Minor refactoring (#36)

Some minor compiler error and parsing refactoring.

src/lib.rs

@@ -240,7 +240,7 @@ impl Parse for TestHelper {
             .parse()
             .map_err(|err| Error::new(err.span(), "expected helper function"))?;
         let static_args = input.peek(Paren).then(|| input.parse()).transpose()?;
-        let static_return_type = ReturnType::try_parse(input).transpose()?;
+        let static_return_type = input.call(ReturnType::try_parse)?;
         let farrow = input.parse::<Token![=>]>()?;
         let cases;
         let braces = braced!(cases in input);
@@ -405,7 +405,7 @@ impl Parse for CaseArgs {
         }
 
         let args = inner.parse()?;
-        let return_type = ReturnType::try_parse(&inner).transpose()?;
+        let return_type = inner.call(ReturnType::try_parse)?;
 
         let out = CaseArgs {
             braces,
@@ -444,8 +444,9 @@ impl Parse for FnArgs {
         // Using `punctuated::parse_separated_nonempty` in this case,
         // because if this type is being parsed, arguments should be expected,
         // and trailing commas look sloppy...
-        let args = Punctuated::parse_separated_nonempty(&inner)
-            .map_err(|err| Error::new(err.span(), "missing function arguments"))?;
+        let args = inner
+            .call(Punctuated::parse_separated_nonempty)
+            .map_err(|err| Error::new(err.span(), "expected function arguments"))?;
 
         Ok(Self { parens, args })
     }
@@ -469,8 +470,8 @@ impl ReturnType {
     /// Conditionally parses the type, if a right-arrow is peeked from the stream.
     ///
     /// Included, due to the optional nature of return types in this macro.
-    fn try_parse(input: ParseStream) -> Option<Result<Self>> {
-        input.peek(Token![->]).then(|| input.parse())
+    fn try_parse(input: ParseStream) -> Result<Option<Self>> {
+        input.peek(Token![->]).then(|| input.parse()).transpose()
     }
 }