wrap: Add special 'dependency_names' key in [provide] section

The value for that key must be a coma separated list of dependecy names
provided by that subproject, when no variable name is needed because the
subproject uses override_dependency().

docs/markdown/Wrap-dependency-system-manual.md

@@ -145,12 +145,15 @@ wrap-git, the repository must contain all Meson build definitions.
 Wrap files can define the dependencies it provides in the `[provide]` section.
 When a wrap file provides the dependency `foo` any call do `dependency('foo')`
 will automatically fallback to that subproject even if no `fallback` keyword
-argument is given. Each entry in the format `dependency_name = variable_name`,
+argument is given. It is recommended for subprojects to call
+`meson.override_dependency('foo', foo_dep)`, dependency name can then be added into
+the special `dependency_names` entry which takes coma separated list of dependency
+names. For backward compatibility with subprojects that does not call
+`meson.override_dependency()`, the variable name can be provided in the wrap file
+with entries in the format `dependency_name = variable_name`,
 where `dependency_name` usually match the corresponding pkg-config name and
 `variable_name` is the name of a variable defined in the subproject that should
-be returned for that dependency. In the case the subproject uses
-`meson.override_dependency('foo', foo_dep)` the `variable_name` can be left empty
-in the wrap file.
+be returned for that dependency.
 
 For example `glib.wrap` provides `glib-2.0`, `gobject-2.0` and `gio-2.0`. A wrap
 file for glib would look like:
@@ -165,6 +168,17 @@ gobject-2.0=gobject_dep
 gio-2.0=gio_dep
 ```
 
+Alternatively, when using a recent enough version of glib that uses
+`meson.override_dependency()`:
+```ini
+[wrap-git]
+url=https://gitlab.gnome.org/GNOME/glib.git
+revision=glib-2-62
+
+[provide]
+dependency_names = glib-2.0, gobject-2.0, gio-2.0
+```
+
 With such wrap file, `dependency('glib-2.0')` will automatically fallback to use
 `glib.wrap` and return `glib_dep` variable from the subproject.
 

mesonbuild/wrap/wrap.py

@@ -127,7 +127,19 @@ class PackageDefinition:
     def parse_provide_section(self):
         self.provide = {self.name: None}
         if self.config.has_section('provide'):
-            self.provide.update(self.config['provide'])
+            for k, v in self.config['provide'].items():
+                if k == 'dependency_names':
+                    # A coma separated list of dependency names that does not
+                    # need a variable name
+                    names = {n.strip(): None for n in v.split(',')}
+                    self.provide.update(names)
+                    continue
+                if not v:
+                    m = ('Empty dependency variable name for {!r} in {}. '
+                         'If the subproject uses meson.override_dependency() '
+                         'it can be added in the "dependency_names" special key.')
+                    raise WrapException(m.format(k, self.basename))
+                self.provide[k] = v
 
     def get(self, key: str) -> str:
         try:

test cases/common/102 subproject subdir/meson.build

@@ -31,7 +31,7 @@ d = dependency('sub_implicit')
 assert(d.found(), 'Should implicitly fallback')
 
 # Verify that implicit fallback works because sub_implicit.wrap has
-# `sub_implicit_provide1=` and the subproject overrides sub_implicit_provide1.
+# `dependency_names=sub_implicit_provide1` and the subproject overrides sub_implicit_provide1.
 d = dependency('sub_implicit_provide1')
 assert(d.found(), 'Should implicitly fallback')
 

test cases/common/102 subproject subdir/subprojects/sub_implicit.wrap

@@ -1,5 +1,5 @@
 [wrap-file]
 
 [provide]
-sub_implicit_provide1=
+dependency_names = sub_implicit_provide1
 sub_implicit_provide2 = sub_implicit_provide2_dep