Don't panic on parse error at EOF in macro grammar

Fixes #376

peg-macros/lib.rs

@@ -2,6 +2,7 @@ extern crate proc_macro;
 extern crate proc_macro2;
 extern crate quote;
 
+use peg::Parse;
 use quote::quote_spanned;
 
 // This can't use the `peg` crate as it would be a circular dependency, but the generated code in grammar.rs
@@ -23,7 +24,11 @@ pub fn parser(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     let grammar = match grammar::peg::peg_grammar(&tokens) {
         Ok(g) => g,
         Err(err) => {
-            let msg = format!("expected {}", err.expected);
+            let msg = if tokens.is_eof(err.location.1) {
+                format!("expected {} at end of input", err.expected)
+            } else {
+                format!("expected {}", err.expected)
+            };
             return quote_spanned!(err.location.0=> compile_error!(#msg);).into();
         }
     };

peg-macros/tokens.rs

@@ -122,7 +122,12 @@ impl Parse for FlatTokenStream {
     }
 
     fn position_repr(&self, pos: usize) -> Sp {
-        Sp(self.tokens[pos].span(), pos)
+        let span = self.tokens.get(pos)
+            .map_or_else(
+                || Span::call_site(),
+                |t| t.span()
+            );
+        Sp(span, pos)
     }
 }
 

tests/compile-fail/incomplete_grammar.rs

@@ -0,0 +1,8 @@
+
+peg::parser!();
+
+peg::parser!(
+    grammar parser() for str
+);
+
+fn main() {}

tests/compile-fail/incomplete_grammar.stderr

@@ -0,0 +1,17 @@
+error: expected one of "#", "grammar", "pub" at end of input
+ --> tests/compile-fail/incomplete_grammar.rs:2:1
+  |
+2 | peg::parser!();
+  | ^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the macro `peg::parser` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: expected one of "::", "<", "{" at end of input
+ --> tests/compile-fail/incomplete_grammar.rs:4:1
+  |
+4 | / peg::parser!(
+5 | |     grammar parser() for str
+6 | | );
+  | |_^
+  |
+  = note: this error originates in the macro `peg::parser` (in Nightly builds, run with -Z macro-backtrace for more info)