Detect cycles and specialize error reporting for Sized. It is important

to get the `Sized` error usable, since that hits new users
frequently. Further work is needed for the error reporting for non-Sized
cycle cases; those currently just fallback to the old path. Also adjust tests.

Author: nikomatsakis

src/librustc/diagnostics.rs

@@ -712,6 +712,43 @@ There's no easy fix for this, generally code will need to be refactored so that
 you no longer need to derive from `Super<Self>`.
 "####,
 
+E0072: r##"
+When defining a recursive struct or enum, any use of the type being defined
+from inside the definition must occur behind a pointer (like `Box` or `&`).
+This is because structs and enums must have a well-defined size, and without
+the pointer the size of the type would need to be unbounded.
+
+Consider the following erroneous definition of a type for a list of bytes:
+
+```
+// error, invalid recursive struct type
+struct ListNode {
+    head: u8,
+    tail: Option<ListNode>,
+}
+```
+
+This type cannot have a well-defined size, because it needs to be arbitrarily
+large (since we would be able to nest `ListNode`s to any depth). Specifically,
+
+```plain
+size of `ListNode` = 1 byte for `head`
+                   + 1 byte for the discriminant of the `Option`
+                   + size of `ListNode`
+```
+
+One way to fix this is by wrapping `ListNode` in a `Box`, like so:
+
+```
+struct ListNode {
+    head: u8,
+    tail: Option<Box<ListNode>>,
+}
+```
+
+This works because `Box` is a pointer, so its size is well-known.
+"##,
+
 E0109: r##"
 You tried to give a type parameter to a type which doesn't need it. Erroneous
 code example:

src/librustc/middle/traits/error_reporting.rs

@@ -182,7 +182,8 @@ fn report_on_unimplemented<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
 /// if the program type checks or not -- and they are unusual
 /// occurrences in any case.
 pub fn report_overflow_error<'a, 'tcx, T>(infcx: &InferCtxt<'a, 'tcx>,
-                                          obligation: &Obligation<'tcx, T>)
+                                          obligation: &Obligation<'tcx, T>,
+                                          suggest_increasing_limit: bool)
                                           -> !
     where T: fmt::Display + TypeFoldable<'tcx>
 {
@@ -192,7 +193,9 @@ pub fn report_overflow_error<'a, 'tcx, T>(infcx: &InferCtxt<'a, 'tcx>,
                                    "overflow evaluating the requirement `{}`",
                                    predicate);
 
-    suggest_new_overflow_limit(infcx.tcx, &mut err, obligation.cause.span);
+    if suggest_increasing_limit {
+        suggest_new_overflow_limit(infcx.tcx, &mut err, obligation.cause.span);
+    }
 
     note_obligation_cause(infcx, &mut err, obligation);
 
@@ -201,6 +204,141 @@ pub fn report_overflow_error<'a, 'tcx, T>(infcx: &InferCtxt<'a, 'tcx>,
     unreachable!();
 }
 
+/// Reports that a cycle was detected which led to overflow and halts
+/// compilation. This is equivalent to `report_overflow_error` except
+/// that we can give a more helpful error message (and, in particular,
+/// we do not suggest increasing the overflow limit, which is not
+/// going to help).
+pub fn report_overflow_error_cycle<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
+                                             cycle: &Vec<PredicateObligation<'tcx>>)
+                                             -> !
+{
+    assert!(cycle.len() > 1);
+
+    debug!("report_overflow_error_cycle(cycle length = {})", cycle.len());
+
+    let cycle = infcx.resolve_type_vars_if_possible(cycle);
+
+    debug!("report_overflow_error_cycle: cycle={:?}", cycle);
+
+    assert_eq!(&cycle[0].predicate, &cycle.last().unwrap().predicate);
+
+    try_report_overflow_error_type_of_infinite_size(infcx, &cycle);
+    report_overflow_error(infcx, &cycle[0], false);
+}
+
+/// If a cycle results from evaluated whether something is Sized, that
+/// is a particular special case that always results from a struct or
+/// enum definition that lacks indirection (e.g., `struct Foo { x: Foo
+/// }`). We wish to report a targeted error for this case.
+pub fn try_report_overflow_error_type_of_infinite_size<'a, 'tcx>(
+    infcx: &InferCtxt<'a, 'tcx>,
+    cycle: &[PredicateObligation<'tcx>])
+{
+    let sized_trait = match infcx.tcx.lang_items.sized_trait() {
+        Some(v) => v,
+        None => return,
+    };
+    let top_is_sized = {
+        match cycle[0].predicate {
+            ty::Predicate::Trait(ref data) => data.def_id() == sized_trait,
+            _ => false,
+        }
+    };
+    if !top_is_sized {
+        return;
+    }
+
+    // The only way to have a type of infinite size is to have,
+    // somewhere, a struct/enum type involved. Identify all such types
+    // and report the cycle to the user.
+
+    let struct_enum_tys: Vec<_> =
+        cycle.iter()
+             .flat_map(|obligation| match obligation.predicate {
+                 ty::Predicate::Trait(ref data) => {
+                     assert_eq!(data.def_id(), sized_trait);
+                     let self_ty = data.skip_binder().trait_ref.self_ty(); // (*)
+                     // (*) ok to skip binder because this is just
+                     // error reporting and regions don't really
+                     // matter
+                     match self_ty.sty {
+                         ty::TyEnum(..) | ty::TyStruct(..) => Some(self_ty),
+                         _ => None,
+                     }
+                 }
+                 _ => {
+                     infcx.tcx.sess.span_bug(obligation.cause.span,
+                                             &format!("Sized cycle involving non-trait-ref: {:?}",
+                                                      obligation.predicate));
+                 }
+             })
+             .collect();
+
+    assert!(!struct_enum_tys.is_empty());
+
+    // This is a bit tricky. We want to pick a "main type" in the
+    // listing that is local to the current crate, so we can give a
+    // good span to the user. But it might not be the first one in our
+    // cycle list. So find the first one that is local and then
+    // rotate.
+    let (main_index, main_def_id) =
+        struct_enum_tys.iter()
+                       .enumerate()
+                       .filter_map(|(index, ty)| match ty.sty {
+                           ty::TyEnum(adt_def, _) | ty::TyStruct(adt_def, _) if adt_def.did.is_local() =>
+                               Some((index, adt_def.did)),
+                           _ =>
+                               None,
+                       })
+                       .next()
+                       .unwrap(); // should always be SOME local type involved!
+
+    // Rotate so that the "main" type is at index 0.
+    let struct_enum_tys: Vec<_> =
+        struct_enum_tys.iter()
+                       .cloned()
+                       .skip(main_index)
+                       .chain(struct_enum_tys.iter().cloned().take(main_index))
+                       .collect();
+
+    let tcx = infcx.tcx;
+    let mut err = recursive_type_with_infinite_size_error(tcx, main_def_id);
+    let len = struct_enum_tys.len();
+    if len > 2 {
+        let span = tcx.map.span_if_local(main_def_id).unwrap();
+        err.fileline_note(span,
+                          &format!("type `{}` is embedded within `{}`...",
+                                   struct_enum_tys[0],
+                                   struct_enum_tys[1]));
+        for &next_ty in &struct_enum_tys[1..len-1] {
+            err.fileline_note(span,
+                              &format!("...which in turn is embedded within `{}`...", next_ty));
+        }
+        err.fileline_note(span,
+                          &format!("...which in turn is embedded within `{}`, \
+                                    completing the cycle.",
+                                   struct_enum_tys[len-1]));
+    }
+    err.emit();
+    infcx.tcx.sess.abort_if_errors();
+    unreachable!();
+}
+
+pub fn recursive_type_with_infinite_size_error<'tcx>(tcx: &ty::ctxt<'tcx>,
+                                                     type_def_id: DefId)
+                                                     -> DiagnosticBuilder<'tcx>
+{
+    assert!(type_def_id.is_local());
+    let span = tcx.map.span_if_local(type_def_id).unwrap();
+    let mut err = struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size",
+                                   tcx.item_path_str(type_def_id));
+    err.fileline_help(span, &format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
+                                      at some point to make `{}` representable",
+                                     tcx.item_path_str(type_def_id)));
+    err
+}
+
 pub fn report_selection_error<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
                                         obligation: &PredicateObligation<'tcx>,
                                         error: &SelectionError<'tcx>)

src/librustc/middle/traits/fulfill.rs

@@ -11,10 +11,10 @@
 use middle::infer::InferCtxt;
 use middle::ty::{self, Ty, TypeFoldable};
 use rustc_data_structures::obligation_forest::{Backtrace, ObligationForest, Error};
-
+use std::iter;
 use syntax::ast;
 use util::common::ErrorReported;
-use util::nodemap::{FnvHashSet, NodeMap};
+use util::nodemap::{FnvHashMap, FnvHashSet, NodeMap};
 
 use super::CodeAmbiguity;
 use super::CodeProjectionError;
@@ -25,6 +25,7 @@ use super::FulfillmentErrorCode;
 use super::ObligationCause;
 use super::PredicateObligation;
 use super::project;
+use super::report_overflow_error_cycle;
 use super::select::SelectionContext;
 use super::Unimplemented;
 use super::util::predicate_for_builtin_bound;
@@ -357,6 +358,17 @@ fn process_predicate1<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
     }
 
     let obligation = &pending_obligation.obligation;
+
+    // If we exceed the recursion limit, take a moment to look for a
+    // cycle so we can give a better error report from here, where we
+    // have more context.
+    let recursion_limit = selcx.tcx().sess.recursion_limit.get();
+    if obligation.recursion_depth >= recursion_limit {
+        if let Some(cycle) = scan_for_cycle(obligation, &backtrace) {
+            report_overflow_error_cycle(selcx.infcx(), &cycle);
+        }
+    }
+
     match obligation.predicate {
         ty::Predicate::Trait(ref data) => {
             if coinductive_match(selcx, obligation, data, &backtrace) {
@@ -488,11 +500,15 @@ fn coinductive_match<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
                               -> bool
 {
     if selcx.tcx().trait_has_default_impl(top_data.def_id()) {
+        debug!("coinductive_match: top_data={:?}", top_data);
         for bt_obligation in backtrace.clone() {
+            debug!("coinductive_match: bt_obligation={:?}", bt_obligation);
+
             // *Everything* in the backtrace must be a defaulted trait.
             match bt_obligation.obligation.predicate {
                 ty::Predicate::Trait(ref data) => {
                     if !selcx.tcx().trait_has_default_impl(data.def_id()) {
+                        debug!("coinductive_match: trait does not have default impl");
                         break;
                     }
                 }
@@ -501,7 +517,7 @@ fn coinductive_match<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
 
             // And we must find a recursive match.
             if bt_obligation.obligation.predicate == top_obligation.predicate {
-                debug!("process_predicate: found a match in the backtrace");
+                debug!("coinductive_match: found a match in the backtrace");
                 return true;
             }
         }
@@ -510,6 +526,27 @@ fn coinductive_match<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
     false
 }
 
+fn scan_for_cycle<'a,'tcx>(top_obligation: &PredicateObligation<'tcx>,
+                           backtrace: &Backtrace<PendingPredicateObligation<'tcx>>)
+                           -> Option<Vec<PredicateObligation<'tcx>>>
+{
+    let mut map = FnvHashMap();
+    let all_obligations =
+        || iter::once(top_obligation)
+               .chain(backtrace.clone()
+                               .map(|p| &p.obligation));
+    for (index, bt_obligation) in all_obligations().enumerate() {
+        if let Some(&start) = map.get(&bt_obligation.predicate) {
+            // Found a cycle starting at position `start` and running
+            // until the current position (`index`).
+            return Some(all_obligations().skip(start).take(index - start + 1).cloned().collect());
+        } else {
+            map.insert(bt_obligation.predicate.clone(), index);
+        }
+    }
+    None
+}
+
 fn register_region_obligation<'tcx>(t_a: Ty<'tcx>,
                                     r_b: ty::Region,
                                     cause: ObligationCause<'tcx>,

src/librustc/middle/traits/mod.rs

@@ -28,8 +28,10 @@ use syntax::ast;
 use syntax::codemap::{Span, DUMMY_SP};
 
 pub use self::error_reporting::TraitErrorKey;
+pub use self::error_reporting::recursive_type_with_infinite_size_error;
 pub use self::error_reporting::report_fulfillment_errors;
 pub use self::error_reporting::report_overflow_error;
+pub use self::error_reporting::report_overflow_error_cycle;
 pub use self::error_reporting::report_selection_error;
 pub use self::error_reporting::report_object_safety_error;
 pub use self::coherence::orphan_check;

src/librustc/middle/traits/project.rs

@@ -479,7 +479,7 @@ fn project_type<'cx,'tcx>(
     let recursion_limit = selcx.tcx().sess.recursion_limit.get();
     if obligation.recursion_depth >= recursion_limit {
         debug!("project: overflow!");
-        report_overflow_error(selcx.infcx(), &obligation);
+        report_overflow_error(selcx.infcx(), &obligation, true);
     }
 
     let obligation_trait_ref =

src/librustc/middle/traits/select.rs

@@ -711,7 +711,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // not update) the cache.
         let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
         if stack.obligation.recursion_depth >= recursion_limit {
-            report_overflow_error(self.infcx(), &stack.obligation);
+            report_overflow_error(self.infcx(), &stack.obligation, true);
         }
 
         // Check the cache. Note that we skolemize the trait-ref
@@ -2124,6 +2124,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                            nested: ty::Binder<Vec<Ty<'tcx>>>)
                            -> VtableBuiltinData<PredicateObligation<'tcx>>
     {
+        debug!("vtable_builtin_data(obligation={:?}, bound={:?}, nested={:?})",
+               obligation, bound, nested);
+
         let trait_def = match self.tcx().lang_items.from_builtin_kind(bound) {
             Ok(def_id) => def_id,
             Err(_) => {

src/librustc_typeck/check/mod.rs

@@ -4132,7 +4132,7 @@ fn check_const_with_ty<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 pub fn check_representable(tcx: &ty::ctxt,
                            sp: Span,
                            item_id: ast::NodeId,
-                           designation: &str) -> bool {
+                           _designation: &str) -> bool {
     let rty = tcx.node_id_to_type(item_id);
 
     // Check that it is possible to represent this type. This call identifies
@@ -4142,9 +4142,7 @@ pub fn check_representable(tcx: &ty::ctxt,
     // caught by case 1.
     match rty.is_representable(tcx, sp) {
         Representability::SelfRecursive => {
-            struct_span_err!(tcx.sess, sp, E0072, "invalid recursive {} type", designation)
-                .fileline_help(sp, "wrap the inner value in a box to make it representable")
-                .emit();
+            traits::recursive_type_with_infinite_size_error(tcx, tcx.map.local_def_id(item_id)).emit();
             return false
         }
         Representability::Representable | Representability::ContainsRecursive => (),