copy_files: error out if no files found

src/dune_rules/simple_rules.ml

@@ -227,6 +227,8 @@ let copy_files sctx ~dir ~expander ~src_dir (def : Copy_files.t) =
     in
     Build_system.eval_pred (File_selector.of_glob ~dir glob)
   in
+  if Filename_set.is_empty files
+  then User_error.raise ~loc [ Pp.textf "Cannot find any files to copy" ];
   (* CR-someday amokhov: We currently traverse the set [files] twice: first, to
      add the corresponding rules, and then to convert the files to [targets]. To
      do only one traversal we need [Memo.parallel_map_set]. *)

test/blackbox-tests/test-cases/copy_files/test7.t

@@ -0,0 +1,17 @@
+Show that copy_files errors out if no files are found
+
+  $ mkdir -p target foo
+  $ cat >dune-project <<EOF
+  > (lang dune 3.17)
+  > EOF
+  $ cat >target/dune <<EOF
+  > (copy_files
+  >  (files ../foo/*.txt))
+  > EOF
+
+  $ dune build
+  File "target/dune", line 2, characters 8-20:
+  2 |  (files ../foo/*.txt))
+              ^^^^^^^^^^^^
+  Error: Cannot find any files to copy
+  [1]