Can specify a string to print when dep not found. Closes #2407.

4 files changed, 62 insertions, 2 deletions

diff --git a/docs/markdown/Reference-manual.md b/docs/markdown/Reference-manual.md
index 2fc61d5..8d0d123 100644
--- a/docs/markdown/Reference-manual.md
+++ b/docs/markdown/Reference-manual.md
@@ -395,6 +395,8 @@ are also supported. This function supports the following keyword arguments:
   the build machine system rather than the host system (i.e. where the
   cross compiled binary will run on), usually only needed if you build
   a tool to be used during compilation.
+- `not_found_message` *(added 0.50.0)* is an optional string that will
+  be printed as a `message()` if the dependency was not found.
 - `required`, when set to false, Meson will proceed with the build
   even if the dependency is not found. Since *0.47.0* the value of a
   [`feature`](Build-options.md#features) option can also be passed.
diff --git a/docs/markdown/snippets/notfound_message.md b/docs/markdown/snippets/notfound_message.md
new file mode 100644
index 0000000..d73c6b2
--- /dev/null
+++ b/docs/markdown/snippets/notfound_message.md
@@ -0,0 +1,38 @@
+## New `not_found_message` for dependency
+
+You can now specify a `not_found_message` that will be printed if the
+specified dependency was not found. The point is to convert constructs
+that look like this:
+
+```meson
+d = dependency('something', required: false)
+if not d.found()
+  message('Will not be able to do something.')
+endif
+```
+
+Into this:
+
+```meson
+d = dependency('something',
+  required: false,
+  not_found_message: 'Will not be able to do something.')
+```
+
+Or constructs like this:
+
+```meson
+d = dependency('something', required: false)
+if not d.found()
+  error('Install something by doing XYZ.')
+endif
+```
+
+into this:
+
+```meson
+d = dependency('something',
+  not_found_message: 'Install something by doing XYZ.')
+```
+
+Which works, because the default value of `required` is `true`.
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 8012486..6fef4a9 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -1900,6 +1900,7 @@ permitted_kwargs = {'add_global_arguments': {'language', 'native'},
                                    'modules',
                                    'optional_modules',
                                    'native',
+                                   'not_found_message',
                                    'required',
                                    'static',
                                    'version',
@@ -2645,6 +2646,9 @@ external dependencies (including libraries) must go to "dependencies".''')
     @noKwargs
     def func_message(self, node, args, kwargs):
         argstr = self.get_message_string_arg(node)
+        self.message_impl(argstr)
+
+    def message_impl(self, argstr):
         mlog.log(mlog.bold('Message:'), argstr)
 
     @FeatureNew('warning', '0.44.0')
@@ -3013,13 +3017,27 @@ external dependencies (including libraries) must go to "dependencies".''')
     @FeatureNewKwargs('dependency', '0.49.0', ['disabler'])
     @FeatureNewKwargs('dependency', '0.40.0', ['method'])
     @FeatureNewKwargs('dependency', '0.38.0', ['default_options'])
+    @FeatureNewKwargs('dependency', '0.50.0', ['not_found_message'])
     @disablerIfNotFound
     @permittedKwargs(permitted_kwargs['dependency'])
     def func_dependency(self, node, args, kwargs):
         self.validate_arguments(args, 1, [str])
         name = args[0]
         display_name = name if name else '(anonymous)'
+        not_found_message = kwargs.get('not_found_message', '')
+        if not isinstance(not_found_message, str):
+            raise InvalidArguments('The not_found_message must be a string.')
+        try:
+            d = self.dependency_impl(name, display_name, kwargs)
+        except Exception:
+            if not_found_message:
+                self.message_impl(not_found_message)
+            raise
+        if not d.found() and not_found_message:
+            self.message_impl(not_found_message)
+        return d
 
+    def dependency_impl(self, name, display_name, kwargs):
         disabled, required, feature = extract_required_kwarg(kwargs, self.subproject)
         if disabled:
             mlog.log('Dependency', mlog.bold(display_name), 'skipped: feature', mlog.bold(feature), 'disabled')
diff --git a/test cases/linuxlike/1 pkg-config/meson.build b/test cases/linuxlike/1 pkg-config/meson.build
index 17ee192..891fea4 100644
--- a/test cases/linuxlike/1 pkg-config/meson.build
+++ b/test cases/linuxlike/1 pkg-config/meson.build
@@ -2,7 +2,8 @@ project('external dependency', 'c')
 
 # Zlib is probably on all dev machines.
 
-dep = dependency('zlib', version : '>=1.2')
+dep = dependency('zlib', version : '>=1.2',
+  not_found_message: 'DANGER! DANGER! THIS MUST NEVER BE SEEN!')
 exe = executable('zlibprog', 'prog-checkver.c',
   dependencies : dep,
   c_args : '-DFOUND_ZLIB="' + dep.version() + '"')
@@ -29,7 +30,8 @@ test('zlibtest2', exe2)
 
 # Try to find a nonexistent library to ensure requires:false works.
 
-dep = dependency('nvakuhrabnsdfasdf', required : false)
+dep = dependency('nvakuhrabnsdfasdf', required : false,
+  not_found_message : 'This dependency was not found as was expected.')
 
 # Try to compile a test that takes a dep and an include_directories