error messages for values beginnning with #

prior to this commit, error messages that occurred within idents
beginning with interpolation would be swallowed by error messages for
hex (largely, "Expected identifier.")

src/value/parse.rs

@@ -722,15 +722,17 @@ impl Value {
                 })
             }
             '#' => {
-                if let Ok(s) = Self::ident(toks, scope, super_selector) {
-                    s
-                } else {
+                if let Some(Token { kind: '{', .. }) = toks.peek_forward(1) {
+                    toks.reset_view();
+                    return Some(Self::ident(toks, scope, super_selector));
+                }
+                toks.reset_view();
+                toks.next();
                 IntermediateValue::Value(match parse_hex(toks, scope, super_selector, span) {
                     Ok(v) => v,
                     Err(e) => return Some(Err(e)),
                 })
             }
-            }
             q @ '"' | q @ '\'' => {
                 let span_start = toks.next().unwrap().pos();
                 let Spanned { node, span } =

tests/interpolation.rs

@@ -73,3 +73,7 @@ test!(
     "$f: foo;\na {\n  color: #{$f}(a, 1+2, c);\n}\n",
     "a {\n  color: foo(a, 3, c);\n}\n"
 );
+error!(
+    error_message_when_at_start_of_value,
+    "a {\n  color: #{2px*5px};\n}\n", "Error: 10px*px isn't a valid CSS value."
+);