meson: prepare for include rearrangement

Context: we have include paths that are "types.hh" and similarly common
names. We currently have these compatibly available as
"lix/libutil/types.hh" externally but *not yet internally*. This is
because we don't have any way for the src directory to appear as
`"lix/"` from inside of Lix: the lix/ include directory is created by
the install process.

The goal of this whole thing is to make it clearer which component of
Lix that files are a part of, which should hopefully help at least a
little bit to new developers. One disadvantage of un-mixing these is
that it will cause some API changes if we ever move a file between
libraries, but that is not very common, and we don't care that much
about external API users.

This was planned for a while and is why we have a FixIncludes check to
begin with.

Personally I don't see a great benefit in rearranging our source code,
and in fact, it would probably be counterproductive:
- Moving the includes into a separate `include/` directory would just
  make developers have to deal with more directories, when we can
  already generate the desired layout through the build process.
- This would also decouple the .cc and .hh files which currently
  conventionally have each others' definitions and declarations
  respectively, right next to each other, making it easier for them to
  feel decoupled and diverge.

Content: Add ../include as an include directory so that lix/ in include
paths will resolve to src/ within Lix itself, just as it does externally
today. This prepares for a further series of commits applying the actual
change to each library one-by-one by accepting both include versions at
once.

This could have been done with ../ and a symlink called lix, but we
would like to not accept libexpr/foo.hh internally for it would be
broken externally, so we need an otherwise empty directory for the
include.

Change-Id: Ideac17faadae2bcea2dffbab34eb27c582ede399

doc/manual/rl-next/include-rearrangement.md

@@ -0,0 +1,20 @@
+---
+synopsis: "Includes are now qualified with library name"
+category: Development
+cls: [2178]
+credits: jade
+---
+
+The Lix includes have all been rearranged to be of the form `"lix/libexpr/foo.hh"` instead of `"foo.hh"`.
+This was already supported externally for a migration period, but it is now being applied to all the internal usages within Lix itself.
+The goal of this change is to both clarify where a file is from and to avoid polluting global include paths with things like `config.h` that might conflict with other projects.
+
+For other details, see the release notes of Lix 2.90.0, under "Rename all the libraries" in Breaking Changes.
+
+To fix an external project with sources in `src` which has a separate build directory (such that headers are in `../src` relative to where the compiler is running):
+
+```
+lix_root=$HOME/lix
+(cd $lix_root && nix develop -c 'meson setup build && ninja -C build subprojects/lix-clang-tidy/liblix-clang-tidy.so')
+run-clang-tidy -checks='-*,lix-fixincludes' -load=$lix_root/build/subprojects/lix-clang-tidy/liblix-clang-tidy.so -p build/ -header-filter '\.\./src/.*\.h' -fix src
+```

src/include/lix

@@ -0,0 +1 @@
+..

src/libcmd/meson.build

@@ -53,6 +53,8 @@ libcmd = library(
     nlohmann_json,
     liblix_doc,
   ],
+  # '../include' for self references like "lix/libcmd/*.hh"
+  include_directories : [ '../include' ],
   cpp_pch : cpp_pch,
   install : true,
   # FIXME(Qyriad): is this right?
@@ -69,7 +71,8 @@ custom_target(
 )
 
 liblixcmd = declare_dependency(
-  include_directories : '.',
+  # FIXME(jade): remove old '.' include directory
+  include_directories : include_directories('.', '../include'),
   link_with : libcmd,
 )
 

src/libexpr/meson.build

@@ -278,6 +278,7 @@ libexpr = library(
   ],
   # for shared.hh
   include_directories : [
+    '../',
     '../libmain',
   ],
   cpp_pch : cpp_pch,
@@ -293,7 +294,8 @@ install_headers(
 )
 
 liblixexpr = declare_dependency(
-  include_directories : include_directories('.'),
+  # FIXME(jade): remove old '.' include directory
+  include_directories : include_directories('.', '../include'),
   sources : libexpr_settings_header,
   link_with : libexpr,
 )
@@ -301,7 +303,7 @@ liblixexpr = declare_dependency(
 # FIXME: remove when https://git.lix.systems/lix-project/lix/issues/359 is fixed.
 if is_static
   liblixexpr_mstatic = declare_dependency(
-    include_directories : include_directories('.'),
+    include_directories : include_directories('.', '../include'),
     sources : libexpr_settings_header,
     link_whole : libexpr,
   )

src/libfetchers/meson.build

@@ -55,6 +55,7 @@ libfetchers = library(
     liblixutil,
     nlohmann_json,
   ],
+  include_directories : [ '../include' ],
   cpp_pch : cpp_pch,
   install : true,
   # FIXME(Qyriad): is this right?
@@ -78,7 +79,7 @@ configure_file(
 )
 
 liblixfetchers = declare_dependency(
-  include_directories : include_directories('.'),
+  include_directories : include_directories('.', '../include'),
   sources : libfetchers_settings_header,
   link_with : libfetchers,
 )
@@ -86,7 +87,7 @@ liblixfetchers = declare_dependency(
 # FIXME: remove when https://git.lix.systems/lix-project/lix/issues/359 is fixed.
 if is_static
   liblixfetchers_mstatic = declare_dependency(
-    include_directories : include_directories('.'),
+    include_directories : include_directories('.', '../include'),
     sources : libfetchers_settings_header,
     link_whole : libfetchers,
   )

src/libmain/meson.build

@@ -22,6 +22,7 @@ libmain = library(
     liblixutil,
     liblixstore,
   ],
+  include_directories : [ '../include' ],
   cpp_pch : cpp_pch,
   install : true,
   # FIXME(Qyriad): is this right?
@@ -31,7 +32,7 @@ libmain = library(
 install_headers(libmain_headers, subdir : 'lix/libmain', preserve_path : true)
 
 liblixmain = declare_dependency(
-  include_directories : include_directories('.'),
+  include_directories : include_directories('.', '../include'),
   link_with : libmain,
 )
 

src/libstore/meson.build

@@ -349,6 +349,7 @@ libstore = library(
   libstore_sources,
   libstore_settings_headers,
   libstore_generated_headers,
+  include_directories : [ '../include' ],
   dependencies : dependencies,
   cpp_args : cpp_args,
   cpp_pch : cpp_pch,
@@ -361,7 +362,8 @@ install_headers(libstore_headers, subdir : 'lix/libstore', preserve_path : true)
 
 # Used by libfetchers.
 liblixstore = declare_dependency(
-  include_directories : include_directories('.'),
+  # FIXME(jade): remove old '.' include directory
+  include_directories : include_directories('.', '../include'),
   sources : libstore_settings_headers,
   link_with : libstore,
 )
@@ -369,7 +371,7 @@ liblixstore = declare_dependency(
 # FIXME: remove when https://git.lix.systems/lix-project/lix/issues/359 is fixed.
 if is_static
   liblixstore_mstatic = declare_dependency(
-    include_directories : include_directories('.'),
+    include_directories : include_directories('.', '../include'),
     sources : libstore_settings_headers,
     link_whole : libstore,
   )

src/libutil/meson.build

@@ -292,6 +292,7 @@ libutil = library(
     openssl,
     nlohmann_json,
   ],
+  include_directories : [ '../include' ],
   cpp_pch : cpp_pch,
   implicit_include_directories : true,
   install : true,
@@ -315,7 +316,8 @@ configure_file(
 
 # Used by libstore and libfetchers.
 liblixutil = declare_dependency(
-  include_directories : include_directories('.'),
+  # FIXME(jade): remove old "." include-dir
+  include_directories : include_directories('.', '../include'),
   sources : [
     experimental_features_header,
     deprecated_features_header,
@@ -327,7 +329,7 @@ liblixutil = declare_dependency(
 # FIXME: remove when https://git.lix.systems/lix-project/lix/issues/359 is fixed.
 if is_static
   liblixutil_mstatic = declare_dependency(
-    include_directories : include_directories('.'),
+    include_directories : include_directories('.', '../include'),
     sources : [
       experimental_features_header,
       deprecated_features_header,

subprojects/lix-clang-tidy/FixIncludes.cc

@@ -58,7 +58,9 @@ void FixIncludesCallbacks::InclusionDirective(
   };
 
   for (const auto &SourceDir : SourceDirs) {
-    const bool IsAlreadyFixed = FileName.starts_with("lix/lib");
+    // Ignore generated files, since they are often only used internally within
+    // a library anyhow and they are not in the normal source dir.
+    const bool IsAlreadyFixed = FileName.starts_with("lix/lib") || FileName.contains(".gen.") || FileName.ends_with(".md");
     if (File && File->getNameAsRequested().contains(SourceDir) &&
         !IsAlreadyFixed) {
       StringRef Name = File->getNameAsRequested();