cmake module: Better warnings and error messages in some cases.

mesonbuild/cmake/generator.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 from .. import mesonlib
+from .. import mlog
 from .common import cmake_is_debug
 import typing as T
 
@@ -66,6 +67,7 @@ def parse_generator_expressions(
 
     def target_file(arg: str) -> str:
         if arg not in trace.targets:
+            mlog.warning(f"Somewhere in your CMakeLists.txt you have '$<TARGET_FILE:{arg}>'. In cmake, this evaluates to the path to '{arg}'. If '{arg}' does not exist, cmake errors out. We think that '{arg}' does not exist, so we do not know its path and just return an empty string.")
             return ''
         tgt = trace.targets[arg]
 

mesonbuild/cmake/interpreter.py

@@ -667,6 +667,7 @@ class ConverterCustomTarget:
         commands = []  # type: T.List[T.List[T.Union[str, ConverterTarget]]]
         for curr_cmd in self._raw_target.command:
             assert isinstance(curr_cmd, list)
+            assert curr_cmd[0] != '', "An empty string is not a valid executable"
             cmd = []  # type: T.List[T.Union[str, ConverterTarget]]
 
             for j in curr_cmd:

mesonbuild/cmake/traceparser.py

@@ -231,7 +231,14 @@ class CMakeTraceParser:
 
         for ctgt in self.custom_targets:
             ctgt.outputs = pathlist_gen(ctgt._outputs_str)
+            temp = ctgt.command
             ctgt.command = [strlist_gen(x) for x in ctgt.command]
+            for command, src in zip(ctgt.command, temp):
+                if command[0] == "":
+                    raise CMakeException(
+                        "We have a problem: We would like to generate a recipe for a target that should be generated by the custom command %s\nThe problem is that the executable that should be called, here: '%s', is a variable that evaluates to an empty string. An empty string is not a valid path to an executable so ninja will not be able to call it."
+                        % (repr(src), src[0])
+                    )
             ctgt.working_dir = Path(parse_generator_expressions(str(ctgt.working_dir), self)) if ctgt.working_dir is not None else None
 
         # Postprocess