Use colored output with GCC and Clang

The flag -fdiagnostics-color=always is added to the :standard set of
flags for C stubs, which can be easily removed by the user if needed.

Signed-off-by: Antonin Décimo <antonin@tarides.com>

src/dune_rules/cxx_flags.ml

@@ -20,6 +20,11 @@ let base_cxx_flags ~for_ cc =
   | Msvc, Link -> []
   | Other _, (Link | Compile) -> []
 
+let fdiagnostics_color = function
+  | (Gcc | Clang) when Lazy.force Ansi_color.stderr_supports_color ->
+    [ "-fdiagnostics-color=always" ]
+  | _ -> []
+
 let preprocessed_filename = "ccomp"
 
 let ccomp_type build_dir =

src/dune_rules/cxx_flags.mli

@@ -24,3 +24,7 @@ val ccomp_type : Context.t -> ccomp_type Action_builder.t
 (** [get_flags for_:phase ctx] returns the necessary flags to turn this compiler
     into a c++ compiler for some of the most common compilers *)
 val get_flags : for_:phase -> Context.t -> string list Action_builder.t
+
+(** [fdiagnostics_color cc] returns the flags activating color diagnostics for
+    the C/C++ compiler, if supported. *)
+val fdiagnostics_color : ccomp_type -> string list

src/dune_rules/super_context.ml

@@ -7,16 +7,23 @@ let default_context_flags (ctx : Context.t) ~project =
   in
   let c, cxx =
     match Dune_project.use_standard_c_and_cxx_flags project with
-    | None | Some false ->
-      (Action_builder.return cflags, Action_builder.return cxxflags)
+    | None | Some false -> Action_builder.(return cflags, return cxxflags)
     | Some true ->
-      let c = cflags @ Ocaml_config.ocamlc_cppflags ctx.ocaml_config in
+      let open Action_builder.O in
+      let c =
+        let+ cc = Cxx_flags.ccomp_type ctx in
+        let fdiagnostics_color = Cxx_flags.fdiagnostics_color cc in
+        cflags
+        @ Ocaml_config.ocamlc_cppflags ctx.ocaml_config
+        @ fdiagnostics_color
+      in
       let cxx =
-        let open Action_builder.O in
-        let+ db_flags = Cxx_flags.get_flags ~for_:Compile ctx in
-        db_flags @ cxxflags
+        let+ cc = Cxx_flags.ccomp_type ctx
+        and+ db_flags = Cxx_flags.get_flags ~for_:Compile ctx in
+        let fdiagnostics_color = Cxx_flags.fdiagnostics_color cc in
+        db_flags @ cxxflags @ fdiagnostics_color
       in
-      (Action_builder.return c, cxx)
+      (c, cxx)
   in
   Foreign_language.Dict.make ~c ~cxx
 

test/blackbox-tests/test-cases/c-flags-diagnostics-color.t/dune-project

@@ -0,0 +1 @@
+(lang dune 3.5)

test/blackbox-tests/test-cases/c-flags-diagnostics-color.t/run.t

@@ -0,0 +1,47 @@
+We also have to force Dune outputting colors, for that we use
+`CLICOLOR_FORCE=1`.
+
+The flag should be present in the command, as we want it in the
+:standard set.
+
+test, color enabled, color flag default (enabled)
+=================================================
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name test)
+  >  (foreign_stubs (language c) (names stub)))
+  > EOF
+
+  $ CLICOLOR_FORCE=1 dune rules -m stub.o | grep -ce "-fdiagnostics-color=always"
+  1
+
+test, color enabled, color flag disabled
+========================================
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name test)
+  >  (foreign_stubs
+  >   (flags :standard \ -fdiagnostics-color=always)
+  >   (language c) (names stub)))
+  > EOF
+
+  $ CLICOLOR_FORCE=1 dune rules -m stub.o | grep -ce "-fdiagnostics-color=always"
+  0
+  [1]
+
+test, color disabled, color flag default (enabled)
+==================================================
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name test)
+  >  (foreign_stubs
+  >   (flags :standard)
+  >   (language c) (names stub)))
+  > EOF
+
+  $ CLICOLOR=0 dune rules -m stub.o | grep -ce "-fdiagnostics-color=always"
+  0
+  [1]

test/blackbox-tests/test-cases/c-flags-diagnostics-color.t/stub.c

@@ -0,0 +1 @@
+#error "error message"