fix: preserve directory hierarchy for installed headers

If we have foo/bar.h, we'll preserve the hierarchy relative to the
library directory

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: cf542cda-1684-4719-bffd-fdb6a8bc2d63 -->

CHANGES.md

@@ -1,6 +1,10 @@
 Unreleased
 ----------
 
+- Preserve the directory structure of headers installed with the
+  `install_c_headers` field. Previously, they would all be flattened. Now the
+  directories are preserved upto the library stanza. (#7512, @rgrinberg)
+
 - Resolve `ppx_runtime_libraries` in the target context when cross compiling
   (#7450, fixes #2794, @anmonteiro)
 

src/dune_rules/install_rules.ml

@@ -279,8 +279,22 @@ end = struct
     in
     let+ execs = lib_ppxs ctx ~scope ~lib in
     let install_c_headers =
-      List.map lib.install_c_headers ~f:(fun base ->
-          Path.Build.relative dir (base ^ Foreign_language.header_extension))
+      let f =
+        let path base =
+          Path.Build.relative dir (base ^ Foreign_language.header_extension)
+        in
+        let project = Scope.project scope in
+        if Dune_project.dune_version project < (3, 8) then fun base ->
+          make_entry Lib (path base)
+        else fun base ->
+          let sub_dir =
+            let dirname = Filename.dirname base in
+            if String.equal dirname Filename.current_dir_name then None
+            else in_sub_dir (Some dirname)
+          in
+          make_entry ?sub_dir Lib (path base)
+      in
+      List.map lib.install_c_headers ~f
     in
     List.concat
       [ sources
@@ -292,7 +306,7 @@ end = struct
       ; List.map dll_files ~f:(fun a ->
             let entry = Install.Entry.make ~kind:`File Stublibs a in
             Install.Entry.Sourced.create ~loc entry)
-      ; List.map ~f:(make_entry Lib) install_c_headers
+      ; install_c_headers
       ]
 
   let keep_if expander ~scope stanza =

test/blackbox-tests/test-cases/foreign-stubs/installed-headers.t

@@ -16,11 +16,8 @@ Headers with the same filename cannot be installed together:
   $ touch foo.h inc/foo.h
 
   $ dune build mypkg.install && cat _build/default/mypkg.install | grep ".h"
-  Error: Multiple rules generated for _build/install/default/lib/mypkg/foo.h:
-  - dune:1
-  - dune:1
-  -> required by _build/default/mypkg.install
-  [1]
+    "_build/install/default/lib/mypkg/foo.h"
+    "_build/install/default/lib/mypkg/inc/foo.h" {"inc/foo.h"}
 
 Now we demonstrate that header paths get squashed when installed
 
@@ -32,8 +29,8 @@ Now we demonstrate that header paths get squashed when installed
   > EOF
 
   $ dune build mypkg.install && cat _build/default/mypkg.install | grep ".h"
-    "_build/install/default/lib/mypkg/bar.h"
     "_build/install/default/lib/mypkg/foo.h"
+    "_build/install/default/lib/mypkg/inc/bar.h" {"inc/bar.h"}
 
 Now we try to use the installed headers:
 
@@ -61,11 +58,3 @@ Now we try to use the installed headers:
   > #include <inc/bar.h>
   > EOF
   $ dune build bar.exe
-  File "dune", line 6, characters 9-12:
-  6 |   (names foo)))
-               ^^^
-  foo.c:2:10: fatal error: inc/bar.h: No such file or directory
-      2 | #include <inc/bar.h>
-        |          ^~~~~~~~~~~
-  compilation terminated.
-  [1]