allow %expect and %expect-rr modifiers on individual rules

This change allows one to document (and check) which rules participate
in shift/reduce and reduce/reduce conflicts.  This is particularly
important GLR parsers, where conflicts are a normal occurrence.  For
example,

    %glr-parser
    %expect 1
    %%

    ...

    argument_list:
      arguments %expect 1
    | arguments ','
    | %empty
    ;

    arguments:
      expression
    | argument_list ',' expression
    ;

    ...

Looking at the output from -v, one can see that the shift-reduce
conflict here is due to the fact that the parser does not know whether
to reduce arguments to argument_list until it sees the token AFTER the
following ','.  By marking the rule with %expect 1 (because there is a
conflict in one state), we document the source of the 1 overall shift-
reduce conflict.

In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
conflicts.  In this case, we mark each of the conflicting rules.  For
example,

    %glr-parser
    %expect-rr 1

    %%

    stmt:
      target_list '=' expr ';'
    | expr_list ';'
    ;

    target_list:
      target
    | target ',' target_list
    ;

    target:
      ID %expect-rr 1
    ;

    expr_list:
      expr
    | expr ',' expr_list
    ;

    expr:
      ID %expect-rr 1
    | ...
    ;

In a statement such as

    x, y = 3, 4;

the parser must reduce x to a target or an expr, but does not know
which until it sees the '='.  So we notate the two possible reductions
to indicate that each conflicts in one rule.

See https://lists.gnu.org/archive/html/bison-patches/2013-02/msg00105.html.

* doc/bison.texi (Suppressing Conflict Warnings): Document %expect,
%expect-rr in grammar rules.
* src/conflicts.c (count_state_rr_conflicts): Adjust comment.
(rule_has_state_sr_conflicts): New static function.
(count_rule_sr_conflicts): New static function.
(rule_nast_state_rr_conflicts): New static function.
(count_rule_rr_conflicts): New static function.
(rule_conflicts_print): New static function.
(conflicts_print): Also use rule_conflicts_print to report on individual
rules.
* src/gram.h (struct rule): Add new fields expected_sr_conflicts,
expected_rr_conflicts.
* src/reader.c (grammar_midrule_action): Transfer expected_sr_conflicts,
expected_rr_conflicts to new rule, and turn off in current_rule.
(grammar_current_rule_expect_sr): New function.
(grammar_current_rule_expect_rr): New function.
(packgram): Transfer expected_sr_conflicts, expected_rr_conflicts
to new rule.
* src/reader.h (grammar_current_rule_expect_sr): New function.
(grammar_current_rule_expect_rr): New function.
* src/symlist.c (symbol_list_sym_new): Initialize expected_sr_conflicts,
expected_rr_conflicts.
* src/symlist.h (struct symbol_list): Add new fields expected_sr_conflicts,
expected_rr_conflicts.
* tests/conflicts.at: Add tests "%expect in grammar rule not enough",
"%expect in grammar rule right.", "%expect in grammar rule too much."

1 files changed, 65 insertions, 2 deletions

diff --git a/doc/bison.texi b/doc/bison.texi
index 72f28fc2..fe580bde 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -5254,6 +5254,53 @@ in GLR parsers, using the declaration:
 %expect-rr @var{n}
 @end example
 
+You may wish to be more specific in your
+specification of expected conflicts.  To this end, you can also attach
+@code{%expect} and @code{%expect-rr} modifiers to individual rules.
+The interpretation of these modifiers differs from their use as
+declarations.  When attached to rules, they indicate the number of states
+in which the rule is involved in a conflict.  You will need to consult the
+output resulting from @samp{-v} to determine appropriate numbers to use.
+For example, for the following grammar fragment, the first rule for
+@code{empty_dims} appears in two states in which the @samp{[} token is a
+lookahead.  Having determined that, you can document this fact with an
+@code{%expect} modifier as follows:
+
+@example
+dims:
+  empty_dims
+| '[' expr ']' dims
+;
+
+empty_dims:
+  %empty   %expect 2
+| empty_dims '[' ']'
+;
+@end example
+
+Mid-rule actions generate implicit rules that are also subject to conflicts
+(@pxref{Midrule Conflicts,, Conflicts due to Midrule Actions}). To attach
+an @code{%expect} or @code{%expect-rr} annotation to an implicit
+mid-rule action's rule, put it before the action.  For example,
+
+@example
+%glr-parser
+%expect-rr 1
+
+%%
+
+clause:
+  "condition" %expect-rr 1 @{ value_mode(); @} '(' exprs ')'
+| "condition" %expect-rr 1 @{ class_mode(); @} '(' types ')'
+;
+@end example
+
+@noindent
+Here, the appropriate mid-rule action will not be determined until after
+the @samp{(} token is shifted.  Thus,
+the two actions will clash with each other, and we should expect one
+reduce/reduce conflict for each.
+
 In general, using @code{%expect} involves these steps:
 
 @itemize @bullet
@@ -5269,8 +5316,17 @@ go back to the beginning.
 
 @item
 Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed.  With GLR parsers, add an
+number that Bison printed.  With GLR parsers, add an
 @code{%expect-rr} declaration as well.
+
+@item
+Optionally, count up the number of states in which one or more
+conflicted reductions for particular rules appear and add these numbers
+to the affected rules as @code{%expect-rr} or @code{%expect} modifiers
+as appropriate.  Rules that are in conflict appear in the output listing
+surrounded by square brackets or, in the case of reduce/reduce conflicts,
+as reductions having the same lookahead symbol as a square-bracketed
+reduction in the same state.
 @end itemize
 
 Now Bison will report an error if you introduce an unexpected conflict,
@@ -5491,7 +5547,14 @@ Start-Symbol}).
 @end deffn
 
 @deffn {Directive} %expect
-Declare the expected number of shift-reduce conflicts
+Declare the expected number of shift-reduce conflicts, either overall or
+for a given rule
+(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
+@end deffn
+
+@deffn {Directive} %expect-rr
+Declare the expected number of reduce-reduce conflicts, either overall or
+for a given rule
 (@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
 @end deffn