Implement content-exists function

Fixes sass#2266
Spec sass/sass-spec#1100

src/context.cpp

@@ -831,6 +831,7 @@ namespace Sass {
     register_function(ctx, mixin_exists_sig, mixin_exists, env);
     register_function(ctx, feature_exists_sig, feature_exists, env);
     register_function(ctx, call_sig, call, env);
+    register_function(ctx, content_exists_sig, content_exists, env);
     // Boolean Functions
     register_function(ctx, not_sig, sass_not, env);
     register_function(ctx, if_sig, sass_if, env);

src/expand.cpp

@@ -679,7 +679,6 @@ namespace Sass {
 
   Statement_Ptr Expand::operator()(Mixin_Call_Ptr c)
   {
-
     if (recursions > maxRecursion) {
       throw Exception::StackError(*c);
     }
@@ -730,13 +729,14 @@ namespace Sass {
     Block_Obj trace_block = SASS_MEMORY_NEW(Block, c->pstate());
     Trace_Obj trace = SASS_MEMORY_NEW(Trace, c->pstate(), c->name(), trace_block);
 
-
+    env->set_global("is_in_mixin", bool_true);
     block_stack.push_back(trace_block);
     for (auto bb : body->elements()) {
       Statement_Obj ith = bb->perform(this);
       if (ith) trace->block()->append(ith);
     }
     block_stack.pop_back();
+    env->del_global("is_in_mixin");
 
     env_stack.pop_back();
     backtrace_stack.pop_back();

src/expand.hpp

@@ -37,6 +37,8 @@ namespace Sass {
     std::vector<Media_Block_Ptr>   media_block_stack;
     std::vector<Backtrace*>        backtrace_stack;
 
+    Boolean_Obj bool_true;
+
     Statement_Ptr fallback_impl(AST_Node_Ptr n);
 
   private:

src/functions.cpp

@@ -2000,5 +2000,14 @@ namespace Sass {
       List_Obj list = Cast<List>(value);
       return SASS_MEMORY_NEW(Boolean, pstate, list && list->is_bracketed());
     }
+
+    Signature content_exists_sig = "content-exists()";
+    BUILT_IN(content_exists)
+    {
+      if (!d_env.has_global("is_in_mixin")) {
+        error("Cannot call content-exists() except within a mixin.", pstate, backtrace);
+      }
+      return SASS_MEMORY_NEW(Boolean, pstate, d_env.has_lexical("@content[m]"));
+    }
   }
 }

src/functions.hpp

@@ -106,6 +106,7 @@ namespace Sass {
     extern Signature simple_selectors_sig;
     extern Signature selector_parse_sig;
     extern Signature is_bracketed_sig;
+    extern Signature content_exists_sig;
 
     BUILT_IN(rgb);
     BUILT_IN(rgba_4);
@@ -188,6 +189,7 @@ namespace Sass {
     BUILT_IN(simple_selectors);
     BUILT_IN(selector_parse);
     BUILT_IN(is_bracketed);
+    BUILT_IN(content_exists);
   }
 }