Can use plain strings for include_directories.

docs/markdown/Reference-manual.md

@@ -349,7 +349,8 @@ keyword arguments.
 
   - `compile_args`, compile arguments to use
   - `dependencies`, other dependencies needed to use this dependency
-  - `include_directories`, the directories to add to header search path
+  - `include_directories`, the directories to add to header search path,
+    must be include_directories objects or, since 0.50.0, plain strings
   - `link_args`, link arguments to use
   - `link_with`, libraries to link against
   - `link_whole`, libraries to link fully, same as [`executable`](#executable)
@@ -530,7 +531,8 @@ be passed to [shared and static libraries](#library).
   adds the current source and build directories to the include path,
   defaults to `true`, since 0.42.0
 - `include_directories` one or more objects created with the
-  `include_directories` function
+  `include_directories` function, or, since 0.50.0, strings, which
+  will be transparently expanded to include directory objects
 - `install`, when set to true, this executable should be installed
 - `install_dir` override install directory for this file. The value is
   relative to the `prefix` specified. F.ex, if you want to install

docs/markdown/snippets/includestr.md

@@ -0,0 +1,16 @@
+## `include_directories` accepts a string
+
+The `include_directories` keyword argument now accepts plain strings
+rather than an include directory object. Meson will transparently
+expand it so that a declaration like this:
+
+```meson
+executable(..., include_directories: 'foo')
+```
+
+Is equivalent to this:
+
+```meson
+foo_inc = include_directories('foo')
+executable(..., include_directories: inc)
+```

mesonbuild/interpreter.py

@@ -2209,7 +2209,7 @@ class Interpreter(InterpreterBase):
         version = kwargs.get('version', self.project_version)
         if not isinstance(version, str):
             raise InterpreterException('Version must be a string.')
-        incs = extract_as_list(kwargs, 'include_directories', unholder=True)
+        incs = self.entries_to_incdirs(extract_as_list(kwargs, 'include_directories', unholder=True))
         libs = extract_as_list(kwargs, 'link_with', unholder=True)
         libs_whole = extract_as_list(kwargs, 'link_whole', unholder=True)
         sources = extract_as_list(kwargs, 'sources')
@@ -3708,6 +3708,19 @@ This will become a hard error in the future.''' % kwargs['input'])
             self.build.data.append(build.Data([cfile], idir, install_mode))
         return mesonlib.File.from_built_file(self.subdir, output)
 
+    def entries_to_incdirs(self, prospectives):
+        if not isinstance(prospectives, list):
+            return self.entries_to_incdirs([prospectives])[0]
+        result = []
+        for p in prospectives:
+            if isinstance(p, (IncludeDirsHolder, build.IncludeDirs)):
+                result.append(p)
+            elif isinstance(p, str):
+                result.append(self.build_incdir_object([p]))
+            else:
+                raise InterpreterException('Include directory objects can only be created from strings or include directories.')
+        return result
+
     @permittedKwargs(permitted_kwargs['include_directories'])
     @stringArgs
     def func_include_directories(self, node, args, kwargs):
@@ -4069,6 +4082,8 @@ Try setting b_lundef to false instead.'''.format(self.coredata.base_options['b_s
         # passed to library() when default_library == 'static'.
         kwargs = {k: v for k, v in kwargs.items() if k in targetclass.known_kwargs}
 
+        if 'include_directories' in kwargs:
+            kwargs['include_directories'] = self.entries_to_incdirs(kwargs['include_directories'])
         target = targetclass(name, self.subdir, self.subproject, is_cross, sources, objs, self.environment, kwargs)
 
         if is_cross:

test cases/common/19 includedir/src/meson.build

@@ -1,2 +1,5 @@
 exe = executable('prog', 'prog.c', 'func.c', include_directories : inc)
 test('inc test', exe)
+
+exe2 = executable('prog2', 'prog.c', 'func.c', include_directories : '../include')
+test('inc test 2', exe2)

test cases/common/84 declare dep/entity/meson.build

@@ -1,7 +1,7 @@
 entity_lib = static_library('entity', 'entity1.c')
 
 entity_dep = declare_dependency(link_with : [[entity_lib]],
-  include_directories : include_directories('.'),
+  include_directories : '.',
   sources : 'entity2.c',
   compile_args : ['-DUSING_ENT=1'],
   version : '1.2.3',