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, 3 insertions, 3 deletions

diff --git a/src/libsyntax/ext/format.rs b/src/libsyntax/ext/format.rs
index 2fe77bf7a54..1d99a475b32 100644
--- a/src/libsyntax/ext/format.rs
+++ b/src/libsyntax/ext/format.rs
@@ -92,7 +92,7 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
     let fmtstr = p.parse_expr();
     let mut named = false;
     while p.token != token::Eof {
-        if !p.eat(&token::Comma) {
+        if !panictry!(p.eat(&token::Comma)) {
             ecx.span_err(sp, "expected token: `,`");
             return None;
         }
@@ -101,7 +101,7 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
             named = true;
             let ident = match p.token {
                 token::Ident(i, _) => {
-                    p.bump();
+                    panictry!(p.bump());
                     i
                 }
                 _ if named => {
@@ -120,7 +120,7 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
             let interned_name = token::get_ident(ident);
             let name = &interned_name[..];
 
-            p.expect(&token::Eq);
+            panictry!(p.expect(&token::Eq));
             let e = p.parse_expr();
             match names.get(name) {
                 None => {}