Add allow_abs_paths option

The existing error upon detecting use of an absolute include directory
path (within the source tree) is a well-intentioned but blunt tool that
unnecessarily prevents use of valid and safe absolute include dir paths.

Absolute paths can be constructed from build-relative absolute paths
(e.g. src_root = project_source_root(), ... src_root / 'dir/inc') and so
use consistent 'src_root'-relative path construction across multiple
meson.build files while also making each meson.build file easy to move
without breaking through use of pure relative paths.

Discussed with more context and rationale here -
mesonbuild#12244

Setting the option `allow_abs_paths=true` prevents this error check and
is obviously used at your own risk.

docs/markdown/Builtin-options.md

@@ -96,6 +96,7 @@ machine](#specifying-options-per-machine) section for details.
 | wrap_mode {default, nofallback,<br>nodownload, forcefallback, nopromote} | default | Wrap mode to use                   | no             | no                |
 | force_fallback_for                     | []            | Force fallback for those dependencies                          | no             | no                |
 | vsenv                                  | false         | Activate Visual Studio environment                             | no             | no                |
+| allow_abs_paths                        | false         | Suppress errors from use of absolute include paths             | no             | yes               |
 
 #### Details for `backend`
 

mesonbuild/coredata.py

@@ -1380,6 +1380,7 @@ def add_to_argparse(self, name: str, parser: argparse.ArgumentParser, help_suffi
     (OptionKey('wrap_mode'),       BuiltinOption(UserComboOption, 'Wrap mode', 'default', choices=['default', 'nofallback', 'nodownload', 'forcefallback', 'nopromote'])),
     (OptionKey('force_fallback_for'), BuiltinOption(UserArrayOption, 'Force fallback for those subprojects', [])),
     (OptionKey('vsenv'),           BuiltinOption(UserBooleanOption, 'Activate Visual Studio environment', False, readonly=True)),
+    (OptionKey('allow_abs_paths'), BuiltinOption(UserBooleanOption, 'Suppress errors from use of absolute include paths', False, yielding=False)),
 
     # Pkgconfig module
     (OptionKey('relocatable', module='pkgconfig'),

mesonbuild/interpreter/interpreter.py

@@ -2781,11 +2781,12 @@ def build_incdir_object(self, incdir_strings: T.List[str], is_system: bool = Fal
         absbase_build = os.path.join(build_root, self.subdir)
 
         for a in incdir_strings:
-            if path_is_in_root(Path(a), Path(src_root)):
+            if not self.coredata.options[OptionKey('allow_abs_paths')].value and path_is_in_root(Path(a), Path(src_root)):
                 raise InvalidArguments(textwrap.dedent('''\
                     Tried to form an absolute path to a dir in the source tree.
                     You should not do that but use relative paths instead, for
-                    directories that are part of your project.
+                    directories that are part of your project. This is to help avoid
+                    broken paths when directories change locations.
 
                     To get include path to any directory relative to the current dir do
 
@@ -2804,6 +2805,12 @@ def build_incdir_object(self, incdir_strings: T.List[str], is_system: bool = Fal
                     Note that this error message can also be triggered by
                     external dependencies being installed within your source
                     tree - it's not recommended to do this.
+
+                    If you understand the risks and would still prefer to use
+                    relocation-safe absolute paths (e.g. by constructing absolute
+                    paths from project-relative path variables, combined with other
+                    relative paths) then you can suppress this error by setting the
+                    'allow_abs_paths' option to 'true'.
                     '''))
             else:
                 try:

mesonbuild/utils/universal.py

@@ -2185,6 +2185,7 @@ class OptionType(enum.IntEnum):
     'pkg_config_path',
     'cmake_prefix_path',
     'vsenv',
+    'allow_abs_paths',
 }
 
 

test cases/common/18 includedir/meson.build

@@ -5,7 +5,8 @@ subdir('src')
 
 errormsg = '''Tried to form an absolute path to a dir in the source tree.
 You should not do that but use relative paths instead, for
-directories that are part of your project.
+directories that are part of your project. This is to help avoid
+broken paths when directories change locations.
 
 To get include path to any directory relative to the current dir do
 
@@ -24,6 +25,12 @@ different subdirectory.
 Note that this error message can also be triggered by
 external dependencies being installed within your source
 tree - it's not recommended to do this.
+
+If you understand the risks and would still prefer to use
+relocation-safe absolute paths (e.g. by constructing absolute
+paths from project-relative path variables, combined with other
+relative paths) then you can suppress this error by setting the
+'allow_abs_paths' option to 'true'.
 '''
 testcase expect_error(errormsg)
   include_directories(meson.current_source_dir() / 'include')

test cases/common/271 allow_abs_paths/func.c

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

test cases/common/271 allow_abs_paths/include/func.h

@@ -0,0 +1,6 @@
+#ifndef FUNC_H__
+#define FUNC_H__
+
+int func(void);
+
+#endif

test cases/common/271 allow_abs_paths/meson.build

@@ -0,0 +1,6 @@
+project('include dir test', 'c',
+	default_options : [ 'allow_abs_paths=true' ]
+	)
+
+exe = executable('prog', 'prog.c', 'func.c', include_directories : include_directories(meson.current_source_dir() / 'include'))
+test('abs inc test', exe)

test cases/common/271 allow_abs_paths/prog.c

@@ -0,0 +1,5 @@
+#include "func.h"
+
+int main(void) {
+    return func();
+}