Warn when grabbing internals of subprojects with include_directories.

mesonbuild/interpreter.py

@@ -4595,6 +4595,28 @@ remember that the current source and build directories are always
 put in the include directories by default so you only need to do
 include_directories('.') if you intend to use the result in a
 different subdirectory.
+''')
+            else:
+                try:
+                    self.validate_within_subproject(a, '')
+                except InterpreterException:
+                    mlog.warning('include_directories sandbox violation!')
+                    print(f'''The project is trying to access the directory {a} which belongs to a different
+subproject. This is a problem as it hardcodes the relative paths of these two projeccts.
+This makes it impossible to compile the project in any other directory layout and also
+prevents the subproject from changing its own directory layout.
+
+Instead of poking directly at the internals the subproject should be executed and 
+it should set a variable that the caller can then use. Something like:
+
+# In subproject
+some_dep = declare_depencency(include_directories: include_directories('include'))
+
+# In parent project
+some_dep = depencency('some')
+executable(..., dependencies: [some_dep])
+
+This warning will become a hard error in a future Meson release. 
 ''')
             absdir_src = os.path.join(absbase_src, a)
             absdir_build = os.path.join(absbase_build, a)
@@ -4797,6 +4819,10 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
     def validate_within_subproject(self, subdir, fname):
         srcdir = Path(self.environment.source_dir)
         norm = Path(srcdir, subdir, fname).resolve()
+        if os.path.isdir(norm):
+            inputtype = 'directory'
+        else:
+            inputtype = 'file'
         if srcdir not in norm.parents:
             # Grabbing files outside the source tree is ok.
             # This is for vendor stuff like:
@@ -4805,9 +4831,9 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
             return
         project_root = Path(srcdir, self.root_subdir)
         if project_root not in norm.parents:
-            raise InterpreterException(f'Sandbox violation: Tried to grab file {norm.name} outside current (sub)project.')
+            raise InterpreterException(f'Sandbox violation: Tried to grab {inputtype} {norm.name} outside current (sub)project.')
         if project_root / self.subproject_dir in norm.parents:
-            raise InterpreterException(f'Sandbox violation: Tried to grab file {norm.name} from a nested subproject.')
+            raise InterpreterException(f'Sandbox violation: Tried to grab {inputtype} {norm.name} from a nested subproject.')
 
     def source_strings_to_files(self, sources: T.List[str]) -> T.List[mesonlib.File]:
         mesonlib.check_direntry_issues(sources)

test cases/common/240 includedir violation/meson.build

@@ -0,0 +1,7 @@
+project('foo', 'c')
+
+# This is here rather than in failing because this needs a
+# transition period to avoid breaking existing projects.
+# Once this becomes an error, move this under failing tests.
+
+inc = include_directories('subprojects/sub/include')

test cases/common/240 includedir violation/subprojects/sub/include/placeholder.h

@@ -0,0 +1,3 @@
+#pragma once
+
+// Git can not handle empty directories, so there must be something here.

test cases/common/240 includedir violation/subprojects/sub/meson.build

@@ -0,0 +1,3 @@
+project('subproj', 'c')
+
+# This is never actually executed, just here for completeness.

test cases/common/240 includedir violation/test.json

@@ -0,0 +1,7 @@
+{
+  "stdout": [
+    {
+      "line": "WARNING: include_directories sandbox violation!"
+    }
+  ]
+}