Improve description of meson wrapped custom commands

I've always found ninja reporting 'a meson_exe.py custom command'
unclear and confusing. Instead say we are invoking a custom command,
wrapped by meson, and why.

mesonbuild/backend/backends.py

@@ -395,13 +395,30 @@ class Backend:
                 exe_cmd = ['mono'] + exe_cmd
             exe_wrapper = None
 
-        force_serialize = force_serialize or extra_paths or workdir or \
-            exe_wrapper or any('\n' in c for c in cmd_args)
+        reasons = []
+        if extra_paths:
+            reasons.append('to set PATH')
+
+        if exe_wrapper:
+            reasons.append('to use exe_wrapper')
+
+        if workdir:
+            reasons.append('to set workdir')
+
+        if any('\n' in c for c in cmd_args):
+            reasons.append('because command contains newlines')
+
+        force_serialize = force_serialize or bool(reasons)
+
+        if capture:
+            reasons.append('to capture output')
+
         if not force_serialize:
             if not capture:
-                return None
-            return (self.environment.get_build_command() +
-                    ['--internal', 'exe', '--capture', capture, '--'] + exe_cmd + cmd_args)
+                return None, ''
+            return ((self.environment.get_build_command() +
+                    ['--internal', 'exe', '--capture', capture, '--'] + exe_cmd + cmd_args),
+                    ', '.join(reasons))
 
         workdir = workdir or self.environment.get_build_dir()
         env = {}
@@ -425,7 +442,8 @@ class Backend:
                                          exe_wrapper, workdir,
                                          extra_paths, capture)
             pickle.dump(es, f)
-        return self.environment.get_build_command() + ['--internal', 'exe', '--unpickle', exe_data]
+        return (self.environment.get_build_command() + ['--internal', 'exe', '--unpickle', exe_data],
+                ', '.join(reasons))
 
     def serialize_tests(self):
         test_data = os.path.join(self.environment.get_scratch_dir(), 'meson_test_setup.dat')

mesonbuild/backend/ninjabackend.py

@@ -870,7 +870,7 @@ int dummy;
         (srcs, ofilenames, cmd) = self.eval_custom_target_command(target)
         deps = self.unwrap_dep_list(target)
         deps += self.get_custom_target_depend_files(target)
-        desc = 'Generating {0} with a {1} command'
+        desc = 'Generating {0} with a custom command{1}'
         if target.build_always_stale:
             deps.append('PHONY')
         if target.depfile is None:
@@ -884,14 +884,14 @@ int dummy;
             for output in d.get_outputs():
                 elem.add_dep(os.path.join(self.get_target_dir(d), output))
 
-        meson_exe_cmd = self.as_meson_exe_cmdline(target.name, target.command[0], cmd[1:],
+        meson_exe_cmd, reason = self.as_meson_exe_cmdline(target.name, target.command[0], cmd[1:],
                                                           extra_bdeps=target.get_transitive_build_target_deps(),
                                                           capture=ofilenames[0] if target.capture else None)
         if meson_exe_cmd:
             cmd = meson_exe_cmd
-            cmd_type = 'meson_exe.py custom'
+            cmd_type = ' (wrapped by meson {})'.format(reason)
         else:
-            cmd_type = 'custom'
+            cmd_type = ''
         if target.depfile is not None:
             depfile = target.get_dep_outname(elem.infilenames)
             rel_dfile = os.path.join(self.get_target_dir(target), depfile)
@@ -2012,7 +2012,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
                 outfilelist = outfilelist[len(generator.outputs):]
             args = self.replace_paths(target, args, override_subdir=subdir)
             cmdlist = exe_arr + self.replace_extra_args(args, genlist)
-            meson_exe_cmd = self.as_meson_exe_cmdline('generator ' + cmdlist[0],
+            meson_exe_cmd, reason = self.as_meson_exe_cmdline('generator ' + cmdlist[0],
                                                               cmdlist[0], cmdlist[1:],
                                                               capture=outfiles[0] if generator.capture else None)
             if meson_exe_cmd:
@@ -2026,11 +2026,16 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
                 elem.add_item('DEPFILE', depfile)
             if len(extra_dependencies) > 0:
                 elem.add_dep(extra_dependencies)
+
             if len(generator.outputs) == 1:
-                elem.add_item('DESC', 'Generating {!r}.'.format(sole_output))
+                what = '{!r}'.format(sole_output)
             else:
                 # since there are multiple outputs, we log the source that caused the rebuild
-                elem.add_item('DESC', 'Generating source from {!r}.'.format(sole_output))
+                what = 'from {!r}.'.format(sole_output)
+            if reason:
+                reason = ' (wrapped by meson {})'.format(reason)
+            elem.add_item('DESC', 'Generating {}{}.'.format(what, reason))
+
             if isinstance(exe, build.BuildTarget):
                 elem.add_dep(self.get_target_filename(exe))
             elem.add_item('COMMAND', cmdlist)

mesonbuild/backend/vs2010backend.py

@@ -150,7 +150,7 @@ class Vs2010Backend(backends.Backend):
                     # Always use a wrapper because MSBuild eats random characters when
                     # there are many arguments.
                     tdir_abs = os.path.join(self.environment.get_build_dir(), self.get_target_dir(target))
-                    cmd = self.as_meson_exe_cmdline(
+                    cmd, _ = self.as_meson_exe_cmdline(
                         'generator ' + cmd[0],
                         cmd[0],
                         cmd[1:],
@@ -565,7 +565,7 @@ class Vs2010Backend(backends.Backend):
         # there are many arguments.
         tdir_abs = os.path.join(self.environment.get_build_dir(), self.get_target_dir(target))
         extra_bdeps = target.get_transitive_build_target_deps()
-        wrapper_cmd = self.as_meson_exe_cmdline(target.name, target.command[0], cmd[1:],
+        wrapper_cmd, _ = self.as_meson_exe_cmdline(target.name, target.command[0], cmd[1:],
                                                    # All targets run from the target dir
                                                    workdir=tdir_abs,
                                                    extra_bdeps=extra_bdeps,