rust/src/librustc/traits/error_reporting.rs

// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use super::{
    FulfillmentError,
    FulfillmentErrorCode,
    MismatchedProjectionTypes,
    Obligation,
    ObligationCause,
    ObligationCauseCode,
    OutputTypeParameterMismatch,
    TraitNotObjectSafe,
    PredicateObligation,
    SelectionContext,
    SelectionError,
    ObjectSafetyViolation,
    MethodViolationCode,
};

use fmt_macros::{Parser, Piece, Position};
use hir::def_id::DefId;
use infer::InferCtxt;
use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
use ty::fast_reject;
use ty::fold::{TypeFoldable, TypeFolder};
use util::nodemap::{FnvHashMap, FnvHashSet};

use std::cmp;
use std::fmt;
use syntax::attr::{AttributeMethods, AttrMetaMethods};
use syntax::ast;
use syntax::codemap::Span;
use syntax::errors::DiagnosticBuilder;

#[derive(Debug, PartialEq, Eq, Hash)]
pub struct TraitErrorKey<'tcx> {
    span: Span,
    warning_node_id: Option<ast::NodeId>,
    predicate: ty::Predicate<'tcx>
}

impl<'tcx> TraitErrorKey<'tcx> {
    fn from_error<'a>(infcx: &InferCtxt<'a, 'tcx, 'tcx>,
                      e: &FulfillmentError<'tcx>,
                      warning_node_id: Option<ast::NodeId>) -> Self {
        let predicate =
            infcx.resolve_type_vars_if_possible(&e.obligation.predicate);
        TraitErrorKey {
            span: e.obligation.cause.span,
            predicate: infcx.tcx.erase_regions(&predicate),
            warning_node_id: warning_node_id
        }
    }
}

impl<'a, 'tcx> InferCtxt<'a, 'tcx, 'tcx> {
pub fn report_fulfillment_errors(&self, errors: &Vec<FulfillmentError<'tcx>>) {
    for error in errors {
        self.report_fulfillment_error(error, None);
    }
}

pub fn report_fulfillment_errors_as_warnings(&self,
                                             errors: &Vec<FulfillmentError<'tcx>>,
                                             node_id: ast::NodeId) {
    for error in errors {
        self.report_fulfillment_error(error, Some(node_id));
    }
}

fn report_fulfillment_error(&self,
                            error: &FulfillmentError<'tcx>,
                            warning_node_id: Option<ast::NodeId>) {
    let error_key = TraitErrorKey::from_error(self, error, warning_node_id);
    debug!("report_fulfillment_errors({:?}) - key={:?}",
           error, error_key);
    if !self.reported_trait_errors.borrow_mut().insert(error_key) {
        debug!("report_fulfillment_errors: skipping duplicate");
        return;
    }
    match error.code {
        FulfillmentErrorCode::CodeSelectionError(ref e) => {
            self.report_selection_error(&error.obligation, e, warning_node_id);
        }
        FulfillmentErrorCode::CodeProjectionError(ref e) => {
            self.report_projection_error(&error.obligation, e, warning_node_id);
        }
        FulfillmentErrorCode::CodeAmbiguity => {
            self.maybe_report_ambiguity(&error.obligation);
        }
    }
}

fn report_projection_error(&self,
                           obligation: &PredicateObligation<'tcx>,
                           error: &MismatchedProjectionTypes<'tcx>,
                           warning_node_id: Option<ast::NodeId>)
{
    let predicate =
        self.resolve_type_vars_if_possible(&obligation.predicate);

    if !predicate.references_error() {
        if let Some(warning_node_id) = warning_node_id {
            self.tcx.sess.add_lint(
                ::lint::builtin::UNSIZED_IN_TUPLE,
                warning_node_id,
                obligation.cause.span,
                format!("type mismatch resolving `{}`: {}",
                        predicate,
                        error.err));
        } else {
            let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271,
                                           "type mismatch resolving `{}`: {}",
                                           predicate,
                                           error.err);
            self.note_obligation_cause(&mut err, obligation);
            err.emit();
        }
    }
}

fn on_unimplemented_note(&self,
                         trait_ref: ty::PolyTraitRef<'tcx>,
                         span: Span) -> Option<String> {
    let trait_ref = trait_ref.skip_binder();
    let def_id = trait_ref.def_id;
    let mut report = None;
    for item in self.tcx.get_attrs(def_id).iter() {
        if item.check_name("rustc_on_unimplemented") {
            let err_sp = item.meta().span.substitute_dummy(span);
            let def = self.tcx.lookup_trait_def(def_id);
            let trait_str = def.trait_ref.to_string();
            if let Some(ref istring) = item.value_str() {
                let mut generic_map = def.generics.types.iter_enumerated()
                                         .map(|(param, i, gen)| {
                                               (gen.name.as_str().to_string(),
                                                trait_ref.substs.types.get(param, i)
                                                         .to_string())
                                              }).collect::<FnvHashMap<String, String>>();
                generic_map.insert("Self".to_string(),
                                   trait_ref.self_ty().to_string());
                let parser = Parser::new(&istring);
                let mut errored = false;
                let err: String = parser.filter_map(|p| {
                    match p {
                        Piece::String(s) => Some(s),
                        Piece::NextArgument(a) => match a.position {
                            Position::ArgumentNamed(s) => match generic_map.get(s) {
                                Some(val) => Some(val),
                                None => {
                                    span_err!(self.tcx.sess, err_sp, E0272,
                                                   "the #[rustc_on_unimplemented] \
                                                            attribute on \
                                                            trait definition for {} refers to \
                                                            non-existent type parameter {}",
                                                           trait_str, s);
                                    errored = true;
                                    None
                                }
                            },
                            _ => {
                                     span_err!(self.tcx.sess, err_sp, E0273,
                                               "the #[rustc_on_unimplemented] \
                                                        attribute on \
                                                        trait definition for {} must have named \
                                                        format arguments, \
                                                        eg `#[rustc_on_unimplemented = \
                                                        \"foo {{T}}\"]`",
                                                       trait_str);
                                errored = true;
                                None
                            }
                        }
                    }
                }).collect();
                // Report only if the format string checks out
                if !errored {
                    report = Some(err);
                }
            } else {
                span_err!(self.tcx.sess, err_sp, E0274,
                                        "the #[rustc_on_unimplemented] attribute on \
                                                 trait definition for {} must have a value, \
                                                 eg `#[rustc_on_unimplemented = \"foo\"]`",
                                                 trait_str);
            }
            break;
        }
    }
    report
}

fn report_similar_impl_candidates(&self,
                                  trait_ref: ty::PolyTraitRef<'tcx>,
                                  err: &mut DiagnosticBuilder)
{
    let simp = fast_reject::simplify_type(self.tcx,
                                          trait_ref.skip_binder().self_ty(),
                                          true);
    let mut impl_candidates = Vec::new();
    let trait_def = self.tcx.lookup_trait_def(trait_ref.def_id());

    match simp {
        Some(simp) => trait_def.for_each_impl(self.tcx, |def_id| {
            let imp = self.tcx.impl_trait_ref(def_id).unwrap();
            let imp_simp = fast_reject::simplify_type(self.tcx,
                                                      imp.self_ty(),
                                                      true);
            if let Some(imp_simp) = imp_simp {
                if simp != imp_simp {
                    return;
                }
            }
            impl_candidates.push(imp);
        }),
        None => trait_def.for_each_impl(self.tcx, |def_id| {
            impl_candidates.push(
                self.tcx.impl_trait_ref(def_id).unwrap());
        })
    };

    if impl_candidates.is_empty() {
        return;
    }

    err.help(&format!("the following implementations were found:"));

    let end = cmp::min(4, impl_candidates.len());
    for candidate in &impl_candidates[0..end] {
        err.help(&format!("  {:?}", candidate));
    }
    if impl_candidates.len() > 4 {
        err.help(&format!("and {} others", impl_candidates.len()-4));
    }
}

/// Reports that an overflow has occurred and halts compilation. We
/// halt compilation unconditionally because it is important that
/// overflows never be masked -- they basically represent computations
/// whose result could not be truly determined and thus we can't say
/// if the program type checks or not -- and they are unusual
/// occurrences in any case.
pub fn report_overflow_error<T>(&self,
                                obligation: &Obligation<'tcx, T>,
                                suggest_increasing_limit: bool) -> !
    where T: fmt::Display + TypeFoldable<'tcx>
{
    let predicate =
        self.resolve_type_vars_if_possible(&obligation.predicate);
    let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
                                   "overflow evaluating the requirement `{}`",
                                   predicate);

    if suggest_increasing_limit {
        self.suggest_new_overflow_limit(&mut err);
    }

    self.note_obligation_cause(&mut err, obligation);

    err.emit();
    self.tcx.sess.abort_if_errors();
    bug!();
}

/// 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(&self, cycle: &Vec<PredicateObligation<'tcx>>) -> ! {
    assert!(cycle.len() > 1);

    debug!("report_overflow_error_cycle(cycle length = {})", cycle.len());

    let cycle = self.resolve_type_vars_if_possible(cycle);

    debug!("report_overflow_error_cycle: cycle={:?}", cycle);

    assert_eq!(&cycle[0].predicate, &cycle.last().unwrap().predicate);

    self.try_report_overflow_error_type_of_infinite_size(&cycle);
    self.report_overflow_error(&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(&self,
    cycle: &[PredicateObligation<'tcx>])
{
    let sized_trait = match self.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,
                     }
                 }
                 _ => {
                     span_bug!(obligation.cause.span,
                               "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 = self.tcx;
    let mut err = tcx.recursive_type_with_infinite_size_error(main_def_id);
    let len = struct_enum_tys.len();
    if len > 2 {
        err.note(&format!("type `{}` is embedded within `{}`...",
                 struct_enum_tys[0],
                 struct_enum_tys[1]));
        for &next_ty in &struct_enum_tys[1..len-1] {
            err.note(&format!("...which in turn is embedded within `{}`...", next_ty));
        }
        err.note(&format!("...which in turn is embedded within `{}`, \
                           completing the cycle.",
                          struct_enum_tys[len-1]));
    }
    err.emit();
    self.tcx.sess.abort_if_errors();
    bug!();
}

pub fn report_selection_error(&self,
                              obligation: &PredicateObligation<'tcx>,
                              error: &SelectionError<'tcx>,
                              warning_node_id: Option<ast::NodeId>)
{
    let span = obligation.cause.span;
    let mut err = match *error {
        SelectionError::Unimplemented => {
            if let ObligationCauseCode::CompareImplMethodObligation = obligation.cause.code {
                span_err!(
                    self.tcx.sess, span, E0276,
                    "the requirement `{}` appears on the impl \
                     method but not on the corresponding trait method",
                    obligation.predicate);
                return;
            } else {
                match obligation.predicate {
                    ty::Predicate::Trait(ref trait_predicate) => {
                        let trait_predicate =
                            self.resolve_type_vars_if_possible(trait_predicate);

                        if self.tcx.sess.has_errors() && trait_predicate.references_error() {
                            return;
                        } else {
                            let trait_ref = trait_predicate.to_poly_trait_ref();

                            if let Some(warning_node_id) = warning_node_id {
                                self.tcx.sess.add_lint(
                                    ::lint::builtin::UNSIZED_IN_TUPLE,
                                    warning_node_id,
                                    obligation.cause.span,
                                    format!("the trait bound `{}` is not satisfied",
                                            trait_ref.to_predicate()));
                                return;
                            }

                            let mut err = struct_span_err!(
                                self.tcx.sess, span, E0277,
                                "the trait bound `{}` is not satisfied",
                                trait_ref.to_predicate());

                            // Try to report a help message

                            if !trait_ref.has_infer_types() &&
                                self.predicate_can_apply(trait_ref)
                            {
                                // If a where-clause may be useful, remind the
                                // user that they can add it.
                                //
                                // don't display an on-unimplemented note, as
                                // these notes will often be of the form
                                //     "the type `T` can't be frobnicated"
                                // which is somewhat confusing.
                                err.help(&format!("consider adding a `where {}` bound",
                                    trait_ref.to_predicate()
                                    ));
                            } else if let Some(s) = self.on_unimplemented_note(trait_ref, span) {
                                // Otherwise, if there is an on-unimplemented note,
                                // display it.
                                err.note(&s);
                            } else {
                                // If we can't show anything useful, try to find
                                // similar impls.

                                self.report_similar_impl_candidates(trait_ref, &mut err);
                            }
                            err
                        }
                    },
                    ty::Predicate::Equate(ref predicate) => {
                        let predicate = self.resolve_type_vars_if_possible(predicate);
                        let err = self.equality_predicate(span,
                                                          &predicate).err().unwrap();
                        struct_span_err!(self.tcx.sess, span, E0278,
                            "the requirement `{}` is not satisfied (`{}`)",
                            predicate, err)
                    }

                    ty::Predicate::RegionOutlives(ref predicate) => {
                        let predicate = self.resolve_type_vars_if_possible(predicate);
                        let err = self.region_outlives_predicate(span,
                                                                 &predicate).err().unwrap();
                        struct_span_err!(self.tcx.sess, span, E0279,
                            "the requirement `{}` is not satisfied (`{}`)",
                            predicate, err)
                    }

                    ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
                        let predicate =
                            self.resolve_type_vars_if_possible(&obligation.predicate);
                        struct_span_err!(self.tcx.sess, span, E0280,
                            "the requirement `{}` is not satisfied",
                            predicate)
                    }

                    ty::Predicate::ObjectSafe(trait_def_id) => {
                        let violations = self.tcx.object_safety_violations(trait_def_id);
                        let err = self.tcx.report_object_safety_error(span,
                                                                      trait_def_id,
                                                                      warning_node_id,
                                                                      violations);
                        if let Some(err) = err {
                            err
                        } else {
                            return;
                        }
                    }

                    ty::Predicate::ClosureKind(closure_def_id, kind) => {
                        let found_kind = self.closure_kind(closure_def_id).unwrap();
                        let closure_span = self.tcx.map.span_if_local(closure_def_id).unwrap();
                        let mut err = struct_span_err!(
                            self.tcx.sess, closure_span, E0525,
                            "expected a closure that implements the `{}` trait, but this closure \
                             only implements `{}`",
                            kind,
                            found_kind);
                        err.span_note(
                            obligation.cause.span,
                            &format!("the requirement to implement `{}` derives from here", kind));
                        err.emit();
                        return;
                    }

                    ty::Predicate::WellFormed(ty) => {
                        // WF predicates cannot themselves make
                        // errors. They can only block due to
                        // ambiguity; otherwise, they always
                        // degenerate into other obligations
                        // (which may fail).
                        span_bug!(span, "WF predicate not satisfied for {:?}", ty);
                    }

                    ty::Predicate::Rfc1592(ref data) => {
                        span_bug!(
                            obligation.cause.span,
                            "RFC1592 predicate not satisfied for {:?}",
                            data);
                    }
                }
            }
        }

        OutputTypeParameterMismatch(ref expected_trait_ref, ref actual_trait_ref, ref e) => {
            let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
            let actual_trait_ref = self.resolve_type_vars_if_possible(&*actual_trait_ref);
            if actual_trait_ref.self_ty().references_error() {
                return;
            }
            struct_span_err!(self.tcx.sess, span, E0281,
                "type mismatch: the type `{}` implements the trait `{}`, \
                 but the trait `{}` is required ({})",
                expected_trait_ref.self_ty(),
                expected_trait_ref,
                actual_trait_ref,
                e)
        }

        TraitNotObjectSafe(did) => {
            let violations = self.tcx.object_safety_violations(did);
            let err = self.tcx.report_object_safety_error(span, did,
                                                          warning_node_id,
                                                          violations);
            if let Some(err) = err {
                err
            } else {
                return;
            }
        }
    };
    self.note_obligation_cause(&mut err, obligation);
    err.emit();
}
}

impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
pub fn recursive_type_with_infinite_size_error(self,
                                               type_def_id: DefId)
                                               -> DiagnosticBuilder<'tcx>
{
    assert!(type_def_id.is_local());
    let span = self.map.span_if_local(type_def_id).unwrap();
    let mut err = struct_span_err!(self.sess, span, E0072, "recursive type `{}` has infinite size",
                                   self.item_path_str(type_def_id));
    err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
                       at some point to make `{}` representable",
                      self.item_path_str(type_def_id)));
    err
}

pub fn report_object_safety_error(self,
                                  span: Span,
                                  trait_def_id: DefId,
                                  warning_node_id: Option<ast::NodeId>,
                                  violations: Vec<ObjectSafetyViolation>)
                                  -> Option<DiagnosticBuilder<'tcx>>
{
    let mut err = match warning_node_id {
        Some(_) => None,
        None => {
            Some(struct_span_err!(
                self.sess, span, E0038,
                "the trait `{}` cannot be made into an object",
                self.item_path_str(trait_def_id)))
        }
    };

    let mut reported_violations = FnvHashSet();
    for violation in violations {
        if !reported_violations.insert(violation.clone()) {
            continue;
        }
        let buf;
        let note = match violation {
            ObjectSafetyViolation::SizedSelf => {
                "the trait cannot require that `Self : Sized`"
            }

            ObjectSafetyViolation::SupertraitSelf => {
                "the trait cannot use `Self` as a type parameter \
                     in the supertrait listing"
            }

            ObjectSafetyViolation::Method(method,
                                          MethodViolationCode::StaticMethod) => {
                buf = format!("method `{}` has no receiver",
                              method.name);
                &buf
            }

            ObjectSafetyViolation::Method(method,
                                          MethodViolationCode::ReferencesSelf) => {
                buf = format!("method `{}` references the `Self` type \
                                   in its arguments or return type",
                              method.name);
                &buf
            }

            ObjectSafetyViolation::Method(method,
                                          MethodViolationCode::Generic) => {
                buf = format!("method `{}` has generic type parameters",
                              method.name);
                &buf
            }
        };
        match (warning_node_id, &mut err) {
            (Some(node_id), &mut None) => {
                self.sess.add_lint(
                    ::lint::builtin::OBJECT_UNSAFE_FRAGMENT,
                    node_id,
                    span,
                    note.to_string());
            }
            (None, &mut Some(ref mut err)) => {
                err.note(note);
            }
            _ => unreachable!()
        }
    }
    err
}
}

impl<'a, 'tcx> InferCtxt<'a, 'tcx, 'tcx> {
fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>) {
    // Unable to successfully determine, probably means
    // insufficient type information, but could mean
    // ambiguous impls. The latter *ought* to be a
    // coherence violation, so we don't report it here.

    let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);

    debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
           predicate,
           obligation);

    // Ambiguity errors are often caused as fallout from earlier
    // errors. So just ignore them if this infcx is tainted.
    if self.is_tainted_by_errors() {
        return;
    }

    match predicate {
        ty::Predicate::Trait(ref data) => {
            let trait_ref = data.to_poly_trait_ref();
            let self_ty = trait_ref.self_ty();
            let all_types = &trait_ref.substs().types;
            if all_types.references_error() {
            } else {
                // Typically, this ambiguity should only happen if
                // there are unresolved type inference variables
                // (otherwise it would suggest a coherence
                // failure). But given #21974 that is not necessarily
                // the case -- we can have multiple where clauses that
                // are only distinguished by a region, which results
                // in an ambiguity even when all types are fully
                // known, since we don't dispatch based on region
                // relationships.

                // This is kind of a hack: it frequently happens that some earlier
                // error prevents types from being fully inferred, and then we get
                // a bunch of uninteresting errors saying something like "<generic
                // #0> doesn't implement Sized".  It may even be true that we
                // could just skip over all checks where the self-ty is an
                // inference variable, but I was afraid that there might be an
                // inference variable created, registered as an obligation, and
                // then never forced by writeback, and hence by skipping here we'd
                // be ignoring the fact that we don't KNOW the type works
                // out. Though even that would probably be harmless, given that
                // we're only talking about builtin traits, which are known to be
                // inhabited. But in any case I just threw in this check for
                // has_errors() to be sure that compilation isn't happening
                // anyway. In that case, why inundate the user.
                if !self.tcx.sess.has_errors() {
                    if
                        self.tcx.lang_items.sized_trait()
                        .map_or(false, |sized_id| sized_id == trait_ref.def_id())
                    {
                        self.need_type_info(obligation.cause.span, self_ty);
                    } else {
                        let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0283,
                                                       "type annotations required: \
                                                        cannot resolve `{}`",
                                                       predicate);
                        self.note_obligation_cause(&mut err, obligation);
                        err.emit();
                    }
                }
            }
        }

        ty::Predicate::WellFormed(ty) => {
            // Same hacky approach as above to avoid deluging user
            // with error messages.
            if !ty.references_error() && !self.tcx.sess.has_errors() {
                self.need_type_info(obligation.cause.span, ty);
            }
        }

        _ => {
            if !self.tcx.sess.has_errors() {
                let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0284,
                                               "type annotations required: cannot resolve `{}`",
                                               predicate);
                self.note_obligation_cause(&mut err, obligation);
                err.emit();
            }
        }
    }
}

/// Returns whether the trait predicate may apply for *some* assignment
/// to the type parameters.
fn predicate_can_apply(&self, pred: ty::PolyTraitRef<'tcx>) -> bool {
    struct ParamToVarFolder<'a, 'tcx: 'a> {
        infcx: &'a InferCtxt<'a, 'tcx, 'tcx>,
        var_map: FnvHashMap<Ty<'tcx>, Ty<'tcx>>
    }

    impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx>
    {
        fn tcx<'b>(&'b self) -> TyCtxt<'b, 'tcx, 'tcx> { self.infcx.tcx }

        fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
            if let ty::TyParam(..) = ty.sty {
                let infcx = self.infcx;
                self.var_map.entry(ty).or_insert_with(|| infcx.next_ty_var())
            } else {
                ty.super_fold_with(self)
            }
        }
    }

    self.probe(|_| {
        let mut selcx = SelectionContext::new(self);

        let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
            infcx: self,
            var_map: FnvHashMap()
        });

        let cleaned_pred = super::project::normalize(
            &mut selcx,
            ObligationCause::dummy(),
            &cleaned_pred
        ).value;

        let obligation = Obligation::new(
            ObligationCause::dummy(),
            cleaned_pred.to_predicate()
        );

        selcx.evaluate_obligation(&obligation)
    })
}


fn need_type_info(&self, span: Span, ty: Ty<'tcx>) {
    span_err!(self.tcx.sess, span, E0282,
              "unable to infer enough type information about `{}`; \
               type annotations or generic parameter binding required",
              ty);
}

fn note_obligation_cause<T>(&self,
                            err: &mut DiagnosticBuilder,
                            obligation: &Obligation<'tcx, T>)
    where T: fmt::Display
{
    self.note_obligation_cause_code(err,
                                    &obligation.predicate,
                                    &obligation.cause.code);
}

fn note_obligation_cause_code<T>(&self,
                                 err: &mut DiagnosticBuilder,
                                 predicate: &T,
                                 cause_code: &ObligationCauseCode<'tcx>)
    where T: fmt::Display
{
    let tcx = self.tcx;
    match *cause_code {
        ObligationCauseCode::MiscObligation => { }
        ObligationCauseCode::SliceOrArrayElem => {
            err.note("slice and array elements must have `Sized` type");
        }
        ObligationCauseCode::TupleElem => {
            err.note("tuple elements must have `Sized` type");
        }
        ObligationCauseCode::ProjectionWf(data) => {
            err.note(&format!("required so that the projection `{}` is well-formed",
                              data));
        }
        ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
            err.note(&format!("required so that reference `{}` does not outlive its referent",
                              ref_ty));
        }
        ObligationCauseCode::ItemObligation(item_def_id) => {
            let item_name = tcx.item_path_str(item_def_id);
            err.note(&format!("required by `{}`", item_name));
        }
        ObligationCauseCode::ObjectCastObligation(object_ty) => {
            err.note(&format!("required for the cast to the object type `{}`",
                              self.ty_to_string(object_ty)));
        }
        ObligationCauseCode::RepeatVec => {
            err.note("the `Copy` trait is required because the \
                      repeated element will be copied");
        }
        ObligationCauseCode::VariableType(_) => {
            err.note("all local variables must have a statically known size");
        }
        ObligationCauseCode::ReturnType => {
            err.note("the return type of a function must have a \
                      statically known size");
        }
        ObligationCauseCode::AssignmentLhsSized => {
            err.note("the left-hand-side of an assignment must have a statically known size");
        }
        ObligationCauseCode::StructInitializerSized => {
            err.note("structs must have a statically known size to be initialized");
        }
        ObligationCauseCode::ClosureCapture(var_id, _, builtin_bound) => {
            let def_id = tcx.lang_items.from_builtin_kind(builtin_bound).unwrap();
            let trait_name = tcx.item_path_str(def_id);
            let name = tcx.local_var_name_str(var_id);
            err.note(
                &format!("the closure that captures `{}` requires that all captured variables \
                          implement the trait `{}`",
                         name,
                         trait_name));
        }
        ObligationCauseCode::FieldSized => {
            err.note("only the last field of a struct or enum variant \
                      may have a dynamically sized type");
        }
        ObligationCauseCode::SharedStatic => {
            err.note("shared static variables must have a type that implements `Sync`");
        }
        ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
            let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
            err.note(&format!("required because it appears within the type `{}`",
                              parent_trait_ref.0.self_ty()));
            let parent_predicate = parent_trait_ref.to_predicate();
            self.note_obligation_cause_code(err,
                                            &parent_predicate,
                                            &data.parent_code);
        }
        ObligationCauseCode::ImplDerivedObligation(ref data) => {
            let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
            err.note(
                &format!("required because of the requirements on the impl of `{}` for `{}`",
                         parent_trait_ref,
                         parent_trait_ref.0.self_ty()));
            let parent_predicate = parent_trait_ref.to_predicate();
            self.note_obligation_cause_code(err,
                                            &parent_predicate,
                                            &data.parent_code);
        }
        ObligationCauseCode::CompareImplMethodObligation => {
            err.note(
                &format!("the requirement `{}` appears on the impl method \
                          but not on the corresponding trait method",
                         predicate));
        }
    }
}

fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
    let current_limit = self.tcx.sess.recursion_limit.get();
    let suggested_limit = current_limit * 2;
    err.note(&format!(
                      "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
                      suggested_limit));
}
}