Auto merge of #35894 - jseyfried:new_import_semantics, r=nrc

Implement RFC 1560 behind `#![feature(item_like_imports)]`

This implements rust-lang/rfcs#1560 (cc #35120) behind the `item_like_imports` feature gate.

The [RFC text](https://github.com/rust-lang/rfcs/blob/master/text/1560-name-resolution.md#changes-to-name-resolution-rules) describes the changes to name resolution enabled by `#![feature(item_like_imports)` in detail. To summarize,
 - Items and named imports shadow glob imports.
 - Multiple globs can import the same name if the name is unused or the imports are shadowed.
 - Multiple globs can import the same name if the imports are of the same item (following re-exports).
  - The visibility of such a name is the maximum visibility of the imports.
  - Equivalently, adding a glob import will never reduce the visibility of a name, nor will removing one increase it.
 - Non-prelude private imports can be used wherever we currently allow private items to be used.
  - Prelude-imported names are unaffected, i.e. they continue to be usable only in lexical scopes.
 - Globs import all visible names, not just public names.
  - Equivalently, glob importing from an ancestor module imports all of the ancestor's names, and glob importing from other modules is unchanged.

r? @nrc

Author: bors

src/librustc_resolve/build_reduced_graph.rs

@@ -26,6 +26,8 @@ use rustc::hir::def::*;
 use rustc::hir::def_id::{CRATE_DEF_INDEX, DefId};
 use rustc::ty::{self, VariantKind};
 
+use std::cell::Cell;
+
 use syntax::ast::Name;
 use syntax::attr;
 use syntax::parse::token;
@@ -81,7 +83,6 @@ impl<'b> Resolver<'b> {
     /// Constructs the reduced graph for one item.
     fn build_reduced_graph_for_item(&mut self, item: &Item) {
         let parent = self.current_module;
-        let parent_vis = self.current_vis;
         let name = item.ident.name;
         let sp = item.span;
         let vis = self.resolve_visibility(&item.vis);
@@ -177,7 +178,10 @@ impl<'b> Resolver<'b> {
                         }
                     }
                     ViewPathGlob(_) => {
-                        let subclass = GlobImport { is_prelude: is_prelude };
+                        let subclass = GlobImport {
+                            is_prelude: is_prelude,
+                            max_vis: Cell::new(ty::Visibility::PrivateExternal),
+                        };
                         let span = view_path.span;
                         self.add_import_directive(module_path, subclass, span, item.id, vis);
                     }
@@ -204,7 +208,7 @@ impl<'b> Resolver<'b> {
             ItemKind::Mod(..) => {
                 let parent_link = ModuleParentLink(parent, name);
                 let def = Def::Mod(self.definitions.local_def_id(item.id));
-                let module = self.new_module(parent_link, Some(def), false);
+                let module = self.new_module(parent_link, Some(def), Some(item.id));
                 module.no_implicit_prelude.set({
                     parent.no_implicit_prelude.get() ||
                         attr::contains_name(&item.attrs, "no_implicit_prelude")
@@ -214,7 +218,6 @@ impl<'b> Resolver<'b> {
 
                 // Descend into the module.
                 self.current_module = module;
-                self.current_vis = ty::Visibility::Restricted(item.id);
             }
 
             ItemKind::ForeignMod(..) => {}
@@ -243,7 +246,7 @@ impl<'b> Resolver<'b> {
             ItemKind::Enum(ref enum_definition, _) => {
                 let parent_link = ModuleParentLink(parent, name);
                 let def = Def::Enum(self.definitions.local_def_id(item.id));
-                let module = self.new_module(parent_link, Some(def), false);
+                let module = self.new_module(parent_link, Some(def), parent.normal_ancestor_id);
                 self.define(parent, name, TypeNS, (module, sp, vis));
 
                 for variant in &(*enum_definition).variants {
@@ -285,7 +288,8 @@ impl<'b> Resolver<'b> {
                 // Add all the items within to a new module.
                 let parent_link = ModuleParentLink(parent, name);
                 let def = Def::Trait(def_id);
-                let module_parent = self.new_module(parent_link, Some(def), false);
+                let module_parent =
+                    self.new_module(parent_link, Some(def), parent.normal_ancestor_id);
                 self.define(parent, name, TypeNS, (module_parent, sp, vis));
 
                 // Add the names of all the items to the trait info.
@@ -312,7 +316,6 @@ impl<'b> Resolver<'b> {
 
         visit::walk_item(&mut BuildReducedGraphVisitor { resolver: self }, item);
         self.current_module = parent;
-        self.current_vis = parent_vis;
     }
 
     // Constructs the reduced graph for one variant. Variants exist in the
@@ -363,7 +366,7 @@ impl<'b> Resolver<'b> {
                    block_id);
 
             let parent_link = BlockParentLink(parent, block_id);
-            let new_module = self.new_module(parent_link, None, false);
+            let new_module = self.new_module(parent_link, None, parent.normal_ancestor_id);
             self.module_map.insert(block_id, new_module);
             self.current_module = new_module; // Descend into the block.
         }
@@ -395,7 +398,7 @@ impl<'b> Resolver<'b> {
                 debug!("(building reduced graph for external crate) building module {} {:?}",
                        name, vis);
                 let parent_link = ModuleParentLink(parent, name);
-                let module = self.new_module(parent_link, Some(def), true);
+                let module = self.new_module(parent_link, Some(def), None);
                 let _ = self.try_define(parent, name, TypeNS, (module, DUMMY_SP, vis));
             }
             Def::Variant(_, variant_id) => {
@@ -437,7 +440,7 @@ impl<'b> Resolver<'b> {
                 }
 
                 let parent_link = ModuleParentLink(parent, name);
-                let module = self.new_module(parent_link, Some(def), true);
+                let module = self.new_module(parent_link, Some(def), None);
                 let _ = self.try_define(parent, name, TypeNS, (module, DUMMY_SP, vis));
             }
             Def::TyAlias(..) | Def::AssociatedTy(..) => {