error_reporting.rs

// Copyright 2012-2013 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.

/*!

Error Reporting Code for the inference engine

Because of the way inference, and in particular region inference,
works, it often happens that errors are not detected until far after
the relevant line of code has been type-checked. Therefore, there is
an elaborate system to track why a particular constraint in the
inference graph arose so that we can explain to the user what gave
rise to a patricular error.

The basis of the system are the "origin" types. An "origin" is the
reason that a constraint or inference variable arose. There are
different "origin" enums for different kinds of constraints/variables
(e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
a span, but also more information so that we can generate a meaningful
error message.

Having a catalogue of all the different reasons an error can arise is
also useful for other reasons, like cross-referencing FAQs etc, though
we are not really taking advantage of this yet.

# Region Inference

Region inference is particularly tricky because it always succeeds "in
the moment" and simply registers a constraint. Then, at the end, we
can compute the full graph and report errors, so we need to be able to
store and later report what gave rise to the conflicting constraints.

# Subtype Trace

Determing whether `T1 <: T2` often involves a number of subtypes and
subconstraints along the way. A "TypeTrace" is an extended version
of an origin that traces the types and other values that were being
compared. It is not necessarily comprehensive (in fact, at the time of
this writing it only tracks the root values being compared) but I'd
like to extend it to include significant "waypoints". For example, if
you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
<: T4` fails, I'd like the trace to include enough information to say
"in the 2nd element of the tuple". Similarly, failures when comparing
arguments or return types in fn types should be able to cite the
specific position, etc.

# Reality vs plan

Of course, there is still a LOT of code in typeck that has yet to be
ported to this system, and which relies on string concatenation at the
time of error detection.

*/

use collections::HashSet;
use middle::ty;
use middle::ty::{Region, ReFree};
use middle::typeck::infer;
use middle::typeck::infer::InferCtxt;
use middle::typeck::infer::TypeTrace;
use middle::typeck::infer::SubregionOrigin;
use middle::typeck::infer::RegionVariableOrigin;
use middle::typeck::infer::ValuePairs;
use middle::typeck::infer::region_inference::RegionResolutionError;
use middle::typeck::infer::region_inference::ConcreteFailure;
use middle::typeck::infer::region_inference::SubSupConflict;
use middle::typeck::infer::region_inference::SupSupConflict;
use middle::typeck::infer::region_inference::ProcessedErrors;
use middle::typeck::infer::region_inference::SameRegions;
use std::cell::{Cell, RefCell};
use std::char::from_u32;
use syntax::ast;
use syntax::ast_map;
use syntax::ast_util;
use syntax::ast_util::name_to_dummy_lifetime;
use syntax::parse::token;
use syntax::print::pprust;
use util::ppaux::UserString;
use util::ppaux::bound_region_to_str;
use util::ppaux::note_and_explain_region;

pub trait ErrorReporting {
    fn report_region_errors(&self,
                            errors: &Vec<RegionResolutionError>);

    fn process_errors(&self, errors: &Vec<RegionResolutionError>)
                      -> Vec<RegionResolutionError>;

    fn report_type_error(&self, trace: TypeTrace, terr: &ty::type_err);

    fn report_and_explain_type_error(&self,
                                     trace: TypeTrace,
                                     terr: &ty::type_err);

    fn values_str(&self, values: &ValuePairs) -> Option<~str>;

    fn expected_found_str<T:UserString+Resolvable>(
        &self,
        exp_found: &ty::expected_found<T>)
        -> Option<~str>;

    fn report_concrete_failure(&self,
                               origin: SubregionOrigin,
                               sub: Region,
                               sup: Region);

    fn report_sub_sup_conflict(&self,
                               var_origin: RegionVariableOrigin,
                               sub_origin: SubregionOrigin,
                               sub_region: Region,
                               sup_origin: SubregionOrigin,
                               sup_region: Region);

    fn report_sup_sup_conflict(&self,
                               var_origin: RegionVariableOrigin,
                               origin1: SubregionOrigin,
                               region1: Region,
                               origin2: SubregionOrigin,
                               region2: Region);

    fn report_processed_errors(&self,
                               var_origin: &[RegionVariableOrigin],
                               trace_origin: &[(TypeTrace, ty::type_err)],
                               same_regions: &[SameRegions]);

    fn give_suggestion(&self, same_regions: &[SameRegions]);
}

trait ErrorReportingHelpers {
    fn report_inference_failure(&self,
                                var_origin: RegionVariableOrigin);

    fn note_region_origin(&self,
                          origin: SubregionOrigin);

    fn give_expl_lifetime_param(&self,
                                inputs: Vec<ast::Arg>,
                                output: ast::P<ast::Ty>,
                                item: ast::P<ast::Item>,
                                generics: ast::Generics);
}

impl<'a> ErrorReporting for InferCtxt<'a> {
    fn report_region_errors(&self,
                            errors: &Vec<RegionResolutionError>) {
        let p_errors = self.process_errors(errors);
        let errors = if p_errors.is_empty() { errors } else { &p_errors };
        for error in errors.iter() {
            match *error {
                ConcreteFailure(origin, sub, sup) => {
                    self.report_concrete_failure(origin, sub, sup);
                }

                SubSupConflict(var_origin,
                               sub_origin, sub_r,
                               sup_origin, sup_r) => {
                    self.report_sub_sup_conflict(var_origin,
                                                 sub_origin, sub_r,
                                                 sup_origin, sup_r);
                }

                SupSupConflict(var_origin,
                               origin1, r1,
                               origin2, r2) => {
                    self.report_sup_sup_conflict(var_origin,
                                                 origin1, r1,
                                                 origin2, r2);
                }

                ProcessedErrors(ref var_origins,
                                ref trace_origins,
                                ref same_regions) => {
                    if !same_regions.is_empty() {
                        self.report_processed_errors(var_origins.as_slice(),
                                                     trace_origins.as_slice(),
                                                     same_regions.as_slice());
                    }
                }
            }
        }
    }

    // This method goes through all the errors and try to group certain types
    // of error together, for the purpose of suggesting explicit lifetime
    // parameters to the user. This is done so that we can have a more
    // complete view of what lifetimes should be the same.
    // If the return value is an empty vector, it means that processing
    // failed (so the return value of this method should not be used)
    fn process_errors(&self, errors: &Vec<RegionResolutionError>)
                      -> Vec<RegionResolutionError> {
        debug!("process_errors()");
        let mut var_origins = Vec::new();
        let mut trace_origins = Vec::new();
        let mut same_regions = Vec::new();
        let mut processed_errors = Vec::new();
        for error in errors.iter() {
            match *error {
                ConcreteFailure(origin, sub, sup) => {
                    debug!("processing ConcreteFailure")
                    let trace = match origin {
                        infer::Subtype(trace) => Some(trace),
                        _ => None,
                    };
                    match free_regions_from_same_fn(self.tcx, sub, sup) {
                        Some(ref same_frs) if trace.is_some() => {
                            let trace = trace.unwrap();
                            let terr = ty::terr_regions_does_not_outlive(sup,
                                                                         sub);
                            trace_origins.push((trace, terr));
                            append_to_same_regions(&mut same_regions, same_frs);
                        }
                        _ => processed_errors.push((*error).clone()),
                    }
                }
                SubSupConflict(var_origin, _, sub_r, _, sup_r) => {
                    debug!("processing SubSupConflict")
                    match free_regions_from_same_fn(self.tcx, sub_r, sup_r) {
                        Some(ref same_frs) => {
                            var_origins.push(var_origin);
                            append_to_same_regions(&mut same_regions, same_frs);
                        }
                        None => processed_errors.push((*error).clone()),
                    }
                }
                SupSupConflict(..) => processed_errors.push((*error).clone()),
                _ => ()  // This shouldn't happen
            }
        }
        if !same_regions.is_empty() {
            let common_scope_id = same_regions.get(0).scope_id;
            for sr in same_regions.iter() {
                // Since ProcessedErrors is used to reconstruct the function
                // declaration, we want to make sure that they are, in fact,
                // from the same scope
                if sr.scope_id != common_scope_id {
                    debug!("returning empty result from process_errors because
                            {} != {}", sr.scope_id, common_scope_id);
                    return vec!();
                }
            }
            let pe = ProcessedErrors(var_origins, trace_origins, same_regions);
            debug!("errors processed: {:?}", pe);
            processed_errors.push(pe);
        }
        return processed_errors;


        struct FreeRegionsFromSameFn {
            sub_fr: ty::FreeRegion,
            sup_fr: ty::FreeRegion,
            scope_id: ast::NodeId
        }

        fn free_regions_from_same_fn(tcx: &ty::ctxt,
                                     sub: Region,
                                     sup: Region)
                                     -> Option<FreeRegionsFromSameFn> {
            debug!("free_regions_from_same_fn(sub={:?}, sup={:?})", sub, sup);
            let (scope_id, fr1, fr2) = match (sub, sup) {
                (ReFree(fr1), ReFree(fr2)) => {
                    if fr1.scope_id != fr2.scope_id {
                        return None
                    }
                    assert!(fr1.scope_id == fr2.scope_id);
                    (fr1.scope_id, fr1, fr2)
                },
                _ => return None
            };
            let parent = tcx.map.get_parent(scope_id);
            let parent_node = tcx.map.find(parent);
            match parent_node {
                Some(node) => match node {
                    ast_map::NodeItem(item) => match item.node {
                        // FIXME: handle method
                        ast::ItemFn(..) => {
                            let fr_from_same_fn = FreeRegionsFromSameFn {
                                sub_fr: fr1,
                                sup_fr: fr2,
                                scope_id: scope_id
                            };
                            Some(fr_from_same_fn)
                        },
                        _ => None
                    },
                    _ => None
                },
                None => {
                    debug!("no parent node of scope_id {}", scope_id)
                    None
                }
            }
        }

        fn append_to_same_regions(same_regions: &mut Vec<SameRegions>,
                                  same_frs: &FreeRegionsFromSameFn) {
            let scope_id = same_frs.scope_id;
            let (sub_fr, sup_fr) = (same_frs.sub_fr, same_frs.sup_fr);
            for sr in same_regions.mut_iter() {
                if sr.contains(&sup_fr.bound_region)
                   && scope_id == sr.scope_id {
                    sr.push(sub_fr.bound_region);
                    return
                }
            }
            same_regions.push(SameRegions {
                scope_id: scope_id,
                regions: vec!(sub_fr.bound_region, sup_fr.bound_region)
            })
        }
    }

    fn report_type_error(&self, trace: TypeTrace, terr: &ty::type_err) {
        let expected_found_str = match self.values_str(&trace.values) {
            Some(v) => v,
            None => {
                return; /* derived error */
            }
        };

        let message_root_str = match trace.origin {
            infer::Misc(_) => "mismatched types",
            infer::MethodCompatCheck(_) => "method not compatible with trait",
            infer::ExprAssignable(_) => "mismatched types",
            infer::RelateTraitRefs(_) => "mismatched traits",
            infer::RelateSelfType(_) => "mismatched types",
            infer::MatchExpression(_) => "match arms have incompatible types",
            infer::IfExpression(_) => "if and else have incompatible types",
        };

        self.tcx.sess.span_err(
            trace.origin.span(),
            format!("{}: {} ({})",
                 message_root_str,
                 expected_found_str,
                 ty::type_err_to_str(self.tcx, terr)));
    }

    fn report_and_explain_type_error(&self,
                                     trace: TypeTrace,
                                     terr: &ty::type_err) {
        self.report_type_error(trace, terr);
        ty::note_and_explain_type_err(self.tcx, terr);
    }

    fn values_str(&self, values: &ValuePairs) -> Option<~str> {
        /*!
         * Returns a string of the form "expected `{}` but found `{}`",
         * or None if this is a derived error.
         */
        match *values {
            infer::Types(ref exp_found) => {
                self.expected_found_str(exp_found)
            }
            infer::TraitRefs(ref exp_found) => {
                self.expected_found_str(exp_found)
            }
        }
    }

    fn expected_found_str<T:UserString+Resolvable>(
        &self,
        exp_found: &ty::expected_found<T>)
        -> Option<~str>
    {
        let expected = exp_found.expected.resolve(self);
        if expected.contains_error() {
            return None;
        }

        let found = exp_found.found.resolve(self);
        if found.contains_error() {
            return None;
        }

        Some(format!("expected `{}` but found `{}`",
                  expected.user_string(self.tcx),
                  found.user_string(self.tcx)))
    }

    fn report_concrete_failure(&self,
                               origin: SubregionOrigin,
                               sub: Region,
                               sup: Region) {
        match origin {
            infer::Subtype(trace) => {
                let terr = ty::terr_regions_does_not_outlive(sup, sub);
                self.report_and_explain_type_error(trace, &terr);
            }
            infer::Reborrow(span) => {
                self.tcx.sess.span_err(
                    span,
                    "lifetime of reference outlines \
                     lifetime of borrowed content...");
                note_and_explain_region(
                    self.tcx,
                    "...the reference is valid for ",
                    sub,
                    "...");
                note_and_explain_region(
                    self.tcx,
                    "...but the borrowed content is only valid for ",
                    sup,
                    "");
            }
            infer::ReborrowUpvar(span, ref upvar_id) => {
                self.tcx.sess.span_err(
                    span,
                    format!("lifetime of borrowed pointer outlives \
                            lifetime of captured variable `{}`...",
                            ty::local_var_name_str(self.tcx, upvar_id.var_id).get().to_str()));
                note_and_explain_region(
                    self.tcx,
                    "...the borrowed pointer is valid for ",
                    sub,
                    "...");
                note_and_explain_region(
                    self.tcx,
                    format!("...but `{}` is only valid for ",
                            ty::local_var_name_str(self.tcx, upvar_id.var_id).get().to_str()),
                    sup,
                    "");
            }
            infer::InfStackClosure(span) => {
                self.tcx.sess.span_err(
                    span,
                    "closure outlives stack frame");
                note_and_explain_region(
                    self.tcx,
                    "...the closure must be valid for ",
                    sub,
                    "...");
                note_and_explain_region(
                    self.tcx,
                    "...but the closure's stack frame is only valid for ",
                    sup,
                    "");
            }
            infer::InvokeClosure(span) => {
                self.tcx.sess.span_err(
                    span,
                    "cannot invoke closure outside of its lifetime");
                note_and_explain_region(
                    self.tcx,
                    "the closure is only valid for ",
                    sup,
                    "");
            }
            infer::DerefPointer(span) => {
                self.tcx.sess.span_err(
                    span,
                    "dereference of reference outside its lifetime");
                note_and_explain_region(
                    self.tcx,
                    "the reference is only valid for ",
                    sup,
                    "");
            }
            infer::FreeVariable(span, id) => {
                self.tcx.sess.span_err(
                    span,
                    format!("captured variable `{}` does not \
                            outlive the enclosing closure",
                            ty::local_var_name_str(self.tcx, id).get().to_str()));
                note_and_explain_region(
                    self.tcx,
                    "captured variable is valid for ",
                    sup,
                    "");
                note_and_explain_region(
                    self.tcx,
                    "closure is valid for ",
                    sub,
                    "");
            }
            infer::IndexSlice(span) => {
                self.tcx.sess.span_err(
                    span,
                    format!("index of slice outside its lifetime"));
                note_and_explain_region(
                    self.tcx,
                    "the slice is only valid for ",
                    sup,
                    "");
            }
            infer::RelateObjectBound(span) => {
                self.tcx.sess.span_err(
                    span,
                    "lifetime of the source pointer does not outlive \
                     lifetime bound of the object type");
                note_and_explain_region(
                    self.tcx,
                    "object type is valid for ",
                    sub,
                    "");
                note_and_explain_region(
                    self.tcx,
                    "source pointer is only valid for ",
                    sup,
                    "");
            }
            infer::CallRcvr(span) => {
                self.tcx.sess.span_err(
                    span,
                    "lifetime of method receiver does not outlive \
                     the method call");
                note_and_explain_region(
                    self.tcx,
                    "the receiver is only valid for ",
                    sup,
                    "");
            }
            infer::CallArg(span) => {
                self.tcx.sess.span_err(
                    span,
                    "lifetime of function argument does not outlive \
                     the function call");
                note_and_explain_region(
                    self.tcx,
                    "the function argument is only valid for ",
                    sup,
                    "");
            }
            infer::CallReturn(span) => {
                self.tcx.sess.span_err(
                    span,
                    "lifetime of return value does not outlive \
                     the function call");
                note_and_explain_region(
                    self.tcx,
                    "the return value is only valid for ",
                    sup,
                    "");
            }
            infer::AddrOf(span) => {
                self.tcx.sess.span_err(
                    span,
                    "reference is not valid \
                     at the time of borrow");
                note_and_explain_region(
                    self.tcx,
                    "the borrow is only valid for ",
                    sup,
                    "");
            }
            infer::AutoBorrow(span) => {
                self.tcx.sess.span_err(
                    span,
                    "automatically reference is not valid \
                     at the time of borrow");
                note_and_explain_region(
                    self.tcx,
                    "the automatic borrow is only valid for ",
                    sup,
                    "");
            }
            infer::BindingTypeIsNotValidAtDecl(span) => {
                self.tcx.sess.span_err(
                    span,
                    "lifetime of variable does not enclose its declaration");
                note_and_explain_region(
                    self.tcx,
                    "the variable is only valid for ",
                    sup,
                    "");
            }
            infer::ReferenceOutlivesReferent(ty, span) => {
                self.tcx.sess.span_err(
                    span,
                    format!("in type `{}`, pointer has a longer lifetime than \
                          the data it references",
                         ty.user_string(self.tcx)));
                note_and_explain_region(
                    self.tcx,
                    "the pointer is valid for ",
                    sub,
                    "");
                note_and_explain_region(
                    self.tcx,
                    "but the referenced data is only valid for ",
                    sup,
                    "");
            }
        }
    }

    fn report_sub_sup_conflict(&self,
                               var_origin: RegionVariableOrigin,
                               sub_origin: SubregionOrigin,
                               sub_region: Region,
                               sup_origin: SubregionOrigin,
                               sup_region: Region) {
        self.report_inference_failure(var_origin);

        note_and_explain_region(
            self.tcx,
            "first, the lifetime cannot outlive ",
            sup_region,
            "...");

        self.note_region_origin(sup_origin);

        note_and_explain_region(
            self.tcx,
            "but, the lifetime must be valid for ",
            sub_region,
            "...");

        self.note_region_origin(sub_origin);
    }

    fn report_sup_sup_conflict(&self,
                               var_origin: RegionVariableOrigin,
                               origin1: SubregionOrigin,
                               region1: Region,
                               origin2: SubregionOrigin,
                               region2: Region) {
        self.report_inference_failure(var_origin);

        note_and_explain_region(
            self.tcx,
            "first, the lifetime must be contained by ",
            region1,
            "...");

        self.note_region_origin(origin1);

        note_and_explain_region(
            self.tcx,
            "but, the lifetime must also be contained by ",
            region2,
            "...");

        self.note_region_origin(origin2);
    }

    fn report_processed_errors(&self,
                               var_origins: &[RegionVariableOrigin],
                               trace_origins: &[(TypeTrace, ty::type_err)],
                               same_regions: &[SameRegions]) {
        self.give_suggestion(same_regions);
        for vo in var_origins.iter() {
            self.report_inference_failure(*vo);
        }
        for &(trace, terr) in trace_origins.iter() {
            self.report_type_error(trace, &terr);
        }
    }

    fn give_suggestion(&self, same_regions: &[SameRegions]) {
        let scope_id = same_regions[0].scope_id;
        let parent = self.tcx.map.get_parent(scope_id);
        let parent_node = self.tcx.map.find(parent);
        let node_inner = match parent_node {
            Some(node) => match node {
                ast_map::NodeItem(item) => match item.node {
                    // FIXME: handling method
                    ast::ItemFn(ref fn_decl, _, _, ref gen, _) => {
                        Some((item, fn_decl, gen))
                    },
                    _ => None
                },
                _ => None
            },
            None => None
        };
        let (item, fn_decl, generics) = node_inner.expect("expect item fn");
        let rebuilder = Rebuilder::new(self.tcx, *fn_decl,
                                       generics, same_regions);
        let (inputs, output, generics) = rebuilder.rebuild();
        self.give_expl_lifetime_param(inputs, output, item, generics);
    }
}

struct Rebuilder<'a> {
    tcx: &'a ty::ctxt,
    fn_decl: ast::P<ast::FnDecl>,
    generics: &'a ast::Generics,
    same_regions: &'a [SameRegions],
    life_giver: LifeGiver,
    cur_anon: Cell<uint>,
    inserted_anons: RefCell<HashSet<uint>>,
}

impl<'a> Rebuilder<'a> {
    fn new(tcx: &'a ty::ctxt,
           fn_decl: ast::P<ast::FnDecl>,
           generics: &'a ast::Generics,
           same_regions: &'a [SameRegions])
           -> Rebuilder<'a> {
        Rebuilder {
            tcx: tcx,
            fn_decl: fn_decl,
            generics: generics,
            same_regions: same_regions,
            life_giver: LifeGiver::with_taken(generics.lifetimes.as_slice()),
            cur_anon: Cell::new(0),
            inserted_anons: RefCell::new(HashSet::new()),
        }
    }

    fn rebuild(&self) -> (Vec<ast::Arg>, ast::P<ast::Ty>, ast::Generics) {
        let mut inputs = self.fn_decl.inputs.clone();
        let mut output = self.fn_decl.output;
        for sr in self.same_regions.iter() {
            self.cur_anon.set(0);
            self.offset_cur_anon();
            let (anon_nums, region_names) =
                                self.extract_anon_nums_and_names(sr);
            let lifetime = self.life_giver.give_lifetime();
            inputs = self.rebuild_args_ty(inputs.as_slice(), lifetime,
                                          &anon_nums, &region_names);
            output = self.rebuild_arg_ty_or_output(output, lifetime,
                                                   &anon_nums, &region_names);
        }
        let generated_lifetimes = self.life_giver.get_generated_lifetimes();
        let all_region_names = self.extract_all_region_names();
        let generics = self.rebuild_generics(self.generics,
                                             generated_lifetimes,
                                             &all_region_names);
        (inputs, output, generics)
    }

    fn extract_anon_nums_and_names(&self, same_regions: &SameRegions)
                                   -> (HashSet<uint>, HashSet<ast::Name>) {
        let mut anon_nums = HashSet::new();
        let mut region_names = HashSet::new();
        for br in same_regions.regions.iter() {
            match *br {
                ty::BrAnon(i) => {
                    anon_nums.insert(i);
                }
                ty::BrNamed(_, name) => {
                    region_names.insert(name);
                }
                _ => ()
            }
        }
        (anon_nums, region_names)
    }

    fn extract_all_region_names(&self) -> HashSet<ast::Name> {
        let mut all_region_names = HashSet::new();
        for sr in self.same_regions.iter() {
            for br in sr.regions.iter() {
                match *br {
                    ty::BrNamed(_, name) => {
                        all_region_names.insert(name);
                    }
                    _ => ()
                }
            }
        }
        all_region_names
    }

    fn inc_cur_anon(&self, n: uint) {
        let anon = self.cur_anon.get();
        self.cur_anon.set(anon+n);
    }

    fn offset_cur_anon(&self) {
        let mut anon = self.cur_anon.get();
        while self.inserted_anons.borrow().contains(&anon) {
            anon += 1;
        }
        self.cur_anon.set(anon);
    }

    fn inc_and_offset_cur_anon(&self, n: uint) {
        self.inc_cur_anon(n);
        self.offset_cur_anon();
    }

    fn track_anon(&self, anon: uint) {
        self.inserted_anons.borrow_mut().insert(anon);
    }

    fn rebuild_generics(&self,
                        generics: &ast::Generics,
                        add: Vec<ast::Lifetime>,
                        remove: &HashSet<ast::Name>)
                        -> ast::Generics {
        let mut lifetimes = Vec::new();
        for lt in add.iter() {
            lifetimes.push(*lt);
        }
        for lt in generics.lifetimes.iter() {
            if !remove.contains(&lt.name) {
                lifetimes.push((*lt).clone());
            }
        }
        ast::Generics {
            lifetimes: lifetimes,
            ty_params: generics.ty_params.clone()
        }
    }

    fn rebuild_args_ty(&self,
                       inputs: &[ast::Arg],
                       lifetime: ast::Lifetime,
                       anon_nums: &HashSet<uint>,
                       region_names: &HashSet<ast::Name>)
                       -> Vec<ast::Arg> {
        let mut new_inputs = Vec::new();
        for arg in inputs.iter() {
            let new_ty = self.rebuild_arg_ty_or_output(arg.ty, lifetime,
                                                       anon_nums, region_names);
            let possibly_new_arg = ast::Arg {
                ty: new_ty,
                pat: arg.pat,
                id: arg.id
            };
            new_inputs.push(possibly_new_arg);
        }
        new_inputs
    }

    fn rebuild_arg_ty_or_output(&self,
                                ty: ast::P<ast::Ty>,
                                lifetime: ast::Lifetime,
                                anon_nums: &HashSet<uint>,
                                region_names: &HashSet<ast::Name>)
                                -> ast::P<ast::Ty> {
        let mut new_ty = ty;
        let mut ty_queue = vec!(ty);
        let mut cur_ty;
        while !ty_queue.is_empty() {
            cur_ty = ty_queue.shift().unwrap();
            match cur_ty.node {
                ast::TyRptr(lt_opt, mut_ty) => {
                    match lt_opt {
                        Some(lt) => if region_names.contains(&lt.name) {
                            new_ty = self.rebuild_ty(new_ty, cur_ty,
                                                     lifetime, None);
                        },
                        None => {
                            let anon = self.cur_anon.get();
                            if anon_nums.contains(&anon) {
                                new_ty = self.rebuild_ty(new_ty, cur_ty,
                                                         lifetime, None);
                                self.track_anon(anon);
                            }
                            self.inc_and_offset_cur_anon(1);
                        }
                    }
                    ty_queue.push(mut_ty.ty);
                }
                ast::TyPath(ref path, _, id) => {
                    let a_def = match self.tcx.def_map.borrow().find(&id) {
                        None => self.tcx.sess.fatal(format!("unbound path {}",
                                                    pprust::path_to_str(path))),
                        Some(&d) => d
                    };
                    match a_def {
                        ast::DefTy(did) | ast::DefStruct(did) => {
                            let ty::ty_param_bounds_and_ty {
                                generics: generics,
                                ty: _
                            } = ty::lookup_item_type(self.tcx, did);

                            let expected = generics.region_param_defs().len();
                            let lifetimes = &path.segments.last()
                                                 .unwrap().lifetimes;
                            let mut insert = Vec::new();
                            if lifetimes.len() == 0 {
                                let anon = self.cur_anon.get();
                                for (i, a) in range(anon,
                                                    anon+expected).enumerate() {
                                    if anon_nums.contains(&a) {
                                        insert.push(i);
                                    }
                                    self.track_anon(a);
                                }
                                self.inc_and_offset_cur_anon(expected);
                            } else {
                                for (i, lt) in lifetimes.iter().enumerate() {
                                    if region_names.contains(&lt.name) {
                                        insert.push(i);
                                    }
                                }
                            }
                            for i in insert.iter() {
                                new_ty = self.rebuild_ty(new_ty, cur_ty,
                                                         lifetime,
                                                         Some((*i, expected)));
                            }
                        }
                        _ => ()
                    }

                }
                _ => ty_queue.push_all_move(ast_util::get_inner_tys(cur_ty))
            }
        }
        new_ty
    }

    fn rebuild_ty(&self,
                  from: ast::P<ast::Ty>,
                  to: ast::P<ast::Ty>,
                  lifetime: ast::Lifetime,
                  index_opt: Option<(uint, uint)>)
                  -> ast::P<ast::Ty> {

        fn build_to(from: ast::P<ast::Ty>,
                    to: ast::P<ast::Ty>)
                    -> ast::P<ast::Ty> {
            if from.id == to.id {
                return to;
            }
            let new_node = match from.node {
                ast::TyRptr(ref lifetime, ref mut_ty) => {
                    let new_mut_ty = ast::MutTy {
                        ty: build_to(mut_ty.ty, to),
                        mutbl: mut_ty.mutbl
                    };
                    ast::TyRptr(*lifetime, new_mut_ty)
                }
                ast::TyPtr(ref mut_ty) => {
                    let new_mut_ty = ast::MutTy {
                        ty: build_to(mut_ty.ty, to),
                        mutbl: mut_ty.mutbl
                    };
                    ast::TyPtr(new_mut_ty)
                }
                ast::TyBox(ref ty) => ast::TyBox(build_to(*ty, to)),
                ast::TyVec(ref ty) => ast::TyVec(build_to(*ty, to)),
                ast::TyUniq(ref ty) => ast::TyUniq(build_to(*ty, to)),
                ast::TyFixedLengthVec(ref ty, ref e) => {
                    ast::TyFixedLengthVec(build_to(*ty, to), *e)
                }
                ast::TyTup(ref tys) => {
                    let mut new_tys = Vec::new();
                    for ty in tys.iter() {
                        new_tys.push(build_to(*ty, to));
                    }
                    ast::TyTup(new_tys)
                }
                ref other => other.clone()
            };
            @ast::Ty { id: from.id, node: new_node, span: from.span }
        }

        let new_ty_node = match to.node {
            ast::TyRptr(_, mut_ty) => ast::TyRptr(Some(lifetime), mut_ty),
            ast::TyPath(ref path, ref bounds, id) => {
                let (index, expected) = match index_opt {
                    Some((i, e)) => (i, e),
                    None => fail!("expect index_opt in rebuild_ty/ast::TyPath")
                };
                let new_path = self.rebuild_path(path, index,
                                                 expected, lifetime);
                ast::TyPath(new_path, bounds.clone(), id)
            }
            _ => fail!("expect ast::TyRptr or ast::TyPath")
        };
        let new_ty = @ast::Ty {
            id: to.id,
            node: new_ty_node,
            span: to.span
        };
        build_to(from, new_ty)
    }

    fn rebuild_path(&self,
                    path: &ast::Path,
                    index: uint,
                    expected: uint,
                    lifetime: ast::Lifetime)
                    -> ast::Path {
        let last_seg = path.segments.last().unwrap();
        let mut new_lts = Vec::new();
        if last_seg.lifetimes.len() == 0 {
            for i in range(0, expected) {
                if i == index {
                    new_lts.push(lifetime);
                } else {
                    new_lts.push(self.life_giver.give_lifetime());
                }
            }
        } else {
            for (i, lt) in last_seg.lifetimes.iter().enumerate() {
                if i == index {
                    new_lts.push(lifetime);
                } else {
                    new_lts.push(*lt);
                }
            }
        }
        let new_seg = ast::PathSegment {
            identifier: last_seg.identifier,
            lifetimes: new_lts,
            types: last_seg.types.clone(),
        };
        let mut new_segs = Vec::new();
        new_segs.push_all(path.segments.init());
        new_segs.push(new_seg);
        ast::Path {
            span: path.span,
            global: path.global,
            segments: new_segs
        }
    }
}

impl<'a> ErrorReportingHelpers for InferCtxt<'a> {
    fn give_expl_lifetime_param(&self,
                                inputs: Vec<ast::Arg>,
                                output: ast::P<ast::Ty>,
                                item: ast::P<ast::Item>,
                                generics: ast::Generics) {
        let (fn_decl, purity, ident) = match item.node {
            // FIXME: handling method
            ast::ItemFn(ref fn_decl, ref purity, _, _, _) => {
                (fn_decl, purity, item.ident)
            },
            _ => fail!("Expect function or method")

        };
        let fd = ast::FnDecl {
            inputs: inputs,
            output: output,
            cf: fn_decl.cf,
            variadic: fn_decl.variadic
        };
        let suggested_fn =
            pprust::fun_to_str(&fd, *purity, ident, None, &generics);
        let msg = format!("consider using an explicit lifetime \
                           parameter as shown: {}", suggested_fn);
        self.tcx.sess.span_note(item.span, msg);
    }

    fn report_inference_failure(&self,
                                var_origin: RegionVariableOrigin) {
        let var_description = match var_origin {
            infer::MiscVariable(_) => ~"",
            infer::PatternRegion(_) => ~" for pattern",
            infer::AddrOfRegion(_) => ~" for borrow expression",
            infer::AddrOfSlice(_) => ~" for slice expression",
            infer::Autoref(_) => ~" for autoref",
            infer::Coercion(_) => ~" for automatic coercion",
            infer::LateBoundRegion(_, br) => {
                format!(" for {}in function call",
                        bound_region_to_str(self.tcx, "lifetime parameter ", true, br))
            }
            infer::BoundRegionInFnType(_, br) => {
                format!(" for {}in function type",
                        bound_region_to_str(self.tcx, "lifetime parameter ", true, br))
            }
            infer::EarlyBoundRegion(_, name) => {
                format!(" for lifetime parameter `{}",
                        token::get_name(name).get())
            }
            infer::BoundRegionInCoherence(name) => {
                format!(" for lifetime parameter `{} in coherence check",
                        token::get_name(name).get())
            }
            infer::UpvarRegion(ref upvar_id, _) => {
                format!(" for capture of `{}` by closure",
                        ty::local_var_name_str(self.tcx, upvar_id.var_id).get().to_str())
            }
        };

        self.tcx.sess.span_err(
            var_origin.span(),
            format!("cannot infer an appropriate lifetime{} \
                    due to conflicting requirements",
                    var_description));
    }

    fn note_region_origin(&self, origin: SubregionOrigin) {
        match origin {
            infer::Subtype(ref trace) => {
                let desc = match trace.origin {
                    infer::Misc(_) => {
                        format!("types are compatible")
                    }
                    infer::MethodCompatCheck(_) => {
                        format!("method type is compatible with trait")
                    }
                    infer::ExprAssignable(_) => {
                        format!("expression is assignable")
                    }
                    infer::RelateTraitRefs(_) => {
                        format!("traits are compatible")
                    }
                    infer::RelateSelfType(_) => {
                        format!("type matches impl")
                    }
                    infer::MatchExpression(_) => {
                        format!("match arms have compatible types")
                    }
                    infer::IfExpression(_) => {
                        format!("if and else have compatible types")
                    }
                };

                match self.values_str(&trace.values) {
                    Some(values_str) => {
                        self.tcx.sess.span_note(
                            trace.origin.span(),
                            format!("...so that {} ({})",
                                    desc, values_str));
                    }
                    None => {
                        // Really should avoid printing this error at
                        // all, since it is derived, but that would
                        // require more refactoring than I feel like
                        // doing right now. - nmatsakis
                        self.tcx.sess.span_note(
                            trace.origin.span(),
                            format!("...so that {}", desc));
                    }
                }
            }
            infer::Reborrow(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that reference does not outlive \
                    borrowed content");
            }
            infer::ReborrowUpvar(span, ref upvar_id) => {
                self.tcx.sess.span_note(
                    span,
                    format!("...so that closure can access `{}`",
                            ty::local_var_name_str(self.tcx, upvar_id.var_id).get().to_str()))
            }
            infer::InfStackClosure(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that closure does not outlive its stack frame");
            }
            infer::InvokeClosure(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that closure is not invoked outside its lifetime");
            }
            infer::DerefPointer(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that pointer is not dereferenced \
                    outside its lifetime");
            }
            infer::FreeVariable(span, id) => {
                self.tcx.sess.span_note(
                    span,
                    format!("...so that captured variable `{}` \
                            does not outlive the enclosing closure",
                            ty::local_var_name_str(self.tcx, id).get().to_str()));
            }
            infer::IndexSlice(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that slice is not indexed outside the lifetime");
            }
            infer::RelateObjectBound(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that source pointer does not outlive \
                     lifetime bound of the object type");
            }
            infer::CallRcvr(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that method receiver is valid for the method call");
            }
            infer::CallArg(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that argument is valid for the call");
            }
            infer::CallReturn(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that return value is valid for the call");
            }
            infer::AddrOf(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that reference is valid \
                     at the time of borrow");
            }
            infer::AutoBorrow(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that automatically reference is valid \
                     at the time of borrow");
            }
            infer::BindingTypeIsNotValidAtDecl(span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that variable is valid at time of its declaration");
            }
            infer::ReferenceOutlivesReferent(_, span) => {
                self.tcx.sess.span_note(
                    span,
                    "...so that the pointer does not outlive the \
                    data it points at");
            }
        }
    }
}

trait Resolvable {
    fn resolve(&self, infcx: &InferCtxt) -> Self;
    fn contains_error(&self) -> bool;
}

impl Resolvable for ty::t {
    fn resolve(&self, infcx: &InferCtxt) -> ty::t {
        infcx.resolve_type_vars_if_possible(*self)
    }
    fn contains_error(&self) -> bool {
        ty::type_is_error(*self)
    }
}

impl Resolvable for @ty::TraitRef {
    fn resolve(&self, infcx: &InferCtxt) -> @ty::TraitRef {
        @infcx.resolve_type_vars_in_trait_ref_if_possible(*self)
    }
    fn contains_error(&self) -> bool {
        ty::trait_ref_contains_error(*self)
    }
}

// LifeGiver is responsible for generating fresh lifetime names
struct LifeGiver {
    taken: HashSet<~str>,
    counter: Cell<uint>,
    generated: RefCell<Vec<ast::Lifetime>>,
}

impl LifeGiver {
    // FIXME: `taken` needs to include names from higher scope, too
    fn with_taken(taken: &[ast::Lifetime]) -> LifeGiver {
        let mut taken_ = HashSet::new();
        for lt in taken.iter() {
            let lt_name = token::get_name(lt.name).get().to_owned();
            taken_.insert(lt_name);
        }
        LifeGiver {
            taken: taken_,
            counter: Cell::new(0),
            generated: RefCell::new(Vec::new()),
        }
    }

    fn inc_counter(&self) {
        let c = self.counter.get();
        self.counter.set(c+1);
    }

    fn give_lifetime(&self) -> ast::Lifetime {
        let mut lifetime;
        loop {
            let s = num_to_str(self.counter.get());
            if !self.taken.contains(&s) {
                lifetime = name_to_dummy_lifetime(
                                    token::str_to_ident(s.as_slice()).name);
                self.generated.borrow_mut().push(lifetime);
                break;
            }
            self.inc_counter();
        }
        self.inc_counter();
        return lifetime;

        // 0 .. 25 generates a .. z, 26 .. 51 generates aa .. zz, and so on
        fn num_to_str(counter: uint) -> ~str {
            let mut s = ~"";
            let (n, r) = (counter/26 + 1, counter % 26);
            let letter: char = from_u32((r+97) as u32).unwrap();
            for _ in range(0, n) {
                s.push_char(letter);
            }
            return s;
        }
    }

    fn get_generated_lifetimes(&self) -> Vec<ast::Lifetime> {
        self.generated.get()
    }
}