Allow dashes in identifiers

In Nixpkgs, the attribute in all-packages.nix corresponding to a
package is usually equal to the package name.  However, this doesn't
work if the package contains a dash, which is fairly common.  The
convention is to replace the dash with an underscore (e.g. "dbus-lib"
becomes "dbus_glib"), but that's annoying.  So now dashes are valid in
variable / attribute names, allowing you to write:

  dbus-glib = callPackage ../development/libraries/dbus-glib { };

and

  buildInputs = [ dbus-glib ];

Since we don't have a negation or subtraction operation in Nix, this
is unambiguous.

doc/manual/release-notes.xml

@@ -14,6 +14,11 @@
 
 <itemizedlist>
 
+  <listitem>
+    <para>Dashes are now valid as part of identifiers and attribute
+    names.</para>
+  </listitem>
+
   <listitem>
     <para>Nix no longer sets the immutable bit on files in the Nix
     store.  Instead, the recommended way to guard the Nix store

misc/emacs/nix-mode.el

@@ -78,7 +78,7 @@ The hook `nix-mode-hook' is run when Nix mode is started.
     ("\\<isNull\\>" . font-lock-builtin-face)
     ("[a-zA-Z][a-zA-Z0-9\\+-\\.]*:[a-zA-Z0-9%/\\?:@&=\\+\\$,_\\.!~\\*'-]+"
      . font-lock-constant-face)
-    ("\\<\\([a-zA-Z_][a-zA-Z0-9_'\.]*\\)[ \t]*="
+    ("\\<\\([a-zA-Z_][a-zA-Z0-9_'\-\.]*\\)[ \t]*="
      (1 font-lock-variable-name-face nil nil))
     ("<[a-zA-Z0-9._\\+-]+\\(/[a-zA-Z0-9._\\+-]+\\)*>"
      . font-lock-constant-face)

src/libexpr/lexer.l

@@ -78,7 +78,7 @@ static Expr * unescapeStr(SymbolTable & symbols, const char * s)
 %}
 
 
-ID          [a-zA-Z\_][a-zA-Z0-9\_\']*
+ID          [a-zA-Z\_][a-zA-Z0-9\_\'\-]*
 INT         [0-9]+
 PATH        [a-zA-Z0-9\.\_\-\+]*(\/[a-zA-Z0-9\.\_\-\+]+)+
 SPATH       \<[a-zA-Z0-9\.\_\-\+]+(\/[a-zA-Z0-9\.\_\-\+]+)*\>

tests/lang/eval-okay-attrs5.nix

@@ -4,7 +4,7 @@ let
 
   as = { x.y.z = 123; a.b.c = 456; };
 
-  bs = { foo.bar = "foo"; };
+  bs = { f-o-o.bar = "foo"; };
 
   or = x: y: x || y;
   
@@ -13,7 +13,7 @@ in
     as.foo or "foo"
     as.x.y.bla or as.a.b.c
     as.a.b.c or as.x.y.z
-    as.x.y.bla or bs.foo.bar or "xyzzy"
+    as.x.y.bla or bs.f-o-o.bar or "xyzzy"
     as.x.y.bla or bs.bar.foo or "xyzzy"
     123.bla or null.foo or "xyzzy"
     # Backwards compatibility test.