emitting get-function is an error

src/value/mod.rs

@@ -137,8 +137,13 @@ impl Value {
                 )
                     .into())
             }
-            // TODO: should to_css_string on function fail?
-            Self::Function(func) => format!("get-function(\"{}\")", func.name()),
+            Self::Function(..) => {
+                return Err((
+                    format!("{} isn't a valid CSS value.", self.inspect(span)?),
+                    span,
+                )
+                    .into())
+            }
             Self::List(vals, sep, brackets) => match brackets {
                 Brackets::None => vals
                     .iter()

tests/get-function.rs

@@ -99,3 +99,8 @@ error!(
     non_function_call,
     "a {color: call(4);}", "Error: $function: 4 is not a function reference."
 );
+error!(
+    emit_plain_get_function_is_invalid_css,
+    "a {color: get-function(lighten);}",
+    "Error: get-function(\"lighten\") isn't a valid CSS value."
+);