Allow "bare" dynamic attrs

Now, in addition to a."${b}".c, you can write a.${b}.c (applicable
wherever dynamic attributes are valid).

Signed-off-by: Shea Levy <shea@shealevy.com>

src/libexpr/lexer.l

@@ -117,6 +117,8 @@ or          { return OR_KW; }
               return INT;
             }
 
+\$\{        { return DOLLAR_CURLY; }
+
 \"          { BEGIN(STRING); return '"'; }
 <STRING>([^\$\"\\]|\$[^\{\"]|\\.)+ {
               /* !!! Not quite right: we want a follow restriction on

src/libexpr/parser.y

@@ -509,6 +509,7 @@ attr
 
 string_attr
   : '"' string_parts '"' { $$ = $2; }
+  | DOLLAR_CURLY expr '}' { $$ = $2; }
   ;
 
 expr_list

tests/lang/eval-okay-dynamic-attrs-bare.exp

@@ -0,0 +1 @@
+{ binds = true; hasAttrs = true; multiAttrs = true; recBinds = true; selectAttrs = true; selectOrAttrs = true; }

tests/lang/eval-okay-dynamic-attrs-bare.nix

@@ -0,0 +1,17 @@
+let
+  aString = "a";
+
+  bString = "b";
+in {
+  hasAttrs = { a.b = null; } ? ${aString}.b;
+
+  selectAttrs = { a.b = true; }.a.${bString};
+
+  selectOrAttrs = { }.${aString} or true;
+
+  binds = { ${aString}."${bString}c" = true; }.a.bc;
+
+  recBinds = rec { ${bString} = a; a = true; }.b;
+
+  multiAttrs = { ${aString} = true; ${bString} = false; }.a;
+}