Rework the 'codepeer' target selection process

Ensuring that the codepeer target is only chose when no explicit
target has been provided, and no other toolchains are available in the
current PATH.

Author: HugoGGuerrier

lkql_checker/src/gnatcheck-compiler.adb

@@ -1552,14 +1552,40 @@ package body Gnatcheck.Compiler is
    --------------------------------
 
    function Should_Use_Codepeer_Target return Boolean is
-      Regular_Gnatls : String_Access := Locate_Exec_On_Path ("gnatls");
    begin
+      --  If an explicit toolchain has been provided to the tool (through the
+      --  CLI, the project file of the config file), we need to never fallback
+      --  to the codepeer target.
+      if Gnatcheck_Prj.Tree.Is_Defined
+        and then Gnatcheck_Prj.Tree.Has_Explicit_Target
+      then
+         return Ada.Strings.Unbounded."=" (Target, "codepeer") or else
+           Ada.Strings.Unbounded."=" (Target, "gnatsas");
+      end if;
+
       --  If we could find a regular gnatls, it means there is a native
       --  toolchain, that takes precedence over a potential codepeer toolchain.
-      if Regular_Gnatls /= null then
-         Free (Regular_Gnatls);
-         return False;
-      end if;
+      declare
+         Regular_Gnatls : String_Access := Locate_Exec_On_Path ("gnatls");
+      begin
+         if Regular_Gnatls /= null then
+            Free (Regular_Gnatls);
+            return False;
+         else
+            declare
+               Regular_Gprbuild : String_Access :=
+                 Locate_Exec_On_Path ("gprbuild");
+            begin
+               --  If we could find a regular "gprbuild", it means there is a
+               --  cross toolchain available, thus we don't want to fallback on
+               --  the codepeer target
+               if Regular_Gprbuild /= null then
+                  Free (Regular_Gprbuild);
+                  return False;
+               end if;
+            end;
+         end if;
+      end;
 
       --  If we couldn't, look for a codepeer toolchain.
       declare
@@ -1688,7 +1714,7 @@ package body Gnatcheck.Compiler is
             Args (Num_Args) := new String'("--target=" & To_String (Target));
          elsif Should_Use_Codepeer_Target then
             Num_Args := @ + 1;
-            Args (Num_Args) := new String'("--target=codepeer");
+            Args (Num_Args) := new String'("--target=gnatsas");
          end if;
       else
          --  Target and runtime will be taken from config project anyway
@@ -1833,9 +1859,12 @@ package body Gnatcheck.Compiler is
       Args (8) := new String'("--restricted-to-languages=ada");
       Num_Args := 8;
 
+      Num_Args := @ + 1;
       if Should_Use_Codepeer_Target then
-         Num_Args := @ + 1;
-         Args (Num_Args) := new String'("--target=codepeer");
+         Args (Num_Args) := new String'("--target=gnatsas");
+      else
+         Args (Num_Args) :=
+           new String'("--target=" & Ada.Strings.Unbounded.To_String (Target));
       end if;
 
       if Arg.Jobs.Get > 1 then

lkql_checker/src/gnatcheck-projects.adb

@@ -536,11 +536,6 @@ package body Gnatcheck.Projects is
       use GPR2.Containers;
       use Ada.Strings.Unbounded;
    begin
-      --  In case of autoconf, restrict to the Ada language
-
-      My_Project.Tree.Restrict_Autoconf_To_Languages
-        (Language_Id_Set.To_Set (GPR2.Ada_Language));
-
       --  Apply the options
 
       if My_Project.Source_Prj /= null then
@@ -563,12 +558,22 @@ package body Gnatcheck.Projects is
 
       if Target /= Null_Unbounded_String then
          Project_Options.Add_Switch (GPR2.Options.Target, To_String (Target));
+      elsif not Gnatkp_Mode and then Should_Use_Codepeer_Target then
+         GPR2.KB.Set_Default_Target ("gnatsas");
       end if;
 
       if Arg.Follow_Symbolic_Links.Get then
          Project_Options.Add_Switch (GPR2.Options.Resolve_Links);
       end if;
 
+      --  In case of autoconf, restrict to the Ada language
+      --  We do this just before loading the project because calling this
+      --  procedure will set the `Tree.Is_Defined` to `True`. For now there is
+      --  no other ways to know if a project has been loaded or not.
+      --  (see https://gitlab.adacore-it.com/eng/gpr/gpr-issues/-/issues/627)
+      My_Project.Tree.Restrict_Autoconf_To_Languages
+        (Language_Id_Set.To_Set (GPR2.Ada_Language));
+
       if not My_Project.Tree.Load
                (Project_Options,
                 Reporter         => Gpr2_Reporter,