.get_exe()
                         if isinstance(gen_exe, build.Executable):
diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index c54abbd..d3c0b54 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -425,7 +425,7 @@ class BuildTarget(Target):
                 if s not in added_sources:
                     self.sources.append(s)
                     added_sources[s] = True
-            elif isinstance(s, (GeneratedList, CustomTarget)):
+            elif isinstance(s, (GeneratedList, CustomTarget, CustomTargetIndex)):
                 self.generated.append(s)
             else:
                 msg = 'Bad source of type {!r} in target {!r}.'.format(type(s).__name__, self.name)
@@ -1676,6 +1676,15 @@ class CustomTarget(Target):
     def type_suffix(self):
         return "@cus"
 
+    def __getitem__(self, index):
+        return CustomTargetIndex(self, self.outputs[index])
+
+    def __setitem__(self, index, value):
+        raise NotImplementedError
+
+    def __delitem__(self, index):
+        raise NotImplementedError
+
 class RunTarget(Target):
     def __init__(self, name, command, args, dependencies, subdir):
         super().__init__(name, subdir, False)
@@ -1735,6 +1744,29 @@ class Jar(BuildTarget):
         pass
 
 
+class CustomTargetIndex:
+
+    """A special opaque object returned by indexing a CustomTaget. This object
+    exists in meson, but acts as a proxy in the backends, making targets depend
+    on the CustomTarget it's derived from, but only adding one source file to
+    the sources.
+    """
+
+    def __init__(self, target, output):
+        self.target = target
+        self.output = output
+
+    def __repr__(self):
+        return '<CustomTargetIndex: {!r}[{}]>'.format(
+            self.target, self.target.output.index(self.output))
+
+    def get_outputs(self):
+        return [self.output]
+
+    def get_subdir(self):
+        return self.target.get_subdir()
+
+
 class ConfigureFile:
 
     def __init__(self, subdir, sourcename, targetname, configuration_data):
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 2148bed..012370d 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -566,6 +566,11 @@ class JarHolder(BuildTargetHolder):
     def __init__(self, target, interp):
         super().__init__(target, interp)
 
+class CustomTargetIndexHolder(InterpreterObject):
+    def __init__(self, object_to_hold):
+        super().__init__()
+        self.held_object = object_to_hold
+
 class CustomTargetHolder(TargetHolder):
     def __init__(self, object_to_hold, interp):
         super().__init__()
@@ -582,6 +587,15 @@ class CustomTargetHolder(TargetHolder):
     def full_path_method(self, args, kwargs):
         return self.interpreter.backend.get_target_filename_abs(self.held_object)
 
+    def __getitem__(self, index):
+        return CustomTargetIndexHolder(self.held_object[index])
+
+    def __setitem__(self, index, value):
+        raise InterpreterException('Cannot set a member of a CustomTarget')
+
+    def __delitem__(self, index):
+        raise InterpreterException('Cannot delete a member of a CustomTarget')
+
 class RunTargetHolder(InterpreterObject):
     def __init__(self, name, command, args, dependencies, subdir):
         super().__init__()
@@ -2774,7 +2788,7 @@ different subdirectory.
         results = []
         for s in sources:
             if isinstance(s, (mesonlib.File, GeneratedListHolder,
-                              CustomTargetHolder)):
+                              CustomTargetHolder, CustomTargetIndexHolder)):
                 pass
             elif isinstance(s, str):
                 s = mesonlib.File.from_source_file(self.environment.source_dir, self.subdir, s)
diff --git a/mesonbuild/interpreterbase.py b/mesonbuild/interpreterbase.py
index 1dd2f02..cb82e56 100644
--- a/mesonbuild/interpreterbase.py
+++ b/mesonbuild/interpreterbase.py
@@ -369,14 +369,16 @@ class InterpreterBase:
     def evaluate_indexing(self, node):
         assert(isinstance(node, mparser.IndexNode))
         iobject = self.evaluate_statement(node.iobject)
-        if not isinstance(iobject, list):
-            raise InterpreterException('Tried to index a non-array object.')
+        if not hasattr(iobject, '__getitem__'):
+            raise InterpreterException(
+                'Tried to index an object that doesn\'t support indexing.')
         index = self.evaluate_statement(node.index)
         if not isinstance(index, int):
             raise InterpreterException('Index value is not an integer.')
-        if index < -len(iobject) or index >= len(iobject):
+        try:
+            return iobject[index]
+        except IndexError:
             raise InterpreterException('Index %d out of bounds of array of size %d.' % (index, len(iobject)))
-        return iobject[index]
 
     def function_call(self, node):
         func_name = node.func_name
diff --git a/mesonbuild/modules/gnome.py b/mesonbuild/modules/gnome.py
index 1ab075b..d1d7013 100644
--- a/mesonbuild/modules/gnome.py
+++ b/mesonbuild/modules/gnome.py
@@ -107,12 +107,12 @@ class GnomeModule(ExtensionModule):
         for (ii, dep) in enumerate(dependencies):
             if hasattr(dep, 'held_object'):
                 dependencies[ii] = dep = dep.held_object
-            if not isinstance(dep, (mesonlib.File, build.CustomTarget)):
+            if not isinstance(dep, (mesonlib.File, build.CustomTarget, build.CustomTargetIndex)):
                 m = 'Unexpected dependency type {!r} for gnome.compile_resources() ' \
                     '"dependencies" argument.\nPlease pass the return value of ' \
                     'custom_target() or configure_file()'
                 raise MesonException(m.format(dep))
-            if isinstance(dep, build.CustomTarget):
+            if isinstance(dep, (build.CustomTarget, build.CustomTargetIndex)):
                 if not mesonlib.version_compare(glib_version, gresource_dep_needed_version):
                     m = 'The "dependencies" argument of gnome.compile_resources() can not\n' \
                         'be used with the current version of glib-compile-resources due to\n' \
@@ -131,6 +131,7 @@ class GnomeModule(ExtensionModule):
         elif isinstance(ifile, str):
             ifile = os.path.join(state.subdir, ifile)
         elif isinstance(ifile, (interpreter.CustomTargetHolder,
+                                interpreter.CustomTargetIndexHolder,
                                 interpreter.GeneratedObjectsHolder)):
             m = 'Resource xml files generated at build-time cannot be used ' \
                 'with gnome.compile_resources() because we need to scan ' \
@@ -261,7 +262,7 @@ class GnomeModule(ExtensionModule):
                     dep_files.append(dep)
                     subdirs.append(dep.subdir)
                     break
-                elif isinstance(dep, build.CustomTarget):
+                elif isinstance(dep, (build.CustomTarget, build.CustomTargetIndex)):
                     fname = None
                     outputs = {(o, os.path.basename(o)) for o in dep.get_outputs()}
                     for o, baseo in outputs:
@@ -443,7 +444,7 @@ class GnomeModule(ExtensionModule):
             for s in libsources:
                 if hasattr(s, 'held_object'):
                     s = s.held_object
-                if isinstance(s, build.CustomTarget):
+                if isinstance(s, (build.CustomTarget, build.CustomTargetIndex)):
                     gir_filelist.write(os.path.join(state.environment.get_build_dir(),
                                                     state.backend.get_target_dir(s),
                                                     s.get_outputs()[0]) + '\n')