Add comments suggested by Niko

src/libsyntax/diagnostic.rs

@@ -68,9 +68,9 @@ pub trait Emitter {
                    sp: RenderSpan, msg: &str, lvl: Level);
 }
 
-/// This structure is used to signify that a task has panicked with a fatal error
-/// from the diagnostics. You can use this with the `Any` trait to figure out
-/// how a rustc task died (if so desired).
+/// Used as a return value to signify a fatal error occurred. (It is also
+/// used as the argument to panic at the moment, but that will eventually
+/// not be true.)
 #[derive(Copy, Clone)]
 #[must_use]
 pub struct FatalError;

src/libsyntax/ext/asm.rs

@@ -23,17 +23,6 @@ use parse::token::InternedString;
 use parse::token;
 use ptr::P;
 
-macro_rules! panictry {
-    ($e:expr) => ({
-        use std::result::Result::{Ok, Err};
-
-        match $e {
-            Ok(e) => e,
-            Err(e) => panic!(e),
-        }
-    })
-}
-
 enum State {
     Asm,
     Outputs,

src/libsyntax/ext/format.rs

@@ -24,17 +24,6 @@ use ptr::P;
 use std::collections::HashMap;
 use std::iter::repeat;
 
-macro_rules! panictry {
-    ($e:expr) => ({
-        use std::result::Result::{Ok, Err};
-
-        match $e {
-            Ok(e) => e,
-            Err(e) => { panic!(e); }
-        }
-    })
-}
-
 #[derive(PartialEq)]
 enum ArgumentType {
     Known(String),

src/libsyntax/lib.rs

@@ -50,13 +50,17 @@ 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(e) => panic!(e)
+            Err(FatalError) => panic!(FatalError)
         }
     })
 }