resolve_imports.rs

// Copyright 2015 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 self::ImportDirectiveSubclass::*;

use {AmbiguityError, CrateLint, Module, ModuleOrUniformRoot, PerNS};
use Namespace::{self, TypeNS, MacroNS};
use {NameBinding, NameBindingKind, ToNameBinding, PathResult, PrivacyError};
use {Resolver, Segment};
use {names_to_string, module_to_string};
use {resolve_error, ResolutionError};
use macros::ParentScope;

use rustc_data_structures::ptr_key::PtrKey;
use rustc::ty;
use rustc::lint::builtin::BuiltinLintDiagnostics;
use rustc::lint::builtin::{DUPLICATE_MACRO_EXPORTS, PUB_USE_OF_PRIVATE_EXTERN_CRATE};
use rustc::hir::def_id::DefId;
use rustc::hir::def::*;
use rustc::session::DiagnosticMessageId;
use rustc::util::nodemap::FxHashSet;

use syntax::ast::{Ident, Name, NodeId, CRATE_NODE_ID};
use syntax::ext::base::Determinacy::{self, Determined, Undetermined};
use syntax::ext::hygiene::Mark;
use syntax::symbol::keywords;
use syntax::util::lev_distance::find_best_match_for_name;
use syntax_pos::{MultiSpan, Span};

use std::cell::{Cell, RefCell};
use std::collections::BTreeMap;
use std::fmt::Write;
use std::{mem, ptr};

/// Contains data for specific types of import directives.
#[derive(Clone, Debug)]
pub enum ImportDirectiveSubclass<'a> {
    SingleImport {
        target: Ident,
        source: Ident,
        result: PerNS<Cell<Result<&'a NameBinding<'a>, Determinacy>>>,
        type_ns_only: bool,
    },
    GlobImport {
        is_prelude: bool,
        max_vis: Cell<ty::Visibility>, // The visibility of the greatest re-export.
        // n.b. `max_vis` is only used in `finalize_import` to check for re-export errors.
    },
    ExternCrate {
        source: Option<Name>,
        target: Ident,
    },
    MacroUse,
}

/// One import directive.
#[derive(Debug,Clone)]
pub struct ImportDirective<'a> {
    /// The id of the `extern crate`, `UseTree` etc that imported this `ImportDirective`.
    ///
    /// In the case where the `ImportDirective` was expanded from a "nested" use tree,
    /// this id is the id of the leaf tree. For example:
    ///
    /// ```ignore (pacify the mercilous tidy)
    /// use foo::bar::{a, b}
    /// ```
    ///
    /// If this is the import directive for `foo::bar::a`, we would have the id of the `UseTree`
    /// for `a` in this field.
    pub id: NodeId,

    /// The `id` of the "root" use-kind -- this is always the same as
    /// `id` except in the case of "nested" use trees, in which case
    /// it will be the `id` of the root use tree. e.g., in the example
    /// from `id`, this would be the id of the `use foo::bar`
    /// `UseTree` node.
    pub root_id: NodeId,

    /// Span of this use tree.
    pub span: Span,

    /// Span of the *root* use tree (see `root_id`).
    pub root_span: Span,

    pub parent_scope: ParentScope<'a>,
    pub module_path: Vec<Segment>,
    /// The resolution of `module_path`.
    pub imported_module: Cell<Option<ModuleOrUniformRoot<'a>>>,
    pub subclass: ImportDirectiveSubclass<'a>,
    pub vis: Cell<ty::Visibility>,
    pub used: Cell<bool>,

    /// Whether this import is a "canary" for the `uniform_paths` feature,
    /// i.e. `use x::...;` results in an `use self::x as _;` canary.
    /// This flag affects diagnostics: an error is reported if and only if
    /// the import resolves successfully and an external crate with the same
    /// name (`x` above) also exists; any resolution failures are ignored.
    pub is_uniform_paths_canary: bool,
}

impl<'a> ImportDirective<'a> {
    pub fn is_glob(&self) -> bool {
        match self.subclass { ImportDirectiveSubclass::GlobImport { .. } => true, _ => false }
    }

    crate fn crate_lint(&self) -> CrateLint {
        CrateLint::UsePath { root_id: self.root_id, root_span: self.root_span }
    }
}

#[derive(Clone, Default, Debug)]
/// Records information about the resolution of a name in a namespace of a module.
pub struct NameResolution<'a> {
    /// Single imports that may define the name in the namespace.
    /// Import directives are arena-allocated, so it's ok to use pointers as keys.
    single_imports: FxHashSet<PtrKey<'a, ImportDirective<'a>>>,
    /// The least shadowable known binding for this name, or None if there are no known bindings.
    pub binding: Option<&'a NameBinding<'a>>,
    shadowed_glob: Option<&'a NameBinding<'a>>,
}

impl<'a> NameResolution<'a> {
    // Returns the binding for the name if it is known or None if it not known.
    fn binding(&self) -> Option<&'a NameBinding<'a>> {
        self.binding.and_then(|binding| {
            if !binding.is_glob_import() ||
               self.single_imports.is_empty() { Some(binding) } else { None }
        })
    }
}

impl<'a, 'crateloader> Resolver<'a, 'crateloader> {
    fn resolution(&self, module: Module<'a>, ident: Ident, ns: Namespace)
                  -> &'a RefCell<NameResolution<'a>> {
        *module.resolutions.borrow_mut().entry((ident.modern(), ns))
               .or_insert_with(|| self.arenas.alloc_name_resolution())
    }

    /// Attempts to resolve `ident` in namespaces `ns` of `module`.
    /// Invariant: if `record_used` is `Some`, expansion and import resolution must be complete.
    pub fn resolve_ident_in_module_unadjusted(&mut self,
                                              module: ModuleOrUniformRoot<'a>,
                                              ident: Ident,
                                              ns: Namespace,
                                              restricted_shadowing: bool,
                                              record_used: bool,
                                              path_span: Span)
                                              -> Result<&'a NameBinding<'a>, Determinacy> {
        let module = match module {
            ModuleOrUniformRoot::Module(module) => module,
            ModuleOrUniformRoot::UniformRoot(root) => {
                // HACK(eddyb): `resolve_path` uses `keywords::Invalid` to indicate
                // paths of length 0, and currently these are relative `use` paths.
                let can_be_relative = !ident.is_path_segment_keyword() &&
                    root == keywords::Invalid.name();
                if can_be_relative {
                    // Relative paths should only get here if the feature-gate is on.
                    assert!(self.session.rust_2018() &&
                            self.session.features_untracked().uniform_paths);

                    // Try first to resolve relatively.
                    let mut ctxt = ident.span.ctxt().modern();
                    let self_module = self.resolve_self(&mut ctxt, self.current_module);

                    let binding = self.resolve_ident_in_module_unadjusted(
                        ModuleOrUniformRoot::Module(self_module),
                        ident,
                        ns,
                        restricted_shadowing,
                        record_used,
                        path_span,
                    );

                    // FIXME(eddyb) This may give false negatives, specifically
                    // if a crate with the same name is found in `extern_prelude`,
                    // preventing the check below this one from returning `binding`
                    // in all cases.
                    //
                    // That is, if there's no crate with the same name, `binding`
                    // is always returned, which is the result of doing the exact
                    // same lookup of `ident`, in the `self` module.
                    // But when a crate does exist, it will get chosen even when
                    // macro expansion could result in a success from the lookup
                    // in the `self` module, later on.
                    //
                    // NB. This is currently alleviated by the "ambiguity canaries"
                    // (see `is_uniform_paths_canary`) that get introduced for the
                    // maybe-relative imports handled here: if the false negative
                    // case were to arise, it would *also* cause an ambiguity error.
                    if binding.is_ok() {
                        return binding;
                    }

                    // Fall back to resolving to an external crate.
                    if !(
                        ns == TypeNS &&
                        !ident.is_path_segment_keyword() &&
                        self.extern_prelude.contains_key(&ident.modern())
                    ) {
                        // ... unless the crate name is not in the `extern_prelude`.
                        return binding;
                    }
                }

                let crate_root = if
                    ns == TypeNS &&
                    root != keywords::Extern.name() &&
                    (
                        ident.name == keywords::Crate.name() ||
                        ident.name == keywords::DollarCrate.name()
                    )
                {
                    self.resolve_crate_root(ident)
                } else if
                    ns == TypeNS &&
                    !ident.is_path_segment_keyword()
                {
                    if let Some(binding) = self.extern_prelude_get(ident, !record_used, false) {
                        let module = self.get_module(binding.def().def_id());
                        self.populate_module_if_necessary(module);
                        return Ok(binding);
                    } else {
                        return Err(Determined);
                    }
                } else {
                    return Err(Determined);
                };
                self.populate_module_if_necessary(crate_root);
                let binding = (crate_root, ty::Visibility::Public,
                               ident.span, Mark::root()).to_name_binding(self.arenas);
                return Ok(binding);
            }
        };

        self.populate_module_if_necessary(module);

        let resolution = self.resolution(module, ident, ns)
            .try_borrow_mut()
            .map_err(|_| Determined)?; // This happens when there is a cycle of imports

        if let Some(binding) = resolution.binding {
            if !restricted_shadowing && binding.expansion != Mark::root() {
                if let NameBindingKind::Def(_, true) = binding.kind {
                    self.macro_expanded_macro_export_errors.insert((path_span, binding.span));
                }
            }
        }

        if record_used {
            if let Some(binding) = resolution.binding {
                if let Some(shadowed_glob) = resolution.shadowed_glob {
                    // Forbid expanded shadowing to avoid time travel.
                    if restricted_shadowing &&
                       binding.expansion != Mark::root() &&
                       ns != MacroNS && // In MacroNS, `try_define` always forbids this shadowing
                       binding.def() != shadowed_glob.def() {
                        self.ambiguity_errors.push(AmbiguityError {
                            ident,
                            b1: binding,
                            b2: shadowed_glob,
                        });
                    }
                }
                if self.record_use(ident, ns, binding) {
                    return Ok(self.dummy_binding);
                }
                if !self.is_accessible(binding.vis) {
                    self.privacy_errors.push(PrivacyError(path_span, ident.name, binding));
                }
            }

            return resolution.binding.ok_or(Determined);
        }

        let check_usable = |this: &mut Self, binding: &'a NameBinding<'a>| {
            // `extern crate` are always usable for backwards compatibility, see issue #37020.
            let usable = this.is_accessible(binding.vis) || binding.is_extern_crate();
            if usable { Ok(binding) } else { Err(Determined) }
        };

        // Items and single imports are not shadowable, if we have one, then it's determined.
        if let Some(binding) = resolution.binding {
            if !binding.is_glob_import() {
                return check_usable(self, binding);
            }
        }

        // --- From now on we either have a glob resolution or no resolution. ---

        // Check if one of single imports can still define the name,
        // if it can then our result is not determined and can be invalidated.
        for single_import in &resolution.single_imports {
            if !self.is_accessible(single_import.vis.get()) {
                continue;
            }
            let module = unwrap_or!(single_import.imported_module.get(), return Err(Undetermined));
            let ident = match single_import.subclass {
                SingleImport { source, .. } => source,
                _ => unreachable!(),
            };
            match self.resolve_ident_in_module(module, ident, ns, false, path_span) {
                Err(Determined) => continue,
                Ok(binding) if !self.is_accessible_from(
                    binding.vis, single_import.parent_scope.module
                ) => continue,
                Ok(_) | Err(Undetermined) => return Err(Undetermined),
            }
        }

        // So we have a resolution that's from a glob import. This resolution is determined
        // if it cannot be shadowed by some new item/import expanded from a macro.
        // This happens either if there are no unexpanded macros, or expanded names cannot
        // shadow globs (that happens in macro namespace or with restricted shadowing).
        //
        // Additionally, any macro in any module can plant names in the root module if it creates
        // `macro_export` macros, so the root module effectively has unresolved invocations if any
        // module has unresolved invocations.
        // However, it causes resolution/expansion to stuck too often (#53144), so, to make
        // progress, we have to ignore those potential unresolved invocations from other modules
        // and prohibit access to macro-expanded `macro_export` macros instead (unless restricted
        // shadowing is enabled, see `macro_expanded_macro_export_errors`).
        let unexpanded_macros = !module.unresolved_invocations.borrow().is_empty();
        if let Some(binding) = resolution.binding {
            if !unexpanded_macros || ns == MacroNS || restricted_shadowing {
                return check_usable(self, binding);
            } else {
                return Err(Undetermined);
            }
        }

        // --- From now on we have no resolution. ---

        // Now we are in situation when new item/import can appear only from a glob or a macro
        // expansion. With restricted shadowing names from globs and macro expansions cannot
        // shadow names from outer scopes, so we can freely fallback from module search to search
        // in outer scopes. To continue search in outer scopes we have to lie a bit and return
        // `Determined` to `early_resolve_ident_in_lexical_scope` even if the correct answer
        // for in-module resolution could be `Undetermined`.
        if restricted_shadowing {
            return Err(Determined);
        }

        // Check if one of unexpanded macros can still define the name,
        // if it can then our "no resolution" result is not determined and can be invalidated.
        if unexpanded_macros {
            return Err(Undetermined);
        }

        // Check if one of glob imports can still define the name,
        // if it can then our "no resolution" result is not determined and can be invalidated.
        for glob_import in module.globs.borrow().iter() {
            if !self.is_accessible(glob_import.vis.get()) {
                continue
            }
            let module = match glob_import.imported_module.get() {
                Some(ModuleOrUniformRoot::Module(module)) => module,
                Some(ModuleOrUniformRoot::UniformRoot(_)) => continue,
                None => return Err(Undetermined),
            };
            let (orig_current_module, mut ident) = (self.current_module, ident.modern());
            match ident.span.glob_adjust(module.expansion, glob_import.span.ctxt().modern()) {
                Some(Some(def)) => self.current_module = self.macro_def_scope(def),
                Some(None) => {}
                None => continue,
            };
            let result = self.resolve_ident_in_module_unadjusted(
                ModuleOrUniformRoot::Module(module),
                ident,
                ns,
                false,
                false,
                path_span,
            );
            self.current_module = orig_current_module;

            match result {
                Err(Determined) => continue,
                Ok(binding) if !self.is_accessible_from(
                    binding.vis, glob_import.parent_scope.module
                ) => continue,
                Ok(_) | Err(Undetermined) => return Err(Undetermined),
            }
        }

        // No resolution and no one else can define the name - determinate error.
        Err(Determined)
    }

    // Add an import directive to the current module.
    pub fn add_import_directive(&mut self,
                                module_path: Vec<Segment>,
                                subclass: ImportDirectiveSubclass<'a>,
                                span: Span,
                                id: NodeId,
                                root_span: Span,
                                root_id: NodeId,
                                vis: ty::Visibility,
                                parent_scope: ParentScope<'a>,
                                is_uniform_paths_canary: bool) {
        let current_module = parent_scope.module;
        let directive = self.arenas.alloc_import_directive(ImportDirective {
            parent_scope,
            module_path,
            imported_module: Cell::new(None),
            subclass,
            span,
            id,
            root_span,
            root_id,
            vis: Cell::new(vis),
            used: Cell::new(false),
            is_uniform_paths_canary,
        });

        debug!("add_import_directive({:?})", directive);

        self.indeterminate_imports.push(directive);
        match directive.subclass {
            SingleImport { target, type_ns_only, .. } => {
                self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
                    let mut resolution = this.resolution(current_module, target, ns).borrow_mut();
                    resolution.single_imports.insert(PtrKey(directive));
                });
            }
            // We don't add prelude imports to the globs since they only affect lexical scopes,
            // which are not relevant to import resolution.
            GlobImport { is_prelude: true, .. } => {}
            GlobImport { .. } => current_module.globs.borrow_mut().push(directive),
            _ => unreachable!(),
        }
    }

    // Given a binding and an import directive that resolves to it,
    // return the corresponding binding defined by the import directive.
    pub fn import(&self, binding: &'a NameBinding<'a>, directive: &'a ImportDirective<'a>)
                  -> &'a NameBinding<'a> {
        let vis = if binding.pseudo_vis().is_at_least(directive.vis.get(), self) ||
                     // c.f. `PUB_USE_OF_PRIVATE_EXTERN_CRATE`
                     !directive.is_glob() && binding.is_extern_crate() {
            directive.vis.get()
        } else {
            binding.pseudo_vis()
        };

        if let GlobImport { ref max_vis, .. } = directive.subclass {
            if vis == directive.vis.get() || vis.is_at_least(max_vis.get(), self) {
                max_vis.set(vis)
            }
        }

        self.arenas.alloc_name_binding(NameBinding {
            kind: NameBindingKind::Import {
                binding,
                directive,
                used: Cell::new(false),
            },
            span: directive.span,
            vis,
            expansion: directive.parent_scope.expansion,
        })
    }

    crate fn check_reserved_macro_name(&self, ident: Ident, ns: Namespace) {
        // Reserve some names that are not quite covered by the general check
        // performed on `Resolver::builtin_attrs`.
        if ns == MacroNS &&
           (ident.name == "cfg" || ident.name == "cfg_attr" || ident.name == "derive") {
            self.session.span_err(ident.span,
                                  &format!("name `{}` is reserved in macro namespace", ident));
        }
    }

    // Define the name or return the existing binding if there is a collision.
    pub fn try_define(&mut self,
                      module: Module<'a>,
                      ident: Ident,
                      ns: Namespace,
                      binding: &'a NameBinding<'a>)
                      -> Result<(), &'a NameBinding<'a>> {
        self.check_reserved_macro_name(ident, ns);
        self.set_binding_parent_module(binding, module);
        self.update_resolution(module, ident, ns, |this, resolution| {
            if let Some(old_binding) = resolution.binding {
                if binding.is_glob_import() {
                    if !old_binding.is_glob_import() &&
                       !(ns == MacroNS && old_binding.expansion != Mark::root()) {
                        resolution.shadowed_glob = Some(binding);
                    } else if binding.def() != old_binding.def() {
                        resolution.binding = Some(this.ambiguity(old_binding, binding));
                    } else if !old_binding.vis.is_at_least(binding.vis, &*this) {
                        // We are glob-importing the same item but with greater visibility.
                        resolution.binding = Some(binding);
                    }
                } else if old_binding.is_glob_import() {
                    if ns == MacroNS && binding.expansion != Mark::root() &&
                       binding.def() != old_binding.def() {
                        resolution.binding = Some(this.ambiguity(binding, old_binding));
                    } else {
                        resolution.binding = Some(binding);
                        resolution.shadowed_glob = Some(old_binding);
                    }
                } else if let (&NameBindingKind::Def(_, true), &NameBindingKind::Def(_, true)) =
                        (&old_binding.kind, &binding.kind) {

                    this.session.buffer_lint_with_diagnostic(
                        DUPLICATE_MACRO_EXPORTS,
                        CRATE_NODE_ID,
                        binding.span,
                        &format!("a macro named `{}` has already been exported", ident),
                        BuiltinLintDiagnostics::DuplicatedMacroExports(
                            ident, old_binding.span, binding.span));

                    resolution.binding = Some(binding);
                } else {
                    return Err(old_binding);
                }
            } else {
                resolution.binding = Some(binding);
            }

            Ok(())
        })
    }

    pub fn ambiguity(&self, b1: &'a NameBinding<'a>, b2: &'a NameBinding<'a>)
                     -> &'a NameBinding<'a> {
        self.arenas.alloc_name_binding(NameBinding {
            kind: NameBindingKind::Ambiguity { b1, b2 },
            vis: if b1.vis.is_at_least(b2.vis, self) { b1.vis } else { b2.vis },
            span: b1.span,
            expansion: Mark::root(),
        })
    }

    // Use `f` to mutate the resolution of the name in the module.
    // If the resolution becomes a success, define it in the module's glob importers.
    fn update_resolution<T, F>(&mut self, module: Module<'a>, ident: Ident, ns: Namespace, f: F)
                               -> T
        where F: FnOnce(&mut Resolver<'a, 'crateloader>, &mut NameResolution<'a>) -> T
    {
        // Ensure that `resolution` isn't borrowed when defining in the module's glob importers,
        // during which the resolution might end up getting re-defined via a glob cycle.
        let (binding, t) = {
            let resolution = &mut *self.resolution(module, ident, ns).borrow_mut();
            let old_binding = resolution.binding();

            let t = f(self, resolution);

            match resolution.binding() {
                _ if old_binding.is_some() => return t,
                None => return t,
                Some(binding) => match old_binding {
                    Some(old_binding) if ptr::eq(old_binding, binding) => return t,
                    _ => (binding, t),
                }
            }
        };

        // Define `binding` in `module`s glob importers.
        for directive in module.glob_importers.borrow_mut().iter() {
            let mut ident = ident.modern();
            let scope = match ident.span.reverse_glob_adjust(module.expansion,
                                                             directive.span.ctxt().modern()) {
                Some(Some(def)) => self.macro_def_scope(def),
                Some(None) => directive.parent_scope.module,
                None => continue,
            };
            if self.is_accessible_from(binding.vis, scope) {
                let imported_binding = self.import(binding, directive);
                let _ = self.try_define(directive.parent_scope.module, ident, ns, imported_binding);
            }
        }

        t
    }

    // Define a "dummy" resolution containing a Def::Err as a placeholder for a
    // failed resolution
    fn import_dummy_binding(&mut self, directive: &'a ImportDirective<'a>) {
        if let SingleImport { target, .. } = directive.subclass {
            let dummy_binding = self.dummy_binding;
            let dummy_binding = self.import(dummy_binding, directive);
            self.per_ns(|this, ns| {
                let _ = this.try_define(directive.parent_scope.module, target, ns, dummy_binding);
            });
        }
    }
}

pub struct ImportResolver<'a, 'b: 'a, 'c: 'a + 'b> {
    pub resolver: &'a mut Resolver<'b, 'c>,
}

impl<'a, 'b: 'a, 'c: 'a + 'b> ::std::ops::Deref for ImportResolver<'a, 'b, 'c> {
    type Target = Resolver<'b, 'c>;
    fn deref(&self) -> &Resolver<'b, 'c> {
        self.resolver
    }
}

impl<'a, 'b: 'a, 'c: 'a + 'b> ::std::ops::DerefMut for ImportResolver<'a, 'b, 'c> {
    fn deref_mut(&mut self) -> &mut Resolver<'b, 'c> {
        self.resolver
    }
}

impl<'a, 'b: 'a, 'c: 'a + 'b> ty::DefIdTree for &'a ImportResolver<'a, 'b, 'c> {
    fn parent(self, id: DefId) -> Option<DefId> {
        self.resolver.parent(id)
    }
}

impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
    // Import resolution
    //
    // This is a fixed-point algorithm. We resolve imports until our efforts
    // are stymied by an unresolved import; then we bail out of the current
    // module and continue. We terminate successfully once no more imports
    // remain or unsuccessfully when no forward progress in resolving imports
    // is made.

    /// Resolves all imports for the crate. This method performs the fixed-
    /// point iteration.
    pub fn resolve_imports(&mut self) {
        let mut prev_num_indeterminates = self.indeterminate_imports.len() + 1;
        while self.indeterminate_imports.len() < prev_num_indeterminates {
            prev_num_indeterminates = self.indeterminate_imports.len();
            for import in mem::replace(&mut self.indeterminate_imports, Vec::new()) {
                match self.resolve_import(&import) {
                    true => self.determined_imports.push(import),
                    false => self.indeterminate_imports.push(import),
                }
            }
        }
    }

    pub fn finalize_imports(&mut self) {
        for module in self.arenas.local_modules().iter() {
            self.finalize_resolutions_in(module);
        }

        struct UniformPathsCanaryResults<'a> {
            name: Name,
            module_scope: Option<&'a NameBinding<'a>>,
            block_scopes: Vec<&'a NameBinding<'a>>,
        }

        // Collect all tripped `uniform_paths` canaries separately.
        let mut uniform_paths_canaries: BTreeMap<
            (Span, NodeId, Namespace),
            UniformPathsCanaryResults,
        > = BTreeMap::new();

        let mut errors = false;
        let mut seen_spans = FxHashSet::default();
        let mut error_vec = Vec::new();
        let mut prev_root_id: NodeId = NodeId::new(0);
        for i in 0 .. self.determined_imports.len() {
            let import = self.determined_imports[i];
            let error = self.finalize_import(import);

            // For a `#![feature(uniform_paths)]` `use self::x as _` canary,
            // failure is ignored, while success may cause an ambiguity error.
            if import.is_uniform_paths_canary {
                if error.is_some() {
                    continue;
                }

                let (name, result) = match import.subclass {
                    SingleImport { source, ref result, .. } => (source.name, result),
                    _ => bug!(),
                };

                let has_explicit_self =
                    !import.module_path.is_empty() &&
                    import.module_path[0].ident.name == keywords::SelfValue.name();

                self.per_ns(|_, ns| {
                    if let Some(result) = result[ns].get().ok() {
                        let canary_results =
                            uniform_paths_canaries.entry((import.span, import.id, ns))
                                .or_insert(UniformPathsCanaryResults {
                                    name,
                                    module_scope: None,
                                    block_scopes: vec![],
                                });

                        // All the canaries with the same `id` should have the same `name`.
                        assert_eq!(canary_results.name, name);

                        if has_explicit_self {
                            // There should only be one `self::x` (module-scoped) canary.
                            assert!(canary_results.module_scope.is_none());
                            canary_results.module_scope = Some(result);
                        } else {
                            canary_results.block_scopes.push(result);
                        }
                    }
                });
            } else if let Some((span, err)) = error {
                errors = true;

                if let SingleImport { source, ref result, .. } = import.subclass {
                    if source.name == "self" {
                        // Silence `unresolved import` error if E0429 is already emitted
                        if let Err(Determined) = result.value_ns.get() {
                            continue;
                        }
                    }
                }

                // If the error is a single failed import then create a "fake" import
                // resolution for it so that later resolve stages won't complain.
                self.import_dummy_binding(import);
                if prev_root_id.as_u32() != 0 &&
                    prev_root_id.as_u32() != import.root_id.as_u32() &&
                    !error_vec.is_empty(){
                    // in case of new import line, throw diagnostic message
                    // for previous line.
                    let mut empty_vec = vec![];
                    mem::swap(&mut empty_vec, &mut error_vec);
                    self.throw_unresolved_import_error(empty_vec, None);
                }
                if !seen_spans.contains(&span) {
                    let path = import_path_to_string(
                        &import.module_path.iter().map(|seg| seg.ident).collect::<Vec<_>>(),
                        &import.subclass,
                        span,
                    );
                    error_vec.push((span, path, err));
                    seen_spans.insert(span);
                    prev_root_id = import.root_id;
                }
            }
        }

        let uniform_paths_feature = self.session.features_untracked().uniform_paths;
        for ((span, _, ns), results) in uniform_paths_canaries {
            let name = results.name;
            let external_crate = if ns == TypeNS {
                self.extern_prelude_get(Ident::with_empty_ctxt(name), true, false)
                    .map(|binding| binding.def())
            } else {
                None
            };

            // Currently imports can't resolve in non-module scopes,
            // we only have canaries in them for future-proofing.
            if external_crate.is_none() && results.module_scope.is_none() {
                continue;
            }

            {
                let mut all_results = external_crate.into_iter().chain(
                    results.module_scope.iter()
                        .chain(&results.block_scopes)
                        .map(|binding| binding.def())
                );
                let first = all_results.next().unwrap();

                // An ambiguity requires more than one *distinct* possible resolution.
                let possible_resultions =
                    1 + all_results.filter(|&def| def != first).count();
                if possible_resultions <= 1 {
                    continue;
                }
            }

            errors = true;

            let msg = format!("`{}` import is ambiguous", name);
            let mut err = self.session.struct_span_err(span, &msg);
            let mut suggestion_choices = String::new();
            if external_crate.is_some() {
                write!(suggestion_choices, "`::{}`", name);
                err.span_label(span,
                    format!("can refer to external crate `::{}`", name));
            }
            if let Some(result) = results.module_scope {
                if !suggestion_choices.is_empty() {
                    suggestion_choices.push_str(" or ");
                }
                write!(suggestion_choices, "`self::{}`", name);
                if uniform_paths_feature {
                    err.span_label(result.span,
                        format!("can refer to `self::{}`", name));
                } else {
                    err.span_label(result.span,
                        format!("may refer to `self::{}` in the future", name));
                }
            }
            for result in results.block_scopes {
                err.span_label(result.span,
                    format!("shadowed by block-scoped `{}`", name));
            }
            err.help(&format!("write {} explicitly instead", suggestion_choices));
            if uniform_paths_feature {
                err.note("relative `use` paths enabled by `#![feature(uniform_paths)]`");
            } else {
                err.note("in the future, `#![feature(uniform_paths)]` may become the default");
            }
            err.emit();
        }

        if !error_vec.is_empty() {
            self.throw_unresolved_import_error(error_vec.clone(), None);
        }

        // Report unresolved imports only if no hard error was already reported
        // to avoid generating multiple errors on the same import.
        if !errors {
            for import in &self.indeterminate_imports {
                if import.is_uniform_paths_canary {
                    continue;
                }
                self.throw_unresolved_import_error(error_vec, Some(MultiSpan::from(import.span)));
                break;
            }
        }
    }

    fn throw_unresolved_import_error(&self, error_vec: Vec<(Span, String, String)>,
                                     span: Option<MultiSpan>) {
        let max_span_label_msg_count = 10;  // upper limit on number of span_label message.
        let (span, msg) = if error_vec.is_empty() {
            (span.unwrap(), "unresolved import".to_string())
        } else {
            let span = MultiSpan::from_spans(error_vec.clone().into_iter()
                .map(|elem: (Span, String, String)| { elem.0 })
                .collect());
            let path_vec: Vec<String> = error_vec.clone().into_iter()
                .map(|elem: (Span, String, String)| { format!("`{}`", elem.1) })
                .collect();
            let path = path_vec.join(", ");
            let msg = format!("unresolved import{} {}",
                if path_vec.len() > 1 { "s" } else { "" }, path);
            (span, msg)
        };
        let mut err = struct_span_err!(self.resolver.session, span, E0432, "{}", &msg);
        for span_error in error_vec.into_iter().take(max_span_label_msg_count) {
            err.span_label(span_error.0, span_error.2);
        }
        err.emit();
    }

    /// Attempts to resolve the given import, returning true if its resolution is determined.
    /// If successful, the resolved bindings are written into the module.
    fn resolve_import(&mut self, directive: &'b ImportDirective<'b>) -> bool {
        debug!("(resolving import for module) resolving import `{}::...` in `{}`",
               Segment::names_to_string(&directive.module_path[..]),
               module_to_string(self.current_module).unwrap_or_else(|| "???".to_string()));

        self.current_module = directive.parent_scope.module;

        let module = if let Some(module) = directive.imported_module.get() {
            module
        } else {
            let vis = directive.vis.get();
            // For better failure detection, pretend that the import will not define any names
            // while resolving its module path.
            directive.vis.set(ty::Visibility::Invisible);
            let result = self.resolve_path(
                Some(if directive.is_uniform_paths_canary {
                    ModuleOrUniformRoot::Module(directive.parent_scope.module)
                } else {
                    ModuleOrUniformRoot::UniformRoot(keywords::Invalid.name())
                }),
                &directive.module_path[..],
                None,
                false,
                directive.span,
                directive.crate_lint(),
            );
            directive.vis.set(vis);

            match result {
                PathResult::Module(module) => module,
                PathResult::Indeterminate => return false,
                _ => return true,
            }
        };

        directive.imported_module.set(Some(module));
        let (source, target, result, type_ns_only) = match directive.subclass {
            SingleImport { source, target, ref result, type_ns_only } =>
                (source, target, result, type_ns_only),
            GlobImport { .. } => {
                self.resolve_glob_import(directive);
                return true;
            }
            _ => unreachable!(),
        };

        let mut indeterminate = false;
        self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
            if let Err(Undetermined) = result[ns].get() {
                result[ns].set(this.resolve_ident_in_module(module,
                                                            source,
                                                            ns,
                                                            false,
                                                            directive.span));
            } else {
                return
            };

            let parent = directive.parent_scope.module;
            match result[ns].get() {
                Err(Undetermined) => indeterminate = true,
                Err(Determined) => {
                    this.update_resolution(parent, target, ns, |_, resolution| {
                        resolution.single_imports.remove(&PtrKey(directive));
                    });
                }
                Ok(binding) if !binding.is_importable() => {
                    let msg = format!("`{}` is not directly importable", target);
                    struct_span_err!(this.session, directive.span, E0253, "{}", &msg)
                        .span_label(directive.span, "cannot be imported directly")
                        .emit();
                    // Do not import this illegal binding. Import a dummy binding and pretend
                    // everything is fine
                    this.import_dummy_binding(directive);
                }
                Ok(binding) => {
                    let imported_binding = this.import(binding, directive);
                    let conflict = this.try_define(parent, target, ns, imported_binding);
                    if let Err(old_binding) = conflict {
                        this.report_conflict(parent, target, ns, imported_binding, old_binding);
                    }
                }
            }
        });

        !indeterminate
    }

    // If appropriate, returns an error to report.
    fn finalize_import(&mut self, directive: &'b ImportDirective<'b>) -> Option<(Span, String)> {
        self.current_module = directive.parent_scope.module;
        let ImportDirective { ref module_path, span, .. } = *directive;

        let module_result = self.resolve_path(
            Some(if directive.is_uniform_paths_canary {
                ModuleOrUniformRoot::Module(directive.parent_scope.module)
            } else {
                ModuleOrUniformRoot::UniformRoot(keywords::Invalid.name())
            }),
            &module_path,
            None,
            true,
            span,
            directive.crate_lint(),
        );
        let module = match module_result {
            PathResult::Module(module) => module,
            PathResult::Failed(span, msg, false) => {
                resolve_error(self, span, ResolutionError::FailedToResolve(&msg));
                return None;
            }
            PathResult::Failed(span, msg, true) => {
                return if let Some(suggested_path) = self.make_path_suggestion(
                    span, module_path.clone()
                ) {
                    Some((
                        span,
                        format!("Did you mean `{}`?", Segment::names_to_string(&suggested_path))
                    ))
                } else {
                    Some((span, msg))
                };
            },
            _ => return None,
        };

        let (ident, result, type_ns_only) = match directive.subclass {
            SingleImport { source, ref result, type_ns_only, .. } => (source, result, type_ns_only),
            GlobImport { is_prelude, ref max_vis } => {
                if module_path.len() <= 1 {
                    // HACK(eddyb) `lint_if_path_starts_with_module` needs at least
                    // 2 segments, so the `resolve_path` above won't trigger it.
                    let mut full_path = module_path.clone();
                    full_path.push(Segment::from_ident(keywords::Invalid.ident()));
                    self.lint_if_path_starts_with_module(
                        directive.crate_lint(),
                        &full_path,
                        directive.span,
                        None,
                    );
                }

                if let ModuleOrUniformRoot::Module(module) = module {
                    if module.def_id() == directive.parent_scope.module.def_id() {
                        // Importing a module into itself is not allowed.
                        return Some((directive.span,
                            "Cannot glob-import a module into itself.".to_string()));
                    }
                }
                if !is_prelude &&
                   max_vis.get() != ty::Visibility::Invisible && // Allow empty globs.
                   !max_vis.get().is_at_least(directive.vis.get(), &*self) {
                    let msg = "A non-empty glob must import something with the glob's visibility";
                    self.session.span_err(directive.span, msg);
                }
                return None;
            }
            _ => unreachable!(),
        };

        // Do not record uses from canaries, to avoid interfering with other
        // diagnostics or suggestions that rely on some items not being used.
        let record_used = !directive.is_uniform_paths_canary;

        let mut all_ns_err = true;
        self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
            if let Ok(binding) = result[ns].get() {
                all_ns_err = false;
                if record_used && this.record_use(ident, ns, binding) {
                    if let ModuleOrUniformRoot::Module(module) = module {
                        this.resolution(module, ident, ns).borrow_mut().binding =
                            Some(this.dummy_binding);
                    }
                }
                if record_used && ns == TypeNS {
                    if let ModuleOrUniformRoot::UniformRoot(..) = module {
                        // Make sure single-segment import is resolved non-speculatively
                        // at least once to report the feature error.
                        this.extern_prelude_get(ident, false, false);
                    }
                }
            }
        });

        if all_ns_err {
            let mut all_ns_failed = true;
            self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
                if this.resolve_ident_in_module(module, ident, ns, record_used, span).is_ok() {
                    all_ns_failed = false;
                }
            });

            return if all_ns_failed {
                let resolutions = match module {
                    ModuleOrUniformRoot::Module(module) =>
                        Some(module.resolutions.borrow()),
                    ModuleOrUniformRoot::UniformRoot(_) => None,
                };
                let resolutions = resolutions.as_ref().into_iter().flat_map(|r| r.iter());
                let names = resolutions.filter_map(|(&(ref i, _), resolution)| {
                    if *i == ident { return None; } // Never suggest the same name
                    match *resolution.borrow() {
                        NameResolution { binding: Some(name_binding), .. } => {
                            match name_binding.kind {
                                NameBindingKind::Import { binding, .. } => {
                                    match binding.kind {
                                        // Never suggest the name that has binding error
                                        // i.e. the name that cannot be previously resolved
                                        NameBindingKind::Def(Def::Err, _) => return None,
                                        _ => Some(&i.name),
                                    }
                                },
                                _ => Some(&i.name),
                            }
                        },
                        NameResolution { ref single_imports, .. }
                            if single_imports.is_empty() => None,
                        _ => Some(&i.name),
                    }
                });
                let lev_suggestion =
                    match find_best_match_for_name(names, &ident.as_str(), None) {
                        Some(name) => format!(". Did you mean to use `{}`?", name),
                        None => String::new(),
                    };
                let msg = match module {
                    ModuleOrUniformRoot::Module(module) => {
                        let module_str = module_to_string(module);
                        if let Some(module_str) = module_str {
                            format!("no `{}` in `{}`{}", ident, module_str, lev_suggestion)
                        } else {
                            format!("no `{}` in the root{}", ident, lev_suggestion)
                        }
                    }
                    ModuleOrUniformRoot::UniformRoot(_) => {
                        if !ident.is_path_segment_keyword() {
                            format!("no `{}` external crate{}", ident, lev_suggestion)
                        } else {
                            // HACK(eddyb) this shows up for `self` & `super`, which
                            // should work instead - for now keep the same error message.
                            format!("no `{}` in the root{}", ident, lev_suggestion)
                        }
                    }
                };
                Some((span, msg))
            } else {
                // `resolve_ident_in_module` reported a privacy error.
                self.import_dummy_binding(directive);
                None
            }
        }

        let mut reexport_error = None;
        let mut any_successful_reexport = false;
        self.per_ns(|this, ns| {
            if let Ok(binding) = result[ns].get() {
                let vis = directive.vis.get();
                if !binding.pseudo_vis().is_at_least(vis, &*this) {
                    reexport_error = Some((ns, binding));
                } else {
                    any_successful_reexport = true;
                }
            }
        });

        // All namespaces must be re-exported with extra visibility for an error to occur.
        if !any_successful_reexport {
            let (ns, binding) = reexport_error.unwrap();
            if ns == TypeNS && binding.is_extern_crate() {
                let msg = format!("extern crate `{}` is private, and cannot be \
                                   re-exported (error E0365), consider declaring with \
                                   `pub`",
                                   ident);
                self.session.buffer_lint(PUB_USE_OF_PRIVATE_EXTERN_CRATE,
                                         directive.id,
                                         directive.span,
                                         &msg);
            } else if ns == TypeNS {
                struct_span_err!(self.session, directive.span, E0365,
                                 "`{}` is private, and cannot be re-exported", ident)
                    .span_label(directive.span, format!("re-export of private `{}`", ident))
                    .note(&format!("consider declaring type or module `{}` with `pub`", ident))
                    .emit();
            } else {
                let msg = format!("`{}` is private, and cannot be re-exported", ident);
                let note_msg =
                    format!("consider marking `{}` as `pub` in the imported module", ident);
                struct_span_err!(self.session, directive.span, E0364, "{}", &msg)
                    .span_note(directive.span, &note_msg)
                    .emit();
            }
        }

        if module_path.len() <= 1 {
            // HACK(eddyb) `lint_if_path_starts_with_module` needs at least
            // 2 segments, so the `resolve_path` above won't trigger it.
            let mut full_path = module_path.clone();
            full_path.push(Segment::from_ident(ident));
            self.per_ns(|this, ns| {
                if let Ok(binding) = result[ns].get() {
                    this.lint_if_path_starts_with_module(
                        directive.crate_lint(),
                        &full_path,
                        directive.span,
                        Some(binding),
                    );
                }
            });
        }

        // Record what this import resolves to for later uses in documentation,
        // this may resolve to either a value or a type, but for documentation
        // purposes it's good enough to just favor one over the other.
        self.per_ns(|this, ns| if let Some(binding) = result[ns].get().ok() {
            let import = this.import_map.entry(directive.id).or_default();
            import[ns] = Some(PathResolution::new(binding.def()));
        });

        debug!("(resolving single import) successfully resolved import");
        None
    }

    fn resolve_glob_import(&mut self, directive: &'b ImportDirective<'b>) {
        let module = match directive.imported_module.get().unwrap() {
            ModuleOrUniformRoot::Module(module) => module,
            ModuleOrUniformRoot::UniformRoot(_) => {
                self.session.span_err(directive.span,
                    "cannot glob-import all possible crates");
                return;
            }
        };

        self.populate_module_if_necessary(module);

        if let Some(Def::Trait(_)) = module.def() {
            self.session.span_err(directive.span, "items in traits are not importable.");
            return;
        } else if module.def_id() == directive.parent_scope.module.def_id()  {
            return;
        } else if let GlobImport { is_prelude: true, .. } = directive.subclass {
            self.prelude = Some(module);
            return;
        }

        // Add to module's glob_importers
        module.glob_importers.borrow_mut().push(directive);

        // Ensure that `resolutions` isn't borrowed during `try_define`,
        // since it might get updated via a glob cycle.
        let bindings = module.resolutions.borrow().iter().filter_map(|(&ident, resolution)| {
            resolution.borrow().binding().map(|binding| (ident, binding))
        }).collect::<Vec<_>>();
        for ((mut ident, ns), binding) in bindings {
            let scope = match ident.span.reverse_glob_adjust(module.expansion,
                                                             directive.span.ctxt().modern()) {
                Some(Some(def)) => self.macro_def_scope(def),
                Some(None) => self.current_module,
                None => continue,
            };
            if self.is_accessible_from(binding.pseudo_vis(), scope) {
                let imported_binding = self.import(binding, directive);
                let _ = self.try_define(directive.parent_scope.module, ident, ns, imported_binding);
            }
        }

        // Record the destination of this import
        self.record_def(directive.id, PathResolution::new(module.def().unwrap()));
    }

    // Miscellaneous post-processing, including recording re-exports,
    // reporting conflicts, and reporting unresolved imports.
    fn finalize_resolutions_in(&mut self, module: Module<'b>) {
        // Since import resolution is finished, globs will not define any more names.
        *module.globs.borrow_mut() = Vec::new();

        let mut reexports = Vec::new();

        for (&(ident, ns), resolution) in module.resolutions.borrow().iter() {
            let resolution = &mut *resolution.borrow_mut();
            let binding = match resolution.binding {
                Some(binding) => binding,
                None => continue,
            };

            // Don't reexport `uniform_path` canaries.
            let non_canary_import = match binding.kind {
                NameBindingKind::Import { directive, .. } => {
                    !directive.is_uniform_paths_canary
                }
                _ => false,
            };

            if non_canary_import || binding.is_macro_def() {
                let def = binding.def();
                if def != Def::Err {
                    if !def.def_id().is_local() {
                        self.cstore.export_macros_untracked(def.def_id().krate);
                    }
                    reexports.push(Export {
                        ident: ident.modern(),
                        def: def,
                        span: binding.span,
                        vis: binding.vis,
                    });
                }
            }

            if let NameBindingKind::Import { binding: orig_binding, directive, .. } = binding.kind {
                if ns == TypeNS && orig_binding.is_variant() &&
                    !orig_binding.vis.is_at_least(binding.vis, &*self) {
                        let msg = match directive.subclass {
                            ImportDirectiveSubclass::SingleImport { .. } => {
                                format!("variant `{}` is private and cannot be re-exported",
                                        ident)
                            },
                            ImportDirectiveSubclass::GlobImport { .. } => {
                                let msg = "enum is private and its variants \
                                           cannot be re-exported".to_owned();
                                let error_id = (DiagnosticMessageId::ErrorId(0), // no code?!
                                                Some(binding.span),
                                                msg.clone());
                                let fresh = self.session.one_time_diagnostics
                                    .borrow_mut().insert(error_id);
                                if !fresh {
                                    continue;
                                }
                                msg
                            },
                            ref s @ _ => bug!("unexpected import subclass {:?}", s)
                        };
                        let mut err = self.session.struct_span_err(binding.span, &msg);

                        let imported_module = match directive.imported_module.get() {
                            Some(ModuleOrUniformRoot::Module(module)) => module,
                            _ => bug!("module should exist"),
                        };
                        let resolutions = imported_module.parent.expect("parent should exist")
                            .resolutions.borrow();
                        let enum_path_segment_index = directive.module_path.len() - 1;
                        let enum_ident = directive.module_path[enum_path_segment_index].ident;

                        let enum_resolution = resolutions.get(&(enum_ident, TypeNS))
                            .expect("resolution should exist");
                        let enum_span = enum_resolution.borrow()
                            .binding.expect("binding should exist")
                            .span;
                        let enum_def_span = self.session.source_map().def_span(enum_span);
                        let enum_def_snippet = self.session.source_map()
                            .span_to_snippet(enum_def_span).expect("snippet should exist");
                        // potentially need to strip extant `crate`/`pub(path)` for suggestion
                        let after_vis_index = enum_def_snippet.find("enum")
                            .expect("`enum` keyword should exist in snippet");
                        let suggestion = format!("pub {}",
                                                 &enum_def_snippet[after_vis_index..]);

                        self.session
                            .diag_span_suggestion_once(&mut err,
                                                       DiagnosticMessageId::ErrorId(0),
                                                       enum_def_span,
                                                       "consider making the enum public",
                                                       suggestion);
                        err.emit();
                }
            }
        }

        if reexports.len() > 0 {
            if let Some(def_id) = module.def_id() {
                self.export_map.insert(def_id, reexports);
            }
        }
    }
}

fn import_path_to_string(names: &[Ident],
                         subclass: &ImportDirectiveSubclass,
                         span: Span) -> String {
    let pos = names.iter()
        .position(|p| span == p.span && p.name != keywords::CrateRoot.name());
    let global = !names.is_empty() && names[0].name == keywords::CrateRoot.name();
    if let Some(pos) = pos {
        let names = if global { &names[1..pos + 1] } else { &names[..pos + 1] };
        names_to_string(names)
    } else {
        let names = if global { &names[1..] } else { names };
        if names.is_empty() {
            import_directive_subclass_to_string(subclass)
        } else {
            format!("{}::{}",
                    names_to_string(names),
                    import_directive_subclass_to_string(subclass))
        }
    }
}

fn import_directive_subclass_to_string(subclass: &ImportDirectiveSubclass) -> String {
    match *subclass {
        SingleImport { source, .. } => source.to_string(),
        GlobImport { .. } => "*".to_string(),
        ExternCrate { .. } => "<extern crate>".to_string(),
        MacroUse => "#[macro_use]".to_string(),
    }
}