configure_file: Don't optimize away substitutions

It's possible that the configuration data object has components added
conditionally, and that sometimes an empty configuration data object
is passed on purpose.

Instead, we do the substitution and also warn if no tokens were found
that could've been substituted.

Closes https://github.com/mesonbuild/meson/issues/3826

2 files changed, 17 insertions, 19 deletions

diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 06d80ab..3a7716d 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -3444,20 +3444,6 @@ root and issuing %s.
             raise InterpreterException('Output file name must not contain a subdirectory.')
         (ofile_path, ofile_fname) = os.path.split(os.path.join(self.subdir, output))
         ofile_abs = os.path.join(self.environment.build_dir, ofile_path, ofile_fname)
-        # Optimize copies by not doing substitution if there's nothing to
-        # substitute, and warn about this legacy hack
-        if 'configuration' in kwargs:
-            conf = kwargs['configuration']
-            if not isinstance(conf, ConfigurationDataHolder):
-                raise InterpreterException('Argument "configuration" must be of type configuration_data')
-            if ifile_abs and not conf.keys():
-                del kwargs['configuration']
-                kwargs['copy'] = True
-                mlog.warning('Got an empty configuration_data() object: '
-                             'optimizing copy automatically; if you want to '
-                             'copy a file to the build dir, use the \'copy:\' '
-                             'keyword argument added in 0.47.0', location=node)
-                conf.mark_used()
         # Perform the appropriate action
         if 'configuration' in kwargs:
             conf = kwargs['configuration']
@@ -3467,15 +3453,21 @@ root and issuing %s.
             if inputfile is not None:
                 os.makedirs(os.path.join(self.environment.build_dir, self.subdir), exist_ok=True)
                 file_encoding = kwargs.setdefault('encoding', 'utf-8')
-                missing_variables = mesonlib.do_conf_file(ifile_abs, ofile_abs,
-                                                          conf.held_object, fmt,
-                                                          file_encoding)
+                missing_variables, confdata_useless = \
+                    mesonlib.do_conf_file(ifile_abs, ofile_abs, conf.held_object,
+                                          fmt, file_encoding)
                 if missing_variables:
                     var_list = ", ".join(map(repr, sorted(missing_variables)))
                     mlog.warning(
-                        "The variable(s) %s in the input file %s are not "
+                        "The variable(s) %s in the input file '%s' are not "
                         "present in the given configuration data." % (
                             var_list, inputfile), location=node)
+                if confdata_useless:
+                    ifbase = os.path.basename(ifile_abs)
+                    mlog.warning('Got an empty configuration_data() object and found no '
+                                 'substitutions in the input file {!r}. If you want to '
+                                 'copy a file to the build dir, use the \'copy:\' keyword '
+                                 'argument added in 0.47.0'.format(ifbase), location=node)
             else:
                 mesonlib.dump_conf_header(ofile_abs, conf.held_object, output_format)
             conf.mark_used()
diff --git a/mesonbuild/mesonlib.py b/mesonbuild/mesonlib.py
index 5f9b98a..6b1fb76 100644
--- a/mesonbuild/mesonlib.py
+++ b/mesonbuild/mesonlib.py
@@ -653,12 +653,18 @@ def do_conf_file(src, dst, confdata, format, encoding='utf-8'):
 
     result = []
     missing_variables = set()
+    # Detect when the configuration data is empty and no tokens were found
+    # during substitution so we can warn the user to use the `copy:` kwarg.
+    confdata_useless = not confdata.keys()
     for line in data:
         if line.startswith(search_token):
+            confdata_useless = False
             line = do_mesondefine(line, confdata)
         else:
             line, missing = do_replacement(regex, line, format, confdata)
             missing_variables.update(missing)
+            if missing:
+                confdata_useless = False
         result.append(line)
     dst_tmp = dst + '~'
     try:
@@ -668,7 +674,7 @@ def do_conf_file(src, dst, confdata, format, encoding='utf-8'):
         raise MesonException('Could not write output file %s: %s' % (dst, str(e)))
     shutil.copymode(src, dst_tmp)
     replace_if_different(dst, dst_tmp)
-    return missing_variables
+    return missing_variables, confdata_useless
 
 CONF_C_PRELUDE = '''/*
  * Autogenerated by the Meson build system.