warnings: introduce -Wprecedence

[bison.git] / NEWS

diff --git a/NEWS b/NEWS
index 167cc8bd1f41808f8406ff7d6b71894d9dd06165..be3669a91869f7de7b206e39fc2f40a5719ac623 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -2,9 +2,10 @@ GNU Bison NEWS
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
-** Future changes
+** WARNING: Future backward-incompatibilities!
 
-  Bison will stop adding a semicolon at the end of the actions:
+  Bison will stop adding a semicolon at the end of the actions (as announced
+  in the release 2.5):
 
     foo.y:2.22: warning: a ';' might be needed at the end of action code
      exp: "num" { $$ = $1 }
@@ -15,7 +16,7 @@ GNU Bison NEWS
   for its own code, especially the definition of variables after statements.
   The generated C parsers still aim at C90.
 
-** Incompatible changes
+** Backward incompatible changes
 
 *** Obsolete features
 
@@ -295,6 +296,76 @@ GNU Bison NEWS
   when declaring associativity at the same time, with %left (or %right,
   %precedence, %nonassoc), B was inferior to A.
 
+** Useless precedence and associativity
+
+  When developping and maintaining a grammar, useless associativity and
+  precedence directives are common.  They can be a nuisance: new ambiguities
+  arising are sometimes masked because their conflicts are resolved due to
+  the extra precedence or associativity information.  Furthermore, it can
+  hinder the comprehension of a new grammar: one will wonder about the role
+  of a precedence, where in fact it is useless.  The following changes aim
+  at detecting and reporting these extra directives.
+
+*** Precedence warning category
+
+  A new category of warning, -Wprecedence, was introduced. It flags the
+  useless precedence and associativity directives.
+
+*** Useless associativity
+
+  Bison now warns about symbols with a declared associativity that is never
+  used to resolve conflicts.  In that case, using %precedence is sufficient;
+  the parsing tables will remain unchanged.  Solving these warnings may raise
+  useless precedence warnings, as the symbols no longer have associativity.
+  For example:
+
+    %left '+'
+    %left '*'
+    %%
+    exp:
+      "num"
+    | exp '+' "num"
+    | exp '*' exp
+    ;
+
+  will produce a
+
+    warning: useless associativity for '+', use %precedence [-Wprecedence]
+     %left '+'
+           ^^^
+
+*** Useless precedence
+
+  Bison now warns about symbols with a declared precedence and no declared
+  associativity (i.e., declared with %precedence), and whose precedence is
+  never used.  In that case, the symbol can be safely declared with %token
+  instead, without modifying the parsing tables.  For example:
+
+    %precedence '='
+    %%
+    exp: "var" '=' "num";
+
+  will produce a
+
+    warning: useless precedence for '=' [-Wprecedence]
+     %precedence '='
+                 ^^^
+
+*** Useless precedence and associativity
+
+  In case of both useless precedence and associativity, the issue is flagged
+  as follows:
+
+    %nonassoc '='
+    %%
+    exp: "var" '=' "num";
+
+  The warning is:
+
+    warning: useless precedence and associativity for '=' [-Wprecedence]
+     %nonassoc '='
+               ^^^
+
 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
 
 ** Bug fixes
@@ -305,6 +376,8 @@ GNU Bison NEWS
 
 ** Diagnostics are improved
 
+  Contributed by Théophile Ranquet.
+
 *** Changes in the format of error messages
 
   This used to be the format of many error reports:
@@ -390,6 +463,8 @@ GNU Bison NEWS
 
 ** Graph improvements in DOT and XSLT
 
+  Contributed by Théophile Ranquet.
+
   The graphical presentation of the states is more readable: their shape is
   now rectangular, the state number is clearly displayed, and the items are
   numbered and left-justified.
@@ -831,7 +906,9 @@ GNU Bison NEWS
   These features are experimental.  More user feedback will help to
   stabilize them.
 
-** LAC (Lookahead Correction) for syntax error handling:
+** LAC (Lookahead Correction) for syntax error handling
+
+  Contributed by Joel E. Denny.
 
   Canonical LR, IELR, and LALR can suffer from a couple of problems
   upon encountering a syntax error.  First, the parser might perform