doc: document -Wuseless-action

* doc/bison.texi, src/getargs.c, NEWS: here.

NEWS

@@ -8,9 +8,37 @@ GNU Bison NEWS
 
 ** New features
 
+*** Java: disabled debug code
+
   The Java backend no longer emits code and data for parser tracing if the
   %define variable parse.trace is not defined.
 
+*** New warning flag: -Wuseless-action
+
+  A new warning category is introduced: 'useless-action', which reports
+  useless explicit actions.  For instance on the following grammar:
+
+    %type <int> "number" expr term
+    %%
+    expr: expr "+" term { $$ = $1 + $3; }
+        | term          { $$ = $1; }
+    term: "(" expr ")"  { $$ = $2; }
+        | "number"      { $$ = $1; }
+
+  bison diagnoses:
+
+    $ bison -Wuseless-action foo.y
+    foo.y:4.21-32: warning: useless explicit action [-Wuseless-action]
+        4 |     | term          { $$ = $1; }
+          |                     ^~~~~~~~~~~~
+    foo.y:6.21-32: warning: useless explicit action [-Wuseless-action]
+        6 |     | "number"      { $$ = $1; }
+          |                     ^~~~~~~~~~~~
+    foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]
+
+  Running 'bison -Wuseless-action --update foo.y' would remove these
+  actions.
+
 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
 
 ** Bug fixes

doc/bison.texi

@@ -10598,6 +10598,41 @@ One would get the exact same parser with the following directives instead:
 @end group
 @end example
 
+@item useless-action
+Useless explicit actions (@samp{@{ $$ = $1; @}}) are reported.  For instance
+on the following grammar:
+
+@example
+@group
+$ @kbd{cat foo.y}
+%type <int> "number" expr term
+%%
+expr: expr "+" term @{ $$ = $1 + $3; @}
+    | term          @{ $$ = $1; @}
+term: "(" expr ")"  @{ $$ = $2; @}
+    | "number"      @{ $$ = $1; @}
+@end group
+@end example
+
+@noindent
+@command{bison} diagnoses:
+
+@example
+$ @kbd{bison -Wuseless-action foo.y}
+foo.y:4.21-32: warning: useless explicit action [-Wuseless-action]
+     | term          @{ $$ = $1; @}
+                     ^~~~~~~~~~~~
+foo.y:6.21-32: warning: useless explicit action [-Wuseless-action]
+     | "number"      @{ $$ = $1; @}
+                     ^~~~~~~~~~~~
+foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]
+@end example
+
+Running @kbd{bison -Wuseless-action --update foo.y} would remove these actions.
+
+Actions with named references (e.g., @samp{@{ $expr = $term; @}}) are not
+reported.  To disable the warning locally, write @samp{@{ $$ = ($1); @}}.
+
 @item yacc
 Incompatibilities with POSIX Yacc.
 

src/complain.c

@@ -139,6 +139,7 @@ static const argmatch_warning_arg argmatch_warning_args[] =
   { "deprecated",     Wdeprecated },
   { "empty-rule",     Wempty_rule },
   { "precedence",     Wprecedence },
+  { "useless-action", Wuseless_action },
   { "other",          Wother },
   { "all",            Wall },
   { "everything",     Weverything },