Auto merge of #23857 - phildawes:libsyntax_nopanic, r=nikomatsakis

Hello! 

I've been working towards a libsyntax without panics. See:
http://internals.rust-lang.org/t/changing-libsyntax-to-use-result-instead-of-panic/1670

This patch changes the internals of parser.rs to use Result<> rather than panicing. It keeps the following old-style panicing functions as a facade:
parse_expr, parse_item, parse_pat, parse_arm, parse_ty, parse_stmt

I left these functions because I wasn't sure what to do about the quote_* macros or how many syntax-extensions would break if these and quoting macros returned Result.

The gyst of the rest of the patch is:

 - Functions in parse/parser.rs return PResult<> rather than panicing
 - Other functions in libsyntax call panic! explicitly if they rely on panicing behaviour.
 - I added a macro 'panictry!()' to act as scaffolding for callers while converting panicing functions. (This does the same as 'unwrap()' but is easier to grep for and turn into try!()).

Am I on the right track?  I'd quite like to get something merged soon as keeping this rebased in the face of libsyntax changes is a lot of work. Please let me know what changes you'd like to see to make this happen.

Thanks!, Phil

1 files changed, 15 insertions, 0 deletions

diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 0980acd3433..bf95daf8755 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -50,6 +50,21 @@ extern crate libc;
 
 extern crate serialize as rustc_serialize; // used by deriving
 
+// A variant of 'try!' that panics on Err(FatalError). This is used as a
+// crutch on the way towards a non-panic!-prone parser. It should be used
+// for fatal parsing errors; eventually we plan to convert all code using
+// panictry to just use normal try
+macro_rules! panictry {
+    ($e:expr) => ({
+        use std::result::Result::{Ok, Err};
+        use diagnostic::FatalError;
+        match $e {
+            Ok(e) => e,
+            Err(FatalError) => panic!(FatalError)
+        }
+    })
+}
+
 pub mod util {
     pub mod interner;
     #[cfg(test)]