rustc_resolve: ignore uniform_paths canaries that resolve to an import of the same crate.

src/librustc_resolve/build_reduced_graph.rs

@@ -199,22 +199,6 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         if emit_uniform_paths_canary {
             let source = prefix_start.unwrap();
 
-            // HACK(eddyb) For `use x::{self, ...};`, use the ID of the
-            // `self` nested import for the canary. This allows the
-            // ambiguity reporting scope to ignore false positives
-            // in the same way it does for `use x;` (by comparing IDs).
-            let mut canary_id = id;
-            if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
-                for &(ref use_tree, id) in items {
-                    if let ast::UseTreeKind::Simple(..) = use_tree.kind {
-                        if use_tree.ident().name == keywords::SelfValue.name() {
-                            canary_id = id;
-                            break;
-                        }
-                    }
-                }
-            }
-
             // Helper closure to emit a canary with the given base path.
             let emit = |this: &mut Self, base: Option<Ident>| {
                 let subclass = SingleImport {
@@ -234,7 +218,7 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                     base.into_iter().collect(),
                     subclass.clone(),
                     source.span,
-                    canary_id,
+                    id,
                     root_use_tree.span,
                     root_id,
                     ty::Visibility::Invisible,

src/librustc_resolve/resolve_imports.rs

@@ -620,9 +620,9 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         }
 
         #[derive(Default)]
-        struct UniformPathsCanaryResult {
-            module_scope: Option<Span>,
-            block_scopes: Vec<Span>,
+        struct UniformPathsCanaryResult<'a> {
+            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<
@@ -661,20 +661,12 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
 
                 self.per_ns(|_, ns| {
                     if let Some(result) = result[ns].get().ok() {
-                        if let NameBindingKind::Import { directive, .. } = result.kind {
-                            // Skip canaries that resolve to the import itself.
-                            // These come from `use crate_name;`, which isn't really
-                            // ambiguous, as the import can't actually shadow itself.
-                            if directive.id == import.id {
-                                return;
-                            }
-                        }
                         if has_explicit_self {
                             // There should only be one `self::x` (module-scoped) canary.
-                            assert_eq!(canary_results[ns].module_scope, None);
-                            canary_results[ns].module_scope = Some(result.span);
+                            assert!(canary_results[ns].module_scope.is_none());
+                            canary_results[ns].module_scope = Some(result);
                         } else {
-                            canary_results[ns].block_scopes.push(result.span);
+                            canary_results[ns].block_scopes.push(result);
                         }
                     }
                 });
@@ -708,16 +700,36 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         let uniform_paths_feature = self.session.features_untracked().uniform_paths;
         for ((span, _), (name, results)) in uniform_paths_canaries {
             self.per_ns(|this, ns| {
-                let results = &results[ns];
+                let external_crate = if ns == TypeNS && this.extern_prelude.contains(&name) {
+                    let crate_id =
+                        this.crate_loader.process_path_extern(name, span);
+                    Some(DefId { krate: crate_id, index: CRATE_DEF_INDEX })
+                } else {
+                    None
+                };
+                let result_filter = |result: &&NameBinding| {
+                    // Ignore canaries that resolve to an import of the same crate.
+                    // That is, we allow `use crate_name; use crate_name::foo;`.
+                    if let Some(def_id) = external_crate {
+                        if let Some(module) = result.module() {
+                            if module.normal_ancestor_id == def_id {
+                                return false;
+                            }
+                        }
+                    }
 
-                let has_external_crate =
-                    ns == TypeNS && this.extern_prelude.contains(&name);
+                    true
+                };
+                let module_scope = results[ns].module_scope.filter(result_filter);
+                let block_scopes = || {
+                    results[ns].block_scopes.iter().cloned().filter(result_filter)
+                };
 
                 // An ambiguity requires more than one possible resolution.
                 let possible_resultions =
-                    (has_external_crate as usize) +
-                    (results.module_scope.is_some() as usize) +
-                    (!results.block_scopes.is_empty() as usize);
+                    (external_crate.is_some() as usize) +
+                    (module_scope.is_some() as usize) +
+                    (block_scopes().next().is_some() as usize);
                 if possible_resultions <= 1 {
                     return;
                 }
@@ -727,26 +739,26 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
                 let msg = format!("`{}` import is ambiguous", name);
                 let mut err = this.session.struct_span_err(span, &msg);
                 let mut suggestion_choices = String::new();
-                if has_external_crate {
+                if external_crate.is_some() {
                     write!(suggestion_choices, "`::{}`", name);
                     err.span_label(span,
                         format!("can refer to external crate `::{}`", name));
                 }
-                if let Some(span) = results.module_scope {
+                if let Some(result) = module_scope {
                     if !suggestion_choices.is_empty() {
                         suggestion_choices.push_str(" or ");
                     }
                     write!(suggestion_choices, "`self::{}`", name);
                     if uniform_paths_feature {
-                        err.span_label(span,
+                        err.span_label(result.span,
                             format!("can refer to `self::{}`", name));
                     } else {
-                        err.span_label(span,
+                        err.span_label(result.span,
                             format!("may refer to `self::{}` in the future", name));
                     }
                 }
-                for &span in &results.block_scopes {
-                    err.span_label(span,
+                for result in block_scopes() {
+                    err.span_label(result.span,
                         format!("shadowed by block-scoped `{}`", name));
                 }
                 err.help(&format!("write {} explicitly instead", suggestion_choices));

src/test/ui/rust-2018/uniform-paths-forward-compat/redundant.rs

@@ -0,0 +1,30 @@
+// 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.
+
+// run-pass
+// edition:2018
+
+use std;
+use std::io;
+
+mod foo {
+    pub use std as my_std;
+}
+
+mod bar {
+    pub use std::{self};
+}
+
+fn main() {
+    io::stdout();
+    self::std::io::stdout();
+    foo::my_std::io::stdout();
+    bar::std::io::stdout();
+}

src/test/ui/rust-2018/uniform-paths/redundant.rs

@@ -8,11 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// run-pass
 // edition:2018
 
 #![feature(uniform_paths)]
 
 use std;
+use std::io;
 
 mod foo {
     pub use std as my_std;
@@ -23,6 +25,7 @@ mod bar {
 }
 
 fn main() {
+    io::stdout();
     self::std::io::stdout();
     foo::my_std::io::stdout();
     bar::std::io::stdout();