Replace assertions in Puctuated with more helpful panic messages

Debugging proc macros is very difficult as we can't easily get
backtraces into the proc macro code. Improved error messages will
hopefully give some hint to the user on what the problem is.

src/punctuated.rs

@@ -162,7 +162,12 @@ impl<T, P> Punctuated<T, P> {
     /// Panics if the sequence does not already have a trailing punctuation when
     /// this method is called.
     pub fn push_value(&mut self, value: T) {
-        assert!(self.empty_or_trailing());
+        assert!(
+            self.empty_or_trailing(),
+            "Punctuated::push_value: Punctuated is not empty or \
+                does not have a trailing punctuation"
+        );
+
         self.last = Some(Box::new(value));
     }
 
@@ -174,7 +179,10 @@ impl<T, P> Punctuated<T, P> {
     ///
     /// Panics if the sequence is empty or already has a trailing punctuation.
     pub fn push_punct(&mut self, punctuation: P) {
-        assert!(self.last.is_some());
+        assert!(
+            self.last.is_some(),
+            "Punctuated::push_punct: Punctuated doesn't have any items"
+        );
         let last = self.last.take().unwrap();
         self.inner.push((*last, punctuation));
     }
@@ -228,7 +236,10 @@ impl<T, P> Punctuated<T, P> {
     where
         P: Default,
     {
-        assert!(index <= self.len());
+        assert!(
+            index <= self.len(),
+            "Punctuated::insert: index out of range"
+        );
 
         if index == self.len() {
             self.push(value);
@@ -454,7 +465,12 @@ impl<T, P> FromIterator<Pair<T, P>> for Punctuated<T, P> {
 
 impl<T, P> Extend<Pair<T, P>> for Punctuated<T, P> {
     fn extend<I: IntoIterator<Item = Pair<T, P>>>(&mut self, i: I) {
-        assert!(self.empty_or_trailing());
+        assert!(
+            self.empty_or_trailing(),
+            "Punctuated::extend: Punctuated is not empty or \
+                does not have a trailing punctuation"
+        );
+
         let mut nomore = false;
         for pair in i {
             if nomore {