Disable unset/unused mid-rule value warnings by default, and recognize

[bison.git] / NEWS

diff --git a/NEWS b/NEWS
index 75e0284ffaae4a4095637ee216f8147ef9a0d667..b1371ae7fe15f9acacbb9810fddcfba3d9d902ae 100644 (file)
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+++ b/NEWS
@@ -6,6 +6,45 @@ Changes in version 2.3a+ (????-??-??):
 * The -g and --graph options now output graphs in Graphviz DOT format,
   not VCG format.
 
+* Revised warning: unset or unused mid-rule values
+
+  Since Bison 2.2, Bison has warned about mid-rule values that are set but not
+  used within any of the actions of the parent rule.  For example, Bison warns
+  about unused $2 in:
+
+    exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
+
+  Now, Bison also warns about mid-rule values that are used but not set.  For
+  example, Bison warns about unset $$ in the mid-rule action in:
+
+    exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
+
+  However, Bison now disables both of these warnings by default since they
+  sometimes prove to be false alarms in existing grammars employing the Yacc
+  constructs $0 or $-N (where N is some positive integer).
+
+  To enable these warnings, specify the flag `--warnings=midrule-values' or
+  `-W', which is a synonym for `--warnings=all'.
+
+* Bison now recognizes two separate kinds of default %destructor's and
+  %printer's:
+
+    1. Place `<*>' in a %destructor/%printer symbol list to define a default
+       %destructor/%printer for all grammar symbols for which you have formally
+       declared semantic type tags.
+
+    2. Place `<!>' in a %destructor/%printer symbol list to define a default
+       %destructor/%printer for all grammar symbols without declared semantic
+       type tags.
+
+  Bison no longer supports the `%symbol-default' notation from Bison 2.3a.
+  `<*>' and `<!>' combined achieve the same effect with one exception: Bison no
+  longer applies any %destructor to a mid-rule value if that mid-rule value is
+  not actually ever referenced using either $$ or $n in a semantic action.
+
+  See the section `Freeing Discarded Symbols' in the Bison manual for further
+  details.
+
 * The Yacc prologue alternatives from Bison 2.3a have been rewritten as the
   following directives: