Don't unwrap on Span.join.

The method `Span.join` will return `None` if the start and end `Span`s
are from different files. This is currently difficult to observe in
practice due to rust-lang/rust#43081, which causes span information
(including file information) to be lost in many cases.

However, PR rust-lang/rust#73084 will cause `Spans` to be properly
preserved in many more cases. This will cause `rocket` to stop
compiling, as this code will end up getting hit with spans from
different files (one from rocket, and one from the `result` ident
defined in the `pear_try!` macro.

codegen/src/parser.rs

@@ -105,10 +105,9 @@ impl syn::parse::Parse for CallPattern {
 
 impl Spanned for CallPattern {
     fn span(&self) -> Span {
-        match self.name {
-            Some(ref name) => name.span().unstable().join(self.expr.span()).unwrap(),
-            None => self.expr.span()
-        }
+        self.name.as_ref()
+            .and_then(|name| name.span().unstable().join(self.expr.span()))
+            .unwrap_or_else(|| self.expr.span())
     }
 }
 
@@ -207,7 +206,9 @@ impl Parse for Case {
         pattern.validate()?;
         input.parse::<Token![=>]>()?;
         let expr: syn::Expr = input.parse()?;
-        let span = case_span_start.join(input.cursor().span().unstable()).unwrap();
+        let span = case_span_start
+            .join(input.cursor().span().unstable())
+            .unwrap_or(case_span_start);
 
         Ok(Case { pattern, expr, span })
     }