// 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.

//! Give useful errors and suggestions to users when an item can't be
//! found or is otherwise invalid.

use CrateCtxt;

use check::{FnCtxt};
use rustc::hir::map as hir_map;
use rustc::ty::{self, Ty, ToPolyTraitRef, ToPredicate, TypeFoldable};
use middle::cstore;
use hir::def::Def;
use hir::def_id::DefId;
use middle::lang_items::FnOnceTraitLangItem;
use rustc::ty::subst::Substs;
use rustc::traits::{Obligation, SelectionContext};
use util::nodemap::{FnvHashSet};

use syntax::ast;
use errors::DiagnosticBuilder;
use syntax_pos::Span;

use rustc::hir::print as pprust;
use rustc::hir;
use rustc::hir::Expr_;

use std::cell;
use std::cmp::Ordering;

use super::{MethodError, NoMatchData, CandidateSource};
use super::probe::Mode;

impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
    fn is_fn_ty(&self, ty: &Ty<'tcx>, span: Span) -> bool {
        let tcx = self.tcx;
        match ty.sty {
            // Not all of these (e.g. unsafe fns) implement FnOnce
            // so we look for these beforehand
            ty::TyClosure(..) | ty::TyFnDef(..) | ty::TyFnPtr(_) => true,
            // If it's not a simple function, look for things which implement FnOnce
            _ => {
                let fn_once = match tcx.lang_items.require(FnOnceTraitLangItem) {
                    Ok(fn_once) => fn_once,
                    Err(..) => return false
                };

                self.autoderef(span, ty).any(|(ty, _)| self.probe(|_| {
                    let fn_once_substs =
                        Substs::new_trait(vec![self.next_ty_var()], vec![], ty);
                    let trait_ref =
                        ty::TraitRef::new(fn_once,
                                          tcx.mk_substs(fn_once_substs));
                    let poly_trait_ref = trait_ref.to_poly_trait_ref();
                    let obligation = Obligation::misc(span,
                                                      self.body_id,
                                                      poly_trait_ref
                                                      .to_predicate());
                    SelectionContext::new(self).evaluate_obligation(&obligation)
                }))
            }
        }
    }

    pub fn report_method_error(&self,
                               span: Span,
                               rcvr_ty: Ty<'tcx>,
                               item_name: ast::Name,
                               rcvr_expr: Option<&hir::Expr>,
                               error: MethodError<'tcx>)
    {
        // avoid suggestions when we don't know what's going on.
        if rcvr_ty.references_error() {
            return
        }

        let report_candidates = |err: &mut DiagnosticBuilder,
                                 mut sources: Vec<CandidateSource>| {

            sources.sort();
            sources.dedup();
            // Dynamic limit to avoid hiding just one candidate, which is silly.
            let limit = if sources.len() == 5 { 5 } else { 4 };

            for (idx, source) in sources.iter().take(limit).enumerate() {
                match *source {
                    CandidateSource::ImplSource(impl_did) => {
                        // Provide the best span we can. Use the item, if local to crate, else
                        // the impl, if local to crate (item may be defaulted), else nothing.
                        let item = self.impl_item(impl_did, item_name)
                            .or_else(|| {
                                self.trait_item(
                                    self.tcx.impl_trait_ref(impl_did).unwrap().def_id,

                                    item_name
                                )
                            }).unwrap();
                        let note_span = self.tcx.map.span_if_local(item.def_id()).or_else(|| {
                            self.tcx.map.span_if_local(impl_did)
                        });

                        let impl_ty = self.impl_self_ty(span, impl_did).ty;

                        let insertion = match self.tcx.impl_trait_ref(impl_did) {
                            None => format!(""),
                            Some(trait_ref) => {
                                format!(" of the trait `{}`",
                                        self.tcx.item_path_str(trait_ref.def_id))
                            }
                        };

                        let note_str = format!("candidate #{} is defined in an impl{} \
                                                for the type `{}`",
                                               idx + 1,
                                               insertion,
                                               impl_ty);
                        if let Some(note_span) = note_span {
                            // We have a span pointing to the method. Show note with snippet.
                            err.span_note(note_span, &note_str);
                        } else {
                            err.note(&note_str);
                        }
                    }
                    CandidateSource::TraitSource(trait_did) => {
                        let item = self.trait_item(trait_did, item_name).unwrap();
                        let item_span = self.tcx.map.def_id_span(item.def_id(), span);
                        span_note!(err, item_span,
                                   "candidate #{} is defined in the trait `{}`",
                                   idx + 1,
                                   self.tcx.item_path_str(trait_did));
                    }
                }
            }
            if sources.len() > limit {
                err.note(&format!("and {} others", sources.len() - limit));
            }
        };

        match error {
            MethodError::NoMatch(NoMatchData { static_candidates: static_sources,
                                               unsatisfied_predicates,
                                               out_of_scope_traits,
                                               mode, .. }) => {
                let tcx = self.tcx;

                let mut err = self.type_error_struct(
                    span,
                    |actual| {
                        format!("no {} named `{}` found for type `{}` \
                                 in the current scope",
                                if mode == Mode::MethodCall { "method" }
                                else { "associated item" },
                                item_name,
                                actual)
                    },
                    rcvr_ty);

                // If the item has the name of a field, give a help note
                if let (&ty::TyStruct(def, substs), Some(expr)) = (&rcvr_ty.sty, rcvr_expr) {
                    if let Some(field) = def.struct_variant().find_field_named(item_name) {
                        let expr_string = match tcx.sess.codemap().span_to_snippet(expr.span) {
                            Ok(expr_string) => expr_string,
                            _ => "s".into() // Default to a generic placeholder for the
                                            // expression when we can't generate a string
                                            // snippet
                        };

                        let field_ty = field.ty(tcx, substs);

                        if self.is_fn_ty(&field_ty, span) {
                            err.span_note(span,
                                          &format!("use `({0}.{1})(...)` if you meant to call \
                                                   the function stored in the `{1}` field",
                                                   expr_string, item_name));
                        } else {
                            err.span_note(span, &format!("did you mean to write `{0}.{1}`?",
                                                         expr_string, item_name));
                        }
                    }
                }

                if self.is_fn_ty(&rcvr_ty, span) {
                    macro_rules! report_function {
                        ($span:expr, $name:expr) => {
                            err.note(&format!("{} is a function, perhaps you wish to call it",
                                         $name));
                        }
                    }

                    if let Some(expr) = rcvr_expr {
                        if let Ok (expr_string) = tcx.sess.codemap().span_to_snippet(expr.span) {
                            report_function!(expr.span, expr_string);
                        }
                        else if let Expr_::ExprPath(_, path) = expr.node.clone() {
                            if let Some(segment) = path.segments.last() {
                                report_function!(expr.span, segment.name);
                            }
                        }
                    }
                }

                if !static_sources.is_empty() {
                    err.note(
                        "found the following associated functions; to be used as \
                         methods, functions must have a `self` parameter");

                    report_candidates(&mut err, static_sources);
                }

                if !unsatisfied_predicates.is_empty() {
                    let bound_list = unsatisfied_predicates.iter()
                        .map(|p| format!("`{} : {}`",
                                         p.self_ty(),
                                         p))
                        .collect::<Vec<_>>()
                        .join(", ");
                    err.note(
                        &format!("the method `{}` exists but the \
                                 following trait bounds were not satisfied: {}",
                                 item_name,
                                 bound_list));
                }

                self.suggest_traits_to_import(&mut err, span, rcvr_ty, item_name,
                                              rcvr_expr, out_of_scope_traits);
                err.emit();
            }

            MethodError::Ambiguity(sources) => {
                let mut err = struct_span_err!(self.sess(), span, E0034,
                                               "multiple applicable items in scope");

                report_candidates(&mut err, sources);
                err.emit();
            }

            MethodError::ClosureAmbiguity(trait_def_id) => {
                let msg = format!("the `{}` method from the `{}` trait cannot be explicitly \
                                   invoked on this closure as we have not yet inferred what \
                                   kind of closure it is",
                                   item_name,
                                   self.tcx.item_path_str(trait_def_id));
                let msg = if let Some(callee) = rcvr_expr {
                    format!("{}; use overloaded call notation instead (e.g., `{}()`)",
                            msg, pprust::expr_to_string(callee))
                } else {
                    msg
                };
                self.sess().span_err(span, &msg);
            }

            MethodError::PrivateMatch(def) => {
                let msg = format!("{} `{}` is private", def.kind_name(), item_name);
                self.tcx.sess.span_err(span, &msg);
            }
        }
    }

    fn suggest_traits_to_import(&self,
                                err: &mut DiagnosticBuilder,
                                span: Span,
                                rcvr_ty: Ty<'tcx>,
                                item_name: ast::Name,
                                rcvr_expr: Option<&hir::Expr>,
                                valid_out_of_scope_traits: Vec<DefId>)
    {
        if !valid_out_of_scope_traits.is_empty() {
            let mut candidates = valid_out_of_scope_traits;
            candidates.sort();
            candidates.dedup();
            let msg = format!(
                "items from traits can only be used if the trait is in scope; \
                 the following {traits_are} implemented but not in scope, \
                 perhaps add a `use` for {one_of_them}:",
                traits_are = if candidates.len() == 1 {"trait is"} else {"traits are"},
                one_of_them = if candidates.len() == 1 {"it"} else {"one of them"});

            err.help(&msg[..]);

            let limit = if candidates.len() == 5 { 5 } else { 4 };
            for (i, trait_did) in candidates.iter().take(limit).enumerate() {
                err.help(&format!("candidate #{}: `use {}`",
                                  i + 1,
                                  self.tcx.item_path_str(*trait_did)));
            }
            if candidates.len() > limit {
                err.note(&format!("and {} others", candidates.len() - limit));
            }
            return
        }

        let type_is_local = self.type_derefs_to_local(span, rcvr_ty, rcvr_expr);

        // there's no implemented traits, so lets suggest some traits to
        // implement, by finding ones that have the item name, and are
        // legal to implement.
        let mut candidates = all_traits(self.ccx)
            .filter(|info| {
                // we approximate the coherence rules to only suggest
                // traits that are legal to implement by requiring that
                // either the type or trait is local. Multidispatch means
                // this isn't perfect (that is, there are cases when
                // implementing a trait would be legal but is rejected
                // here).
                (type_is_local || info.def_id.is_local())
                    && self.trait_item(info.def_id, item_name).is_some()
            })
            .collect::<Vec<_>>();

        if !candidates.is_empty() {
            // sort from most relevant to least relevant
            candidates.sort_by(|a, b| a.cmp(b).reverse());
            candidates.dedup();

            // FIXME #21673 this help message could be tuned to the case
            // of a type parameter: suggest adding a trait bound rather
            // than implementing.
            let msg = format!(
                "items from traits can only be used if the trait is implemented and in scope; \
                 the following {traits_define} an item `{name}`, \
                 perhaps you need to implement {one_of_them}:",
                traits_define = if candidates.len() == 1 {"trait defines"} else {"traits define"},
                one_of_them = if candidates.len() == 1 {"it"} else {"one of them"},
                name = item_name);

            err.help(&msg[..]);

            for (i, trait_info) in candidates.iter().enumerate() {
                err.help(&format!("candidate #{}: `{}`",
                                  i + 1,
                                  self.tcx.item_path_str(trait_info.def_id)));
            }
        }
    }

    /// Checks whether there is a local type somewhere in the chain of
    /// autoderefs of `rcvr_ty`.
    fn type_derefs_to_local(&self,
                            span: Span,
                            rcvr_ty: Ty<'tcx>,
                            rcvr_expr: Option<&hir::Expr>) -> bool {
        fn is_local(ty: Ty) -> bool {
            match ty.sty {
                ty::TyEnum(def, _) | ty::TyStruct(def, _) => def.did.is_local(),

                ty::TyTrait(ref tr) => tr.principal_def_id().is_local(),

                ty::TyParam(_) => true,

                // everything else (primitive types etc.) is effectively
                // non-local (there are "edge" cases, e.g. (LocalType,), but
                // the noise from these sort of types is usually just really
                // annoying, rather than any sort of help).
                _ => false
            }
        }

        // This occurs for UFCS desugaring of `T::method`, where there is no
        // receiver expression for the method call, and thus no autoderef.
        if rcvr_expr.is_none() {
            return is_local(self.resolve_type_vars_with_obligations(rcvr_ty));
        }

        self.autoderef(span, rcvr_ty).any(|(ty, _)| is_local(ty))
    }
}

pub type AllTraitsVec = Vec<TraitInfo>;

#[derive(Copy, Clone)]
pub struct TraitInfo {
    pub def_id: DefId,
}

impl TraitInfo {
    fn new(def_id: DefId) -> TraitInfo {
        TraitInfo {
            def_id: def_id,
        }
    }
}
impl PartialEq for TraitInfo {
    fn eq(&self, other: &TraitInfo) -> bool {
        self.cmp(other) == Ordering::Equal
    }
}
impl Eq for TraitInfo {}
impl PartialOrd for TraitInfo {
    fn partial_cmp(&self, other: &TraitInfo) -> Option<Ordering> { Some(self.cmp(other)) }
}
impl Ord for TraitInfo {
    fn cmp(&self, other: &TraitInfo) -> Ordering {
        // local crates are more important than remote ones (local:
        // cnum == 0), and otherwise we throw in the defid for totality

        let lhs = (other.def_id.krate, other.def_id);
        let rhs = (self.def_id.krate, self.def_id);
        lhs.cmp(&rhs)
    }
}

/// Retrieve all traits in this crate and any dependent crates.
pub fn all_traits<'a>(ccx: &'a CrateCtxt) -> AllTraits<'a> {
    if ccx.all_traits.borrow().is_none() {
        use rustc::hir::intravisit;

        let mut traits = vec![];

        // Crate-local:
        //
        // meh.
        struct Visitor<'a, 'tcx:'a> {
            map: &'a hir_map::Map<'tcx>,
            traits: &'a mut AllTraitsVec,
        }
        impl<'v, 'a, 'tcx> intravisit::Visitor<'v> for Visitor<'a, 'tcx> {
            fn visit_item(&mut self, i: &'v hir::Item) {
                match i.node {
                    hir::ItemTrait(..) => {
                        let def_id = self.map.local_def_id(i.id);
                        self.traits.push(TraitInfo::new(def_id));
                    }
                    _ => {}
                }
            }
        }
        ccx.tcx.map.krate().visit_all_items(&mut Visitor {
            map: &ccx.tcx.map,
            traits: &mut traits
        });

        // Cross-crate:
        let mut external_mods = FnvHashSet();
        fn handle_external_def(traits: &mut AllTraitsVec,
                               external_mods: &mut FnvHashSet<DefId>,
                               ccx: &CrateCtxt,
                               cstore: &for<'a> cstore::CrateStore<'a>,
                               dl: cstore::DefLike) {
            match dl {
                cstore::DlDef(Def::Trait(did)) => {
                    traits.push(TraitInfo::new(did));
                }
                cstore::DlDef(Def::Mod(did)) => {
                    if !external_mods.insert(did) {
                        return;
                    }
                    for child in cstore.item_children(did) {
                        handle_external_def(traits, external_mods,
                                            ccx, cstore, child.def)
                    }
                }
                _ => {}
            }
        }
        let cstore = &*ccx.tcx.sess.cstore;

        for cnum in ccx.tcx.sess.cstore.crates() {
            for child in cstore.crate_top_level_items(cnum) {
                handle_external_def(&mut traits, &mut external_mods,
                                    ccx, cstore, child.def)
            }
        }

        *ccx.all_traits.borrow_mut() = Some(traits);
    }

    let borrow = ccx.all_traits.borrow();
    assert!(borrow.is_some());
    AllTraits {
        borrow: borrow,
        idx: 0
    }
}

pub struct AllTraits<'a> {
    borrow: cell::Ref<'a, Option<AllTraitsVec>>,
    idx: usize
}

impl<'a> Iterator for AllTraits<'a> {
    type Item = TraitInfo;

    fn next(&mut self) -> Option<TraitInfo> {
        let AllTraits { ref borrow, ref mut idx } = *self;
        // ugh.
        borrow.as_ref().unwrap().get(*idx).map(|info| {
            *idx += 1;
            *info
        })
    }
}