i18n module: provide targets as return value for gettext()

Users may wish to make use of these files for their own purposes.

For example, the -pot and -update-po pseudo targets could be reused in
an alias_target(), and at least one person wanted to reuse the built .mo
files as custom_target input.

Fixes #6227

docs/markdown/i18n-module.md

@@ -38,6 +38,11 @@ This function also defines targets for maintainers to use:
 * `<project_id>-update-po`: regenerates the `.po` files from current `.pot` file
 * `<project_id>-gmo`: builds the translations without installing
 
+(*since 0.60.0*) Returns a list containing:
+* a list of built `.mo` files
+* the maintainer `-pot` target
+* the maintainer `-update-po` target
+
 ### i18n.merge_file()
 
 This merges translations into a text file using `msgfmt`. See

docs/markdown/snippets/i18n-return.md

@@ -0,0 +1,6 @@
+## i18n module now returns gettext targets
+
+`r = i18n.gettext('mydomain')` will now provide access to:
+- a list of built .mo files
+- the mydomain-pot maintainer target which updates .pot files
+- the mydomain-update-po maintainer target which updates .po files

mesonbuild/modules/i18n.py

@@ -143,6 +143,7 @@ class I18nModule(ExtensionModule):
         datadirs = self._get_data_dirs(state, mesonlib.stringlistify(kwargs.get('data_dirs', [])))
         extra_args = mesonlib.stringlistify(kwargs.get('args', []))
         targets = []
+        gmotargets = []
 
         preset = kwargs.pop('preset', None)
         if preset:
@@ -185,6 +186,7 @@ class I18nModule(ExtensionModule):
                           }
             gmotarget = build.CustomTarget(l+'.mo', path.join(state.subdir, l, 'LC_MESSAGES'), state.subproject, gmo_kwargs)
             targets.append(gmotarget)
+            gmotargets.append(gmotarget)
 
         updatepoargs = state.environment.get_build_command() + ['--internal', 'gettext', 'update_po', pkg_arg]
         if lang_arg:
@@ -196,7 +198,7 @@ class I18nModule(ExtensionModule):
         updatepotarget = build.RunTarget(packagename + '-update-po', updatepoargs, [], state.subdir, state.subproject)
         targets.append(updatepotarget)
 
-        return ModuleReturnValue(None, targets)
+        return ModuleReturnValue([gmotargets, pottarget, updatepotarget], targets)
 
 def initialize(*args, **kwargs):
     return I18nModule(*args, **kwargs)