Support ::crate in paths

Author: petrochenkov

src/librustc_passes/ast_validation.rs

@@ -21,7 +21,6 @@ use rustc::session::Session;
 use syntax::ast::*;
 use syntax::attr;
 use syntax::codemap::Spanned;
-use syntax::parse::token;
 use syntax::symbol::keywords;
 use syntax::visit::{self, Visitor};
 use syntax_pos::Span;
@@ -182,18 +181,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         visit::walk_ty(self, ty)
     }
 
-    fn visit_path(&mut self, path: &'a Path, _: NodeId) {
-        if path.segments.len() >= 2 && path.is_global() {
-            let ident = path.segments[1].identifier;
-            if token::Ident(ident).is_path_segment_keyword() {
-                self.err_handler()
-                    .span_err(path.span, &format!("global paths cannot start with `{}`", ident));
-            }
-        }
-
-        visit::walk_path(self, path)
-    }
-
     fn visit_item(&mut self, item: &'a Item) {
         match item.node {
             ItemKind::Use(ref view_path) => {

src/librustc_resolve/build_reduced_graph.rs

@@ -114,7 +114,7 @@ impl<'a> Resolver<'a> {
                 // Extract and intern the module part of the path. For
                 // globs and lists, the path is found directly in the AST;
                 // for simple paths we have to munge the path a little.
-                let mut module_path: Vec<_> = match view_path.node {
+                let module_path: Vec<_> = match view_path.node {
                     ViewPathSimple(_, ref full_path) => {
                         full_path.segments
                                  .split_last()
@@ -134,14 +134,6 @@ impl<'a> Resolver<'a> {
                     }
                 };
 
-                // This can be removed once warning cycle #36888 is complete.
-                if module_path.len() >= 2 &&
-                    module_path[0].node.name == keywords::CrateRoot.name() &&
-                    token::Ident(module_path[1].node).is_path_segment_keyword()
-                {
-                    module_path.remove(0);
-                }
-
                 // Build up the import directives.
                 let is_prelude = attr::contains_name(&item.attrs, "prelude_import");
 

src/librustc_resolve/lib.rs

@@ -2886,12 +2886,18 @@ impl<'a> Resolver<'a> {
             }
             allow_super = false;
 
-            if i == 0 && ns == TypeNS && ident.node.name == keywords::CrateRoot.name() {
-                module = Some(self.resolve_crate_root(ident.node.ctxt.modern()));
-                continue
-            } else if i == 0 && ns == TypeNS && ident.node.name == keywords::DollarCrate.name() {
-                module = Some(self.resolve_crate_root(ident.node.ctxt));
-                continue
+            if ns == TypeNS {
+                if (i == 0 && ident.node.name == keywords::CrateRoot.name()) ||
+                   (i == 1 && ident.node.name == keywords::Crate.name() &&
+                              path[0].node.name == keywords::CrateRoot.name()) {
+                    // `::a::b` or `::crate::a::b`
+                    module = Some(self.resolve_crate_root(ident.node.ctxt.modern()));
+                    continue
+                } else if i == 0 && ident.node.name == keywords::DollarCrate.name() {
+                    // `$crate::a::b`
+                    module = Some(self.resolve_crate_root(ident.node.ctxt));
+                    continue
+                }
             }
 
             let binding = if let Some(module) = module {

src/libsyntax/ast.rs

@@ -96,10 +96,15 @@ impl Path {
         }
     }
 
+    // Add starting "crate root" segment to all paths except those that
+    // already have it or start with `self`, `super`, `Self` or `$crate`.
     pub fn default_to_global(mut self) -> Path {
-        if !self.is_global() &&
-           !::parse::token::Ident(self.segments[0].identifier).is_path_segment_keyword() {
-            self.segments.insert(0, PathSegment::crate_root(self.span));
+        if !self.is_global() {
+            let ident = self.segments[0].identifier;
+            if !::parse::token::Ident(ident).is_path_segment_keyword() ||
+               ident.name == keywords::Crate.name() {
+                self.segments.insert(0, PathSegment::crate_root(self.span));
+            }
         }
         self
     }

src/libsyntax/feature_gate.rs

@@ -33,7 +33,7 @@ use syntax_pos::Span;
 use errors::{DiagnosticBuilder, Handler, FatalError};
 use visit::{self, FnKind, Visitor};
 use parse::ParseSess;
-use symbol::Symbol;
+use symbol::{keywords, Symbol};
 
 use std::env;
 
@@ -418,6 +418,9 @@ declare_features! (
 
     // #![wasm_import_memory] attribute
     (active, wasm_import_memory, "1.22.0", None),
+
+    // `crate` in paths
+    (active, crate_in_paths, "1.23.0", Some(45477)),
 );
 
 declare_features! (
@@ -1628,6 +1631,17 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         visit::walk_impl_item(self, ii);
     }
 
+    fn visit_path(&mut self, path: &'a ast::Path, _id: NodeId) {
+        for segment in &path.segments {
+            if segment.identifier.name == keywords::Crate.name() {
+                gate_feature_post!(&self, crate_in_paths, segment.span,
+                                   "`crate` in paths is experimental");
+            }
+        }
+
+        visit::walk_path(self, path);
+    }
+
     fn visit_vis(&mut self, vis: &'a ast::Visibility) {
         if let ast::Visibility::Crate(span, ast::CrateSugar::JustCrate) = *vis {
             gate_feature_post!(&self, crate_visibility_modifier, span,

src/libsyntax/parse/parser.rs

@@ -3916,6 +3916,10 @@ impl<'a> Parser<'a> {
         self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident())
     }
 
+    fn is_crate_vis(&self) -> bool {
+        self.token.is_keyword(keywords::Crate) && self.look_ahead(1, |t| t != &token::ModSep)
+    }
+
     fn eat_auto_trait(&mut self) -> bool {
         if self.token.is_keyword(keywords::Auto)
             && self.look_ahead(1, |t| t.is_keyword(keywords::Trait))
@@ -4026,10 +4030,15 @@ impl<'a> Parser<'a> {
                 node: StmtKind::Item(macro_def),
                 span: lo.to(self.prev_span),
             }
-        // Starts like a simple path, but not a union item.
+        // Starts like a simple path, but not a union item or item with `crate` visibility.
+        // Our goal here is to parse an arbitrary path `a::b::c` but not something that starts
+        // like a path (1 token), but it fact not a path.
+        // `union::b::c` - path, `union U { ... }` - not a path.
+        // `crate::b::c` - path, `crate struct S;` - not a path.
         } else if self.token.is_path_start() &&
                   !self.token.is_qpath_start() &&
-                  !self.is_union_item() {
+                  !self.is_union_item() &&
+                  !self.is_crate_vis() {
             let pth = self.parse_path(PathStyle::Expr)?;
 
             if !self.eat(&token::Not) {
@@ -5399,7 +5408,9 @@ impl<'a> Parser<'a> {
     pub fn parse_visibility(&mut self, can_take_tuple: bool) -> PResult<'a, Visibility> {
         maybe_whole!(self, NtVis, |x| x);
 
-        if self.eat_keyword(keywords::Crate) {
+        self.expected_tokens.push(TokenType::Keyword(keywords::Crate));
+        if self.is_crate_vis() {
+            self.bump(); // `crate`
             return Ok(Visibility::Crate(self.prev_span, CrateSugar::JustCrate));
         }
 

src/libsyntax/parse/token.rs

@@ -347,6 +347,7 @@ impl Token {
             Some(id) => id.name == keywords::Super.name() ||
                         id.name == keywords::SelfValue.name() ||
                         id.name == keywords::SelfType.name() ||
+                        id.name == keywords::Crate.name() ||
                         id.name == keywords::DollarCrate.name(),
             None => false,
         }

src/test/compile-fail/rfc-2126-crate-paths/crate-path-non-absolute.rs

@@ -0,0 +1,21 @@
+// 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(crate_in_paths)]
+
+struct S;
+
+mod m {
+    fn f() {
+        let s = crate::S; //~ ERROR undeclared type or module `crate`
+    }
+}
+
+fn main() {}

src/test/compile-fail/rfc-2126-crate-paths/crate-visibility-ambiguity.rs

@@ -0,0 +1,23 @@
+// 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(crate_in_paths)]
+#![feature(crate_visibility_modifier)]
+
+mod m {
+    pub struct Z;
+    pub struct S1(crate (::m::Z)); // OK
+    pub struct S2(::crate ::m::Z); // OK
+    pub struct S3(crate ::m::Z); //~ ERROR undeclared type or module `crate`
+}
+
+fn main() {
+    crate struct S; // OK (item in statement position)
+}

src/test/compile-fail/rfc-2126-crate-paths/feature-gate-crate_in_paths.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.
+
+struct S;
+
+fn main() {
+    let _ = ::crate::S; //~ ERROR `crate` in paths is experimental
+}