Resolve absolute paths as extern under a feature flag

src/librustc/middle/cstore.rs

@@ -357,6 +357,7 @@ impl CrateStore for DummyCrateStore {
 pub trait CrateLoader {
     fn process_item(&mut self, item: &ast::Item, defs: &Definitions);
     fn postprocess(&mut self, krate: &ast::Crate);
+    fn resolve_crate_from_path(&mut self, name: Symbol, span: Span) -> CrateNum;
 }
 
 // This method is used when generating the command line to pass through to

src/librustc_metadata/creader.rs

@@ -71,14 +71,6 @@ fn dump_crates(cstore: &CStore) {
     });
 }
 
-#[derive(Debug)]
-struct ExternCrateInfo {
-    ident: Symbol,
-    name: Symbol,
-    id: ast::NodeId,
-    dep_kind: DepKind,
-}
-
 // Extra info about a crate loaded for plugins or exported macros.
 struct ExtensionCrate {
     metadata: PMDSource,
@@ -117,34 +109,6 @@ impl<'a> CrateLoader<'a> {
         }
     }
 
-    fn extract_crate_info(&self, i: &ast::Item) -> Option<ExternCrateInfo> {
-        match i.node {
-            ast::ItemKind::ExternCrate(ref path_opt) => {
-                debug!("resolving extern crate stmt. ident: {} path_opt: {:?}",
-                       i.ident, path_opt);
-                let name = match *path_opt {
-                    Some(name) => {
-                        validate_crate_name(Some(self.sess), &name.as_str(),
-                                            Some(i.span));
-                        name
-                    }
-                    None => i.ident.name,
-                };
-                Some(ExternCrateInfo {
-                    ident: i.ident.name,
-                    name,
-                    id: i.id,
-                    dep_kind: if attr::contains_name(&i.attrs, "no_link") {
-                        DepKind::UnexportedMacrosOnly
-                    } else {
-                        DepKind::Explicit
-                    },
-                })
-            }
-            _ => None
-        }
-    }
-
     fn existing_match(&self, name: Symbol, hash: Option<&Svh>, kind: PathKind)
                       -> Option<CrateNum> {
         let mut ret = None;
@@ -478,17 +442,17 @@ impl<'a> CrateLoader<'a> {
         })).collect()
     }
 
-    fn read_extension_crate(&mut self, span: Span, info: &ExternCrateInfo) -> ExtensionCrate {
-        info!("read extension crate {} `extern crate {} as {}` dep_kind={:?}",
-              info.id, info.name, info.ident, info.dep_kind);
+    fn read_extension_crate(&mut self, span: Span, orig_name: Symbol, rename: Symbol)
+                            -> ExtensionCrate {
+        info!("read extension crate `extern crate {} as {}`", orig_name, rename);
         let target_triple = &self.sess.opts.target_triple[..];
         let is_cross = target_triple != config::host_triple();
         let mut target_only = false;
         let mut locate_ctxt = locator::Context {
             sess: self.sess,
             span,
-            ident: info.ident,
-            crate_name: info.name,
+            ident: orig_name,
+            crate_name: rename,
             hash: None,
             filesearch: self.sess.host_filesearch(PathKind::Crate),
             target: &self.sess.host,
@@ -625,12 +589,8 @@ impl<'a> CrateLoader<'a> {
                                  span: Span,
                                  name: &str)
                                  -> Option<(PathBuf, CrateDisambiguator, DefIndex)> {
-        let ekrate = self.read_extension_crate(span, &ExternCrateInfo {
-             name: Symbol::intern(name),
-             ident: Symbol::intern(name),
-             id: ast::DUMMY_NODE_ID,
-             dep_kind: DepKind::UnexportedMacrosOnly,
-        });
+        let name = Symbol::intern(name);
+        let ekrate = self.read_extension_crate(span, name, name);
 
         if ekrate.target_only {
             // Need to abort before syntax expansion.
@@ -1098,21 +1058,37 @@ impl<'a> middle::cstore::CrateLoader for CrateLoader<'a> {
 
     fn process_item(&mut self, item: &ast::Item, definitions: &Definitions) {
         match item.node {
-            ast::ItemKind::ExternCrate(_) => {
-                let info = self.extract_crate_info(item).unwrap();
+            ast::ItemKind::ExternCrate(rename) => {
+                debug!("resolving extern crate stmt. ident: {} rename: {:?}", item.ident, rename);
+                let rename = match rename {
+                    Some(rename) => {
+                        validate_crate_name(Some(self.sess), &rename.as_str(), Some(item.span));
+                        rename
+                    }
+                    None => item.ident.name,
+                };
+                let dep_kind = if attr::contains_name(&item.attrs, "no_link") {
+                    DepKind::UnexportedMacrosOnly
+                } else {
+                    DepKind::Explicit
+                };
+
                 let (cnum, ..) = self.resolve_crate(
-                    &None, info.ident, info.name, None, item.span, PathKind::Crate, info.dep_kind,
+                    &None, item.ident.name, rename, None, item.span, PathKind::Crate, dep_kind,
                 );
 
                 let def_id = definitions.opt_local_def_id(item.id).unwrap();
-                let len = definitions.def_path(def_id.index).data.len();
+                let path_len = definitions.def_path(def_id.index).data.len();
 
-                let extern_crate =
-                    ExternCrate { def_id: def_id, span: item.span, direct: true, path_len: len };
+                let extern_crate = ExternCrate { def_id, span: item.span, direct: true, path_len };
                 self.update_extern_crate(cnum, extern_crate, &mut FxHashSet());
-                self.cstore.add_extern_mod_stmt_cnum(info.id, cnum);
+                self.cstore.add_extern_mod_stmt_cnum(item.id, cnum);
             }
             _ => {}
         }
     }
+
+    fn resolve_crate_from_path(&mut self, name: Symbol, span: Span) -> CrateNum {
+        self.resolve_crate(&None, name, name, None, span, PathKind::Crate, DepKind::Explicit).0
+    }
 }

src/librustc_resolve/lib.rs

@@ -58,6 +58,7 @@ use syntax::ast::{Item, ItemKind, ImplItem, ImplItemKind};
 use syntax::ast::{Local, Mutability, Pat, PatKind, Path};
 use syntax::ast::{QSelf, TraitItemKind, TraitRef, Ty, TyKind};
 use syntax::feature_gate::{feature_err, emit_feature_err, GateIssue};
+use syntax::parse::token;
 
 use syntax_pos::{Span, DUMMY_SP, MultiSpan};
 use errors::{DiagnosticBuilder, DiagnosticId};
@@ -2959,6 +2960,16 @@ impl<'a> Resolver<'a> {
                     // `$crate::a::b`
                     module = Some(self.resolve_crate_root(ident.node.ctxt));
                     continue
+                } else if i == 1 && self.session.features.borrow().extern_absolute_paths &&
+                                    path[0].node.name == keywords::CrateRoot.name() &&
+                                    !token::Ident(ident.node).is_path_segment_keyword() {
+                    // `::extern_crate::a::b`
+                    let crate_id = self.crate_loader.resolve_crate_from_path(name, ident.span);
+                    let crate_root =
+                        self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
+                    self.populate_module_if_necessary(crate_root);
+                    module = Some(crate_root);
+                    continue
                 }
             }
 

src/librustc_resolve/resolve_imports.rs

@@ -12,14 +12,14 @@ use self::ImportDirectiveSubclass::*;
 
 use {AmbiguityError, Module, PerNS};
 use Namespace::{self, TypeNS, MacroNS};
-use {NameBinding, NameBindingKind, PathResult, PrivacyError};
+use {NameBinding, NameBindingKind, ToNameBinding, PathResult, PrivacyError};
 use Resolver;
 use {names_to_string, module_to_string};
 use {resolve_error, ResolutionError};
 
 use rustc::ty;
 use rustc::lint::builtin::PUB_USE_OF_PRIVATE_EXTERN_CRATE;
-use rustc::hir::def_id::DefId;
+use rustc::hir::def_id::{CRATE_DEF_INDEX, DefId};
 use rustc::hir::def::*;
 use rustc::session::DiagnosticMessageId;
 use rustc::util::nodemap::{FxHashMap, FxHashSet};
@@ -602,8 +602,60 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
     // If appropriate, returns an error to report.
     fn finalize_import(&mut self, directive: &'b ImportDirective<'b>) -> Option<(Span, String)> {
         self.current_module = directive.parent;
-
         let ImportDirective { ref module_path, span, .. } = *directive;
+
+        // Extern crate mode for absolute paths needs some
+        // special support for single-segment imports.
+        let extern_absolute_paths = self.session.features.borrow().extern_absolute_paths;
+        if module_path.len() == 1 && module_path[0].node.name == keywords::CrateRoot.name() {
+            match directive.subclass {
+                GlobImport { .. } if extern_absolute_paths => {
+                    return Some((directive.span,
+                                 "cannot glob-import all possible crates".to_string()));
+                }
+                SingleImport { source, target, .. } => {
+                    let crate_root = if source.name == keywords::Crate.name() {
+                        if target.name == keywords::Crate.name() {
+                            return Some((directive.span,
+                                         "crate root imports need to be explicitly named: \
+                                          `use crate as name;`".to_string()));
+                        } else {
+                            Some(self.resolve_crate_root(source.ctxt.modern()))
+                        }
+                    } else if extern_absolute_paths &&
+                              !token::Ident(source).is_path_segment_keyword() {
+                        let crate_id =
+                            self.crate_loader.resolve_crate_from_path(source.name, directive.span);
+                        let crate_root =
+                            self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
+                        self.populate_module_if_necessary(crate_root);
+                        Some(crate_root)
+                    } else {
+                        None
+                    };
+
+                    if let Some(crate_root) = crate_root {
+                        let binding = (crate_root, ty::Visibility::Public, directive.span,
+                                       directive.expansion).to_name_binding(self.arenas);
+                        let binding = self.arenas.alloc_name_binding(NameBinding {
+                            kind: NameBindingKind::Import {
+                                binding,
+                                directive,
+                                used: Cell::new(false),
+                                legacy_self_import: false,
+                            },
+                            vis: directive.vis.get(),
+                            span: directive.span,
+                            expansion: directive.expansion,
+                        });
+                        let _ = self.try_define(directive.parent, target, TypeNS, binding);
+                        return None;
+                    }
+                }
+                _ => {}
+            }
+        }
+
         let module_result = self.resolve_path(&module_path, None, true, span);
         let module = match module_result {
             PathResult::Module(module) => module,

src/libsyntax/feature_gate.rs

@@ -430,6 +430,9 @@ declare_features! (
 
     // generic associated types (RFC 1598)
     (active, generic_associated_types, "1.23.0", Some(44265)),
+
+    // Resolve absolute paths as paths from other crates
+    (active, extern_absolute_paths, "1.24.0", Some(44660)),
 );
 
 declare_features! (

src/test/compile-fail/rfc-2126-extern-absolute-paths/auxiliary/xcrate.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#[derive(Debug)]
+pub struct S;
+
+#[derive(Debug)]
+pub struct Z;

src/test/compile-fail/rfc-2126-extern-absolute-paths/non-existent-1.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#![feature(extern_absolute_paths)]
+
+use xcrate::S; //~ ERROR can't find crate for `xcrate`
+
+fn main() {}

src/test/compile-fail/rfc-2126-extern-absolute-paths/non-existent-2.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#![feature(extern_absolute_paths)]
+
+fn main() {
+    let s = ::xcrate::S; //~ ERROR can't find crate for `xcrate`
+}

src/test/compile-fail/rfc-2126-extern-absolute-paths/non-existent-3.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#![feature(extern_absolute_paths)]
+
+use ycrate; //~ ERROR can't find crate for `ycrate`
+
+fn main() {}

src/test/compile-fail/rfc-2126-extern-absolute-paths/single-segment.rs

@@ -0,0 +1,24 @@
+// Copyright 2017 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.
+
+// aux-build:xcrate.rs
+
+#![feature(crate_in_paths)]
+#![feature(extern_absolute_paths)]
+
+use crate; //~ ERROR unresolved import `crate`
+           //~^ NOTE crate root imports need to be explicitly named: `use crate as name;`
+use *; //~ ERROR unresolved import `*`
+       //~^ NOTE cannot glob-import all possible crates
+
+fn main() {
+    let s = ::xcrate; //~ ERROR expected value, found module `xcrate`
+                      //~^ NOTE not a value
+}

src/test/run-pass/rfc-2126-crate-paths/crate-path-absolute.rs

@@ -11,10 +11,12 @@
 #![feature(crate_in_paths)]
 
 use crate::m::f;
+use crate as root;
 
 mod m {
     pub fn f() -> u8 { 1 }
     pub fn g() -> u8 { 2 }
+    pub fn h() -> u8 { 3 }
 
     // OK, visibilities are implicitly absolute like imports
     pub(in crate::m) struct S;
@@ -23,14 +25,17 @@ mod m {
 mod n
 {
     use crate::m::f;
+    use crate as root;
     pub fn check() {
         assert_eq!(f(), 1);
         assert_eq!(::crate::m::g(), 2);
+        assert_eq!(root::m::h(), 3);
     }
 }
 
 fn main() {
     assert_eq!(f(), 1);
     assert_eq!(::crate::m::g(), 2);
+    assert_eq!(root::m::h(), 3);
     n::check();
 }

src/test/run-pass/rfc-2126-extern-absolute-paths/auxiliary/xcrate.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#[derive(Debug)]
+pub struct S;
+
+#[derive(Debug)]
+pub struct Z;