Added external_include paths to system includes switch

[LeFrosch]

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Discussion thread for this change

Issue number: #4980

Description of this change

Support for external_includes. Fixes #4980. When --features=external_include_paths is specified includes are added to compilation_context.external_includes and no longer to compilation_context.includes.

[blorente]

The general idea looks good, but could you add tests? Ideally both aspect tests aspect/ and an example project in examples.

[LeFrosch]

I added an example project. But I am not sure how to write a test for this because I need to enable the external_include_paths feature and I need an external repository.
Also I noticed that the external_include_paths feature is a bit unreliably because on Linux I can build and run the example project. However, on mac this project fails to build.

[blorente]

I think the reason the project fails to build on mac is because bazel passes the include directories using -iquote, which means I had to import "simple.h" instead of <simple.h>. Leaving a diff with some changes, feel free to apply as much of it as you want:

diff --git a/examples/cpp/external_includes/project/BUILD b/examples/cpp/external_includes/project/BUILD
index 048db18ecb..cd2df3aeef 100644
--- a/examples/cpp/external_includes/project/BUILD
+++ b/examples/cpp/external_includes/project/BUILD
@@ -1,5 +1,5 @@
 cc_binary(
-    name = "simple",
-    srcs = ["simple.cc"],
-	deps = ["@lib//:simple"],
-)
\ No newline at end of file
+    name = "main",
+    srcs = ["main.cc"],
+    deps = ["@lib//:simple"],
+)
diff --git a/examples/cpp/external_includes/project/simple.cc b/examples/cpp/external_includes/project/main.cc
similarity index 69%
rename from examples/cpp/external_includes/project/simple.cc
rename to examples/cpp/external_includes/project/main.cc
index 3982b98220..4bc95f7584 100644
--- a/examples/cpp/external_includes/project/simple.cc
+++ b/examples/cpp/external_includes/project/main.cc
@@ -1,4 +1,4 @@
-#include <simple.h>
+#include "simple.h"
 
 int main() {
 	simple_header_fun();

[blorente]

But I am not sure how to write a test for this because I need to enable the external_include_paths feature and I need an external repository.

The process can get a little complicated for sure, but there is a way. We can:

• Create a repository_rule that writes the contents of the @lib// example into an external repository.
• Call that repository rule from the WORKSPACE.bzlmod file. Let's say that we give it the name @test_cpp_external_include_paths_example
• Create a fixture and an aspect test, much like this one.