Separate headers from source files

The short answer for why we need to do this is so we can consistently do
`#include "nix/..."`. Without this change, there are ways to still make
that work, but they are hacky, and they have downsides such as making it
harder to make sure headers from the wrong Nix library (e..g.
`libnixexpr` headers in `libnixutil`) aren't being used.

The C API alraedy used `nix_api_*`, so its headers are *not* put in
subdirectories accordingly.

Progress on #7876

We resisted doing this for a while because it would be annoying to not
have the header source file pairs close by / easy to change file
path/name from one to the other. But I am ameliorating that with
symlinks in the next commit.

Author: Ericson2314

doc/manual/source/development/testing.md

@@ -31,7 +31,7 @@ The unit tests are defined using the [googletest] and [rapidcheck] frameworks.
 > ├── libexpr
 > │   ├── meson.build
 > │   ├── value/context.hh
-> │   ├── value/context.cc
+> │   ├── include/nix/value/context.cc
 > │   …
 > │
 > ├── tests
@@ -46,8 +46,12 @@ The unit tests are defined using the [googletest] and [rapidcheck] frameworks.
 > │   │
 > │   ├── libexpr-test-support
 > │   │   ├── meson.build
+> │   │   ├── include/nix
+> │   │   │   ├── meson.build
+> │   │   │   └── tests
+> │   │   │       ├── value/context.hh
+> │   │   │       …
 > │   │   └── tests
-> │   │       ├── value/context.hh
 > │   │       ├── value/context.cc
 > │   │       …
 > │   │
@@ -59,15 +63,15 @@ The unit tests are defined using the [googletest] and [rapidcheck] frameworks.
 > ```
 
 The tests for each Nix library (`libnixexpr`, `libnixstore`, etc..) live inside a directory `src/${library_name_without-nix}-test`.
-Given an interface (header) and implementation pair in the original library, say, `src/libexpr/value/context.{hh,cc}`, we write tests for it in `src/libexpr-tests/value/context.cc`, and (possibly) declare/define additional interfaces for testing purposes in `src/libexpr-test-support/tests/value/context.{hh,cc}`.
+Given an interface (header) and implementation pair in the original library, say, `src/libexpr/include/nix/value/context.hh` and `src/libexpr/value/context.cc`, we write tests for it in `src/libexpr-tests/value/context.cc`, and (possibly) declare/define additional interfaces for testing purposes in `src/libexpr-test-support/include/nix/tests/value/context.hh` and `src/libexpr-test-support/tests/value/context.cc`.
 
 Data for unit tests is stored in a `data` subdir of the directory for each unit test executable.
 For example, `libnixstore` code is in `src/libstore`, and its test data is in `src/libstore-tests/data`.
 The path to the `src/${library_name_without-nix}-test/data` directory is passed to the unit test executable with the environment variable `_NIX_TEST_UNIT_DATA`.
 Note that each executable only gets the data for its tests.
 
 The unit test libraries are in `src/${library_name_without-nix}-test-support`.
-All headers are in a `tests` subdirectory so they are included with `#include "tests/"`.
+All headers are in a `tests` subdirectory so they are included with `#include "nix/tests/"`.
 
 The use of all these separate directories for the unit tests might seem inconvenient, as for example the tests are not "right next to" the part of the code they are testing.
 But organizing the tests this way has one big benefit:

maintainers/flake-module.nix

@@ -16,7 +16,6 @@ import('pkgconfig').generate(
   filebase : meson.project_name(),
   name : 'Nix',
   description : 'Nix Package Manager',
-  subdirs : ['nix'],
   extra_cflags : ['-std=c++2a'],
   requires : requires_public,
   requires_private : requires_private,

nix-meson-build-support/export/meson.build

@@ -9,19 +9,19 @@
 #include <sys/time.h>
 #endif
 
-#include "machines.hh"
-#include "shared.hh"
-#include "plugin.hh"
-#include "pathlocks.hh"
-#include "globals.hh"
-#include "serialise.hh"
-#include "build-result.hh"
-#include "store-api.hh"
-#include "strings.hh"
-#include "derivations.hh"
-#include "local-store.hh"
-#include "legacy.hh"
-#include "experimental-features.hh"
+#include "nix/machines.hh"
+#include "nix/shared.hh"
+#include "nix/plugin.hh"
+#include "nix/pathlocks.hh"
+#include "nix/globals.hh"
+#include "nix/serialise.hh"
+#include "nix/build-result.hh"
+#include "nix/store-api.hh"
+#include "nix/strings.hh"
+#include "nix/derivations.hh"
+#include "nix/local-store.hh"
+#include "nix/legacy.hh"
+#include "nix/experimental-features.hh"
 
 using namespace nix;
 using std::cin;

src/build-remote/build-remote.cc

@@ -1,7 +1,7 @@
-#include "built-path.hh"
-#include "derivations.hh"
-#include "store-api.hh"
-#include "comparator.hh"
+#include "nix/built-path.hh"
+#include "nix/derivations.hh"
+#include "nix/store-api.hh"
+#include "nix/comparator.hh"
 
 #include <nlohmann/json.hpp>
 

src/libcmd/built-path.cc

@@ -1,4 +1,4 @@
-#include "command-installable-value.hh"
+#include "nix/command-installable-value.hh"
 
 namespace nix {
 

src/libcmd/command-installable-value.cc

@@ -1,16 +1,16 @@
 #include <algorithm>
 #include <nlohmann/json.hpp>
 
-#include "command.hh"
-#include "markdown.hh"
-#include "store-api.hh"
-#include "local-fs-store.hh"
-#include "derivations.hh"
-#include "nixexpr.hh"
-#include "profiles.hh"
-#include "repl.hh"
-#include "strings.hh"
-#include "environment-variables.hh"
+#include "nix/command.hh"
+#include "nix/markdown.hh"
+#include "nix/store-api.hh"
+#include "nix/local-fs-store.hh"
+#include "nix/derivations.hh"
+#include "nix/nixexpr.hh"
+#include "nix/profiles.hh"
+#include "nix/repl.hh"
+#include "nix/strings.hh"
+#include "nix/environment-variables.hh"
 
 namespace nix {
 

src/libcmd/command.cc

@@ -1,20 +1,20 @@
-#include "fetch-settings.hh"
-#include "eval-settings.hh"
-#include "common-eval-args.hh"
-#include "shared.hh"
-#include "config-global.hh"
-#include "filetransfer.hh"
-#include "eval.hh"
-#include "fetchers.hh"
-#include "registry.hh"
-#include "flake/flakeref.hh"
-#include "flake/settings.hh"
-#include "store-api.hh"
-#include "command.hh"
-#include "tarball.hh"
-#include "fetch-to-store.hh"
-#include "compatibility-settings.hh"
-#include "eval-settings.hh"
+#include "nix/fetch-settings.hh"
+#include "nix/eval-settings.hh"
+#include "nix/common-eval-args.hh"
+#include "nix/shared.hh"
+#include "nix/config-global.hh"
+#include "nix/filetransfer.hh"
+#include "nix/eval.hh"
+#include "nix/fetchers.hh"
+#include "nix/registry.hh"
+#include "nix/flake/flakeref.hh"
+#include "nix/flake/settings.hh"
+#include "nix/store-api.hh"
+#include "nix/command.hh"
+#include "nix/tarball.hh"
+#include "nix/fetch-to-store.hh"
+#include "nix/compatibility-settings.hh"
+#include "nix/eval-settings.hh"
 
 namespace nix {
 

src/libcmd/common-eval-args.cc

@@ -1,6 +1,6 @@
-#include "editor-for.hh"
-#include "environment-variables.hh"
-#include "source-path.hh"
+#include "nix/editor-for.hh"
+#include "nix/environment-variables.hh"
+#include "nix/source-path.hh"
 
 namespace nix {
 

src/libcmd/editor-for.cc

@@ -1,8 +1,8 @@
 #pragma once
 ///@file
 
-#include "derived-path.hh"
-#include "realisation.hh"
+#include "nix/derived-path.hh"
+#include "nix/realisation.hh"
 
 namespace nix {