doc: document -Wuseless-action

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

NEWS

@@ -2,6 +2,30 @@ GNU Bison NEWS
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+*** Diagnostics for stylistic issues: -Wuseless-action
+
+  A new warning category is introduced: 'style'.  Currently it 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]
+         | 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]
+
+  Running 'bison -Wuseless-action --update foo.y' would remove these actions.
 
 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
 

doc/bison.texi

@@ -10490,6 +10490,41 @@ 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/getargs.c

@@ -344,6 +344,7 @@ Warning categories include:\n\
   'empty-rule'        empty rules without %empty\n\
   'midrule-values'    unset or unused midrule values\n\
   'precedence'        useless precedence and associativity\n\
+  'useless-action'    useless explicit actions\n\
   'yacc'              incompatibilities with POSIX Yacc\n\
   'other'             all other warnings (enabled by default)\n\
   'all'               all the warnings except 'yacc'\n\