add some comments to `Obligation`

[rust.git] / src / librustc / traits / mod.rs

diff --git a/src/librustc/traits/mod.rs b/src/librustc/traits/mod.rs
index 49f43b18e61987fd005f1aec103f9fe530cdbc65..027ad4174bd1526076cf353f76b6727eb081df08 100644 (file)
--- a/src/librustc/traits/mod.rs
+++ b/src/librustc/traits/mod.rs
@@ -8,7 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Trait Resolution. See README.md for an overview of how this works.
+//! Trait Resolution. See [rustc guide] for more info on how this works.
+//!
+//! [rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html
 
 pub use self::SelectionError::*;
 pub use self::FulfillmentErrorCode::*;
@@ -75,10 +77,21 @@ pub enum IntercrateMode {
 /// scope. The eventual result is usually a `Selection` (defined below).
 #[derive(Clone, PartialEq, Eq, Hash)]
 pub struct Obligation<'tcx, T> {
+    /// Why do we have to prove this thing?
     pub cause: ObligationCause<'tcx>,
+
+    /// In which environment should we prove this thing?
     pub param_env: ty::ParamEnv<'tcx>,
-    pub recursion_depth: usize,
+
+    /// What are we trying to prove?
     pub predicate: T,
+
+    /// If we started proving this as a result of trying to prove
+    /// something else, track the total depth to ensure termination.
+    /// If this goes over a certain threshold, we abort compilation --
+    /// in such cases, we can not say whether or not the predicate
+    /// holds for certain. Stupid halting problem. Such a drag.
+    pub recursion_depth: usize,
 }
 
 pub type PredicateObligation<'tcx> = Obligation<'tcx, ty::Predicate<'tcx>>;