Introduce a --warn command line option

Replace the singleton --warn-undefined-variables with infrastructure
to manage multiple warnings: the --warn option can take an action
"ignore", "warn", or "error" (which will apply to all warnings), or
a specific warning type and an action for that type.  Multiple
options can be provided and are consolidated.

* NEWS: Announce the new option.
* doc/make.1: Document in the man page.
* doc/make.texi (Warnings): Document in the user's manual.
* Makefile.am: Add new header warning.h.
* src/warning.h: Define enum for actions and warning types, and
macros to test whether they are set.  Keep the default settings
separate so that we can correctly reconstruct MAKEFLAGS.
* src/makeint.h: Remove deprecated warn_undefined_variables_flag.
* src/main.c: Create global variables to hold warning settings.
(switches): Add a new switch for --warn.
(initialize_warnings): Set the default warning actions.
(main): Call initialize_warnings().
(encode_warning_state, decode_warning_state): Convert warning states
between strings and enums.
(encode_warning_name, decode_warning_name): Convert warning names
between strings and enums.
(decode_warn_flags): Convert a --warn option into enum values.  If
deprecated warn_undefined_variables_flag is set convert it to --warn.
(decode_switches): Don't remove duplicates of --warn since order
matters.  Call decode_warn_flags() to handle --warn.
(define_makeflags): Special-case handling of --warn options written
to MAKEFLAGS: write out the current settings.
* src/read.c (tilde_expand): Use new warning control macros.
* src/variable.c (warn_undefined): Ditto.
* src/job.c (construct_command_argv): Ditto.
* tests/scripts/options/warn: Rename from warn-undefined-variables
and add tests for --warn.
* tests/scripts/variables/MAKEFLAGS: Expect the new behavior.

1 files changed, 60 insertions, 0 deletions

diff --git a/src/warning.h b/src/warning.h
new file mode 100644
index 00000000..778ee407
--- /dev/null
+++ b/src/warning.h
@@ -0,0 +1,60 @@
+/* Control warning output in GNU Make.
+Copyright (C) 2023 Free Software Foundation, Inc.
+This file is part of GNU Make.
+
+GNU Make is free software; you can redistribute it and/or modify it under the
+terms of the GNU General Public License as published by the Free Software
+Foundation; either version 3 of the License, or (at your option) any later
+version.
+
+GNU Make is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Types of warnings we can show.  */
+enum warning_type
+  {
+    wt_undefined_var = 0, /* Reference an undefined variable name.  */
+    wt_max
+  };
+
+/* State of a given warning.  */
+enum warning_state
+  {
+    w_unset = 0,
+    w_ignore,
+    w_warn,
+    w_error
+  };
+
+/* The default state of warnings.  */
+extern enum warning_state default_warnings[wt_max];
+
+/* Current state of warnings.  */
+extern enum warning_state warnings[wt_max];
+
+/* Global warning settings.  */
+extern enum warning_state warn_global;
+
+/* Get the current state of a given warning.  */
+#define warn_get(_w) (warnings[_w] != w_unset ? warnings[_w] \
+                      : warn_global != w_unset ? warn_global \
+                      : default_warnings[_w])
+
+/* Set the current state of a given warning.  Can't use w_unset here.  */
+#define warn_set(_w,_f) do{ warnings[_w] = (_f); } while (0)
+
+/* True if we should check for the warning in the first place.  */
+#define warn_check(_w)  (warn_get (_w) > w_ignore)
+
+/* Check if the warning is ignored.  */
+#define warn_ignored(_w) (warn_get (_w) == w_ignore)
+
+/* Check if the warning is in "warn" mode.  */
+#define warn_warned(_w)  (warn_get (_w) == w_warn)
+
+/* Check if the warning is in "error" mode.  */
+#define warn_error(_w)   (warn_get (_w) == w_error)