Do not add custom target dir automatically when implicit false.

docs/markdown/snippets/customimplicit.md

@@ -0,0 +1,17 @@
+## Do not add custom target dir to header path if `implicit_include_directories` is `false`
+
+If you do the following:
+
+```meson
+# in some subdirectory
+gen_h = custom_target(...)
+# in some other directory
+executable('foo', 'foo.c', gen_h)
+```
+
+then the output directory of the custom target is automatically added
+to the header search path. This is convenient, but sometimes it can
+lead to problems. Starting with this version, the directory will no
+longer be put in the search path if the target has
+`implicit_include_directories: false`. In these cases you need to set
+up the path manually with `include_directories`.

mesonbuild/backend/ninjabackend.py

@@ -2377,9 +2377,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         commands += self.generate_basic_compiler_args(target, compiler, no_warn_args)
         # Add custom target dirs as includes automatically, but before
         # target-specific include directories.
-        # XXX: Not sure if anyone actually uses this? It can cause problems in
-        # situations which increase the likelihood for a header name collision,
-        # such as in subprojects.
+        if target.implicit_include_directories:
             commands += self.get_custom_target_dir_include_args(target, compiler)
         # Add include dirs from the `include_directories:` kwarg on the target
         # and from `include_directories:` of internal deps of the target.

run_unittests.py

@@ -5616,6 +5616,23 @@ class AllPlatformTests(BasePlatformTests):
         self.assertEqual(0, output.count('File reformatted:'))
         self.build('clang-format-check')
 
+    def test_custom_target_implicit_include(self):
+        testdir = os.path.join(self.unit_test_dir, '94 custominc')
+        self.init(testdir)
+        self.build()
+        compdb = self.get_compdb()
+        matches = 0
+        for c in compdb:
+            if 'prog.c' in c['file']:
+                self.assertNotIn('easytogrepfor', c['command'])
+                matches += 1
+        self.assertEqual(matches, 1)
+        matches = 0
+        for c in compdb:
+            if 'prog2.c' in c['file']:
+                self.assertIn('easytogrepfor', c['command'])
+                matches += 1
+        self.assertEqual(matches, 1)
 
 class FailureTests(BasePlatformTests):
     '''

test cases/unit/94 custominc/easytogrepfor/genh.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python3
+
+import sys
+
+f = open(sys.argv[1], 'w')
+f.write('#define RETURN_VALUE 0')
+f.close()

test cases/unit/94 custominc/easytogrepfor/meson.build

@@ -0,0 +1,3 @@
+genh = custom_target('header',
+  output: 'generated.h',
+  command: [find_program('genh.py'), '@OUTPUT@'])

test cases/unit/94 custominc/helper.c

@@ -0,0 +1,5 @@
+#include<generated.h>
+
+int func(void) {
+    return RETURN_VALUE;
+}

test cases/unit/94 custominc/meson.build

@@ -0,0 +1,9 @@
+project('implicit custom dirs', 'c')
+
+subdir('easytogrepfor')
+
+l = static_library('helper', 'helper.c', genh)
+d = declare_dependency(link_with: l, sources: genh)
+executable('prog', 'prog.c', dependencies: d, implicit_include_directories: false)
+
+executable('prog2', 'prog2.c', dependencies: d)

test cases/unit/94 custominc/prog.c

@@ -0,0 +1,9 @@
+#include<stdlib.h>
+
+int func(void);
+
+int main(int argc, char **argv) {
+    (void)argc;
+    (void)(argv);
+    return func();
+}

test cases/unit/94 custominc/prog2.c

@@ -0,0 +1,10 @@
+#include<stdlib.h>
+#include<generated.h>
+
+int func(void);
+
+int main(int argc, char **argv) {
+    (void)argc;
+    (void)(argv);
+    return func() + RETURN_VALUE;
+}