Stop masking overflow and propagate it out more aggressively; also improve error reporting to suggest to user how to fix.

src/librustc/middle/traits/select.rs

@@ -155,10 +155,10 @@ enum BuiltinBoundConditions<'tcx> {
 }
 
 #[deriving(Show)]
-enum EvaluationResult {
+enum EvaluationResult<'tcx> {
     EvaluatedToOk,
-    EvaluatedToErr,
     EvaluatedToAmbig,
+    EvaluatedToErr(SelectionError<'tcx>),
 }
 
 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
@@ -272,7 +272,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                               bound: ty::BuiltinBound,
                                               previous_stack: &ObligationStack<'o, 'tcx>,
                                               ty: Ty<'tcx>)
-                                              -> EvaluationResult
+                                              -> EvaluationResult<'tcx>
     {
         let obligation =
             util::obligation_for_builtin_bound(
@@ -295,7 +295,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn evaluate_obligation_recursively<'o>(&mut self,
                                            previous_stack: Option<&ObligationStack<'o, 'tcx>>,
                                            obligation: &Obligation<'tcx>)
-                                           -> EvaluationResult
+                                           -> EvaluationResult<'tcx>
     {
         debug!("evaluate_obligation_recursively({})",
                obligation.repr(self.tcx()));
@@ -310,7 +310,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn evaluate_stack<'o>(&mut self,
                           stack: &ObligationStack<'o, 'tcx>)
-                          -> EvaluationResult
+                          -> EvaluationResult<'tcx>
     {
         // In intercrate mode, whenever any of the types are unbound,
         // there can always be an impl. Even if there are no impls in
@@ -381,7 +381,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         match self.candidate_from_obligation(stack) {
             Ok(Some(c)) => self.winnow_candidate(stack, &c),
             Ok(None) => EvaluatedToAmbig,
-            Err(_) => EvaluatedToErr,
+            Err(e) => EvaluatedToErr(e),
         }
     }
 
@@ -812,14 +812,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn winnow_candidate<'o>(&mut self,
                             stack: &ObligationStack<'o, 'tcx>,
                             candidate: &Candidate<'tcx>)
-                            -> EvaluationResult
+                            -> EvaluationResult<'tcx>
     {
         debug!("winnow_candidate: candidate={}", candidate.repr(self.tcx()));
         self.infcx.probe(|| {
             let candidate = (*candidate).clone();
             match self.confirm_candidate(stack.obligation, candidate) {
                 Ok(selection) => self.winnow_selection(Some(stack), selection),
-                Err(_) => EvaluatedToErr,
+                Err(error) => EvaluatedToErr(error),
             }
         })
     }
@@ -827,12 +827,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn winnow_selection<'o>(&mut self,
                             stack: Option<&ObligationStack<'o, 'tcx>>,
                             selection: Selection<'tcx>)
-                            -> EvaluationResult
+                            -> EvaluationResult<'tcx>
     {
         let mut result = EvaluatedToOk;
         for obligation in selection.iter_nested() {
             match self.evaluate_obligation_recursively(stack, obligation) {
-                EvaluatedToErr => { return EvaluatedToErr; }
+                EvaluatedToErr(e) => { return EvaluatedToErr(e); }
                 EvaluatedToAmbig => { result = EvaluatedToAmbig; }
                 EvaluatedToOk => { }
             }
@@ -1847,11 +1847,18 @@ impl<'o, 'tcx> Repr<'tcx> for ObligationStack<'o, 'tcx> {
     }
 }
 
-impl EvaluationResult {
+impl<'tcx> EvaluationResult<'tcx> {
     fn may_apply(&self) -> bool {
         match *self {
-            EvaluatedToOk | EvaluatedToAmbig => true,
-            EvaluatedToErr => false,
+            EvaluatedToOk |
+            EvaluatedToAmbig |
+            EvaluatedToErr(Overflow) |
+            EvaluatedToErr(OutputTypeParameterMismatch(..)) => {
+                true
+            }
+            EvaluatedToErr(Unimplemented) => {
+                false
+            }
         }
     }
 }

src/librustc_typeck/check/vtable.rs

@@ -366,6 +366,15 @@ pub fn report_selection_error<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                     "overflow evaluating the trait `{}` for the type `{}`",
                     trait_ref.user_string(fcx.tcx()),
                     self_ty.user_string(fcx.tcx())).as_slice());
+
+            let current_limit = fcx.tcx().sess.recursion_limit.get();
+            let suggested_limit = current_limit * 2;
+            fcx.tcx().sess.span_note(
+                obligation.cause.span,
+                format!(
+                    "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
+                    suggested_limit)[]);
+
             note_obligation_cause(fcx, obligation);
         }
         Unimplemented => {

src/test/compile-fail/recursion_limit.rs

@@ -42,6 +42,10 @@ fn is_send<T:Send>() { }
 
 fn main() {
     is_send::<A>();
-    //~^ ERROR not implemented
-    //~^^ ERROR not implemented
+    //~^ ERROR overflow evaluating
+    //~^^ NOTE consider adding a `#![recursion_limit="20"]` attribute to your crate
+    //~^^^ NOTE must be implemented
+    //~^^^^ ERROR overflow evaluating
+    //~^^^^^ NOTE consider adding a `#![recursion_limit="20"]` attribute to your crate
+    //~^^^^^^ NOTE must be implemented
 }

src/test/compile-fail/unboxed-closures-type-mismatch.rs

@@ -14,6 +14,6 @@ use std::ops::FnMut;
 
 pub fn main() {
     let mut f = |&mut: x: int, y: int| -> int { x + y };
-    let z = f.call_mut((1u, 2));    //~ ERROR not implemented
+    let z = f.call_mut((1u, 2));    //~ ERROR type mismatch
     println!("{}", z);
 }

src/test/compile-fail/unboxed-closures-vtable-mismatch.rs

@@ -18,7 +18,7 @@ fn call_it<F:FnMut<(int,int),int>>(y: int, mut f: F) -> int {
 
 pub fn main() {
     let f = |&mut: x: uint, y: int| -> int { (x as int) + y };
-    let z = call_it(3, f);  //~ ERROR not implemented
+    let z = call_it(3, f);  //~ ERROR type mismatch
     println!("{}", z);
 }