rustc_resolve: also inject canaries to detect block scopes shadowing …

…`uniform_paths` imports.

src/librustc_resolve/build_reduced_graph.rs

@@ -149,6 +149,14 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         // import is injected as a "canary", and an error is emitted if it
         // successfully resolves while an `x` external crate exists.
         //
+        // For each block scope around the `use` item, one special canary
+        // import of the form `use x as _;` is also injected, having its
+        // parent set to that scope; `resolve_imports` will only resolve
+        // it within its appropriate scope; if any of them successfully
+        // resolve, an ambiguity error is emitted, since the original
+        // import can't see the item in the block scope (`self::x` only
+        // looks in the enclosing module), but a non-`use` path could.
+        //
         // Additionally, the canary might be able to catch limitations of the
         // current implementation, where `::x` may be chosen due to `self::x`
         // not existing, but `self::x` could appear later, from macro expansion.
@@ -173,33 +181,57 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                     self.session.features_untracked().uniform_paths);
 
             let source = module_path[0];
-            let subclass = SingleImport {
-                target: Ident {
-                    name: keywords::Underscore.name().gensymed(),
-                    span: source.span,
-                },
-                source,
-                result: PerNS {
-                    type_ns: Cell::new(Err(Undetermined)),
-                    value_ns: Cell::new(Err(Undetermined)),
-                    macro_ns: Cell::new(Err(Undetermined)),
-                },
-                type_ns_only: false,
+            // Helper closure to emit a canary with the given base path.
+            let emit = |this: &mut Self, base: Option<Ident>| {
+                let subclass = SingleImport {
+                    target: Ident {
+                        name: keywords::Underscore.name().gensymed(),
+                        span: source.span,
+                    },
+                    source,
+                    result: PerNS {
+                        type_ns: Cell::new(Err(Undetermined)),
+                        value_ns: Cell::new(Err(Undetermined)),
+                        macro_ns: Cell::new(Err(Undetermined)),
+                    },
+                    type_ns_only: false,
+                };
+                this.add_import_directive(
+                    base.into_iter().collect(),
+                    subclass.clone(),
+                    source.span,
+                    id,
+                    root_use_tree.span,
+                    root_id,
+                    ty::Visibility::Invisible,
+                    expansion,
+                    true, // is_uniform_paths_canary
+                );
             };
-            self.add_import_directive(
-                vec![Ident {
-                    name: keywords::SelfValue.name(),
-                    span: source.span,
-                }],
-                subclass,
-                source.span,
-                id,
-                root_use_tree.span,
-                root_id,
-                ty::Visibility::Invisible,
-                expansion,
-                true, // is_uniform_paths_canary
-            );
+
+            // A single simple `self::x` canary.
+            emit(self, Some(Ident {
+                name: keywords::SelfValue.name(),
+                span: source.span,
+            }));
+
+            // One special unprefixed canary per block scope around
+            // the import, to detect items unreachable by `self::x`.
+            let orig_current_module = self.current_module;
+            let mut span = source.span.modern();
+            loop {
+                match self.current_module.kind {
+                    ModuleKind::Block(..) => emit(self, None),
+                    ModuleKind::Def(..) => break,
+                }
+                match self.hygienic_lexical_parent(self.current_module, &mut span) {
+                    Some(module) => {
+                        self.current_module = module;
+                    }
+                    None => break,
+                }
+            }
+            self.current_module = orig_current_module;
 
             uniform_paths_canary_emitted = true;
         }

src/librustc_resolve/resolve_imports.rs

@@ -11,7 +11,7 @@
 use self::ImportDirectiveSubclass::*;
 
 use {AmbiguityError, CrateLint, Module, ModuleOrUniformRoot, PerNS};
-use Namespace::{self, TypeNS, MacroNS};
+use Namespace::{self, TypeNS, MacroNS, ValueNS};
 use {NameBinding, NameBindingKind, ToNameBinding, PathResult, PrivacyError};
 use Resolver;
 use {names_to_string, module_to_string};
@@ -636,24 +636,32 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
                     continue;
                 }
 
+                let is_explicit_self =
+                    import.module_path.len() > 0 &&
+                    import.module_path[0].name == keywords::SelfValue.name();
                 let extern_crate_exists = self.extern_prelude.contains(&name);
 
                 // A successful `self::x` is ambiguous with an `x` external crate.
-                if !extern_crate_exists {
+                if is_explicit_self && !extern_crate_exists {
                     continue;
                 }
 
                 errors = true;
 
                 let msg = format!("import from `{}` is ambiguous", name);
                 let mut err = self.session.struct_span_err(import.span, &msg);
-                err.span_label(import.span,
-                    format!("could refer to external crate `::{}`", name));
+                if extern_crate_exists {
+                    err.span_label(import.span,
+                        format!("could refer to external crate `::{}`", name));
+                }
                 if let Some(result) = result {
-                    err.span_label(result.span,
-                        format!("could also refer to `self::{}`", name));
+                    if is_explicit_self {
                         err.span_label(result.span,
                             format!("could also refer to `self::{}`", name));
+                    } else {
+                        err.span_label(result.span,
+                            format!("shadowed by block-scoped `{}`", name));
+                    }
                 }
                 err.help(&format!("write `::{0}` or `self::{0}` explicitly instead", name));
                 err.note("relative `use` paths enabled by `#![feature(uniform_paths)]`");
@@ -717,7 +725,11 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
             // while resolving its module path.
             directive.vis.set(ty::Visibility::Invisible);
             let result = self.resolve_path(
-                Some(ModuleOrUniformRoot::UniformRoot(keywords::Invalid.name())),
+                Some(if directive.is_uniform_paths_canary {
+                    ModuleOrUniformRoot::Module(directive.parent)
+                } else {
+                    ModuleOrUniformRoot::UniformRoot(keywords::Invalid.name())
+                }),
                 &directive.module_path[..],
                 None,
                 false,
@@ -792,7 +804,11 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         let ImportDirective { ref module_path, span, .. } = *directive;
 
         let module_result = self.resolve_path(
-            Some(ModuleOrUniformRoot::UniformRoot(keywords::Invalid.name())),
+            Some(if directive.is_uniform_paths_canary {
+                ModuleOrUniformRoot::Module(directive.parent)
+            } else {
+                ModuleOrUniformRoot::UniformRoot(keywords::Invalid.name())
+            }),
             &module_path,
             None,
             true,

src/test/ui/rust-2018/uniform-paths/block-scoped-shadow.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 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.
+
+// edition:2018
+
+#![feature(uniform_paths)]
+
+enum Foo { A, B }
+
+fn main() {
+    enum Foo {}
+    use Foo::*;
+    //~^ ERROR import from `Foo` is ambiguous
+
+    let _ = (A, B);
+}

src/test/ui/rust-2018/uniform-paths/block-scoped-shadow.stderr

@@ -0,0 +1,13 @@
+error: import from `Foo` is ambiguous
+  --> $DIR/block-scoped-shadow.rs:19:9
+   |
+LL |     enum Foo {}
+   |     ----------- shadowed by block-scoped `Foo`
+LL |     use Foo::*;
+   |         ^^^
+   |
+   = help: write `::Foo` or `self::Foo` explicitly instead
+   = note: relative `use` paths enabled by `#![feature(uniform_paths)]`
+
+error: aborting due to previous error
+