setup_rts: Do not require a runtime project when Ada is disabled

Gnatcov required a runtime to be available when loading
gnatcov_rts.gpr in order to be able to choose an appropriate RTS
profile. This is not possible when Ada is not in the list of
available languages.

This commit lifts the runtime requirement and adds a fallback
strategy for when Ada is unavailable, consisting in selecting a
full runtime for the native case, and an embedded runtime for
cross targets.

Change-Id: I3eacf3eee4341565e9ece2ee9eff3ba34d2ce9b7
TN: V913-008

Author: leocreuse

tools/gnatcov/setup_rts.adb

@@ -433,19 +433,26 @@ package body Setup_RTS is
       --  Print all messages in verbose mode, and all but the Information ones
       --  otherwise. Abort on error, or if we failed to get a runtime project
       --  (when we cannot find a toolchain, GPR2 only emits warnings).
+      --
+      --  C-only projects do not require a runtime, so do not complain if there
+      --  is no runtime project in this case.
 
       declare
          Logs      : constant access GPR2.Log.Object := Prj.Log_Messages;
          Has_Error : constant Boolean :=
-           Logs.Has_Error or else not Prj.Has_Runtime_Project;
+           Logs.Has_Error
+           or else (Has_Ada and then not Prj.Has_Runtime_Project);
       begin
          Logs.Output_Messages (Information => Verbose);
          if Has_Error then
             Fatal_Error ("Could not load the coverage runtime project file");
          end if;
       end;
 
-      Actual_RTS := To_Unbounded_String (String (Prj.Runtime (Main_Language)));
+      Actual_RTS :=
+        (if Prj.Has_Runtime_Project
+         then To_Unbounded_String (String (Prj.Runtime (Main_Language)))
+         else Null_Unbounded_String);
 
       --  In verbose mode, show the actual names for the target and the runtime
       --  that GPR2 uses. They can be slightly different from the names users
@@ -461,14 +468,35 @@ package body Setup_RTS is
       --  "full" is to look for an Ada source file that is typically found in
       --  full runtime: "a-comlin.ads" for the Ada.Command_Line unit. If we do
       --  not have it, consider that the runtime is embedded.
+      --
+      --  When only C is enabled and we do not have a runtime at hand to guess
+      --  what is available, fall back to the full profile for native
+      --  applications and the embedded runtime otherwise.
 
       Auto_RTS_Profile := Embedded;
-      for F of Prj.Runtime_Project.Sources loop
-         if F.Path_Name.Simple_Name = "a-comlin.ads" then
-            Auto_RTS_Profile := Full;
-            exit;
-         end if;
-      end loop;
+      if Has_Ada then
+         for F of Prj.Runtime_Project.Sources loop
+            if F.Path_Name.Simple_Name = "a-comlin.ads" then
+               Auto_RTS_Profile := Full;
+               exit;
+            end if;
+         end loop;
+      else
+         declare
+            use GPR2;
+            Tool_Name : constant Name_Type := "gcc";
+         begin
+            --  Add_Tool_Prefix's documentation states that the tool name is
+            --  returned unchanged for native compilation, so use this as a
+            --  proxy to determine if we are setting up the gnatcov RTS for
+            --  a cross target or not.
+
+            Auto_RTS_Profile :=
+              (if Tool_Name = Prj.Add_Tool_Prefix (Tool_Name)
+               then Full
+               else Embedded);
+         end;
+      end if;
 
       --  Query the support for libraries in this configuration. When GPRconfig
       --  fails to find the toolchain for the requested target/RTS, it only