---

yaml
---
r: 213980
b: refs/heads/master
c: d914831
h: refs/heads/master
v: v3

Author: eddyb

[refs]

@@ -1,5 +1,5 @@
 ---
-refs/heads/master: 0b58fdf9252ccd08bf610e13625f49380e769e06
+refs/heads/master: d9148311451d38e831dcaa15548fb6ac6f6450b9
 refs/heads/snap-stage3: ba0e1cd8147d452c356aacb29fb87568ca26f111
 refs/heads/try: b53c0f93eedcdedd4fd89bccc5a3a09d1c5cd23e
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105

trunk/src/librustc/middle/ty.rs

@@ -816,6 +816,12 @@ impl<'tcx> ctxt<'tcx> {
 /// provided ty::ctxt<'tcx>.
 /// This can be done, for example, for Ty<'tcx> or &'tcx Substs<'tcx>
 /// by looking them up in their respective interners.
+/// None is returned if the value or one of the components is not part
+/// of the provided context.
+/// For Ty, None can be returned if either the type interner doesn't
+/// contain the TypeVariants key or if the address of the interned
+/// pointer differs. The latter case is possible if a primitive type,
+/// e.g. `()` or `u8`, was interned in a different context.
 pub trait Lift<'tcx> {
     type Lifted;
     fn lift_to_tcx(&self, tcx: &ctxt<'tcx>) -> Option<Self::Lifted>;
@@ -3016,6 +3022,10 @@ impl<'tcx> CommonTypes<'tcx> {
     }
 }
 
+/// Create a type context and call the closure with a `&ty::ctxt` reference
+/// to the context. The closure enforces that the type context and any interned
+/// value (types, substs, etc.) can only be used while `ty::tls` has a valid
+/// reference to the context, to allow formatting values that need it.
 pub fn with_ctxt<'tcx, F, R>(s: Session,
                              arenas: &'tcx CtxtArenas<'tcx>,
                              def_map: DefMap,