Auto merge of #54011 - eddyb:anchored-in-the-future, r=petrochenkov

rustc_resolve: inject `uniform_paths` canaries regardless of the feature-gate, on Rust 2018.

This PR is an attempt at future-proofing "anchored paths" by emitting the same ambiguity errors that `#![feature(uniform_paths)]` would, with slightly changed phrasing (see added UI tests).

Also, on top of #54005, this PR allows this as well:
```rust
use crate_name;
use crate_name::foo;
```
In that any ambiguity between an extern crate and an import *of that same crate* is ignored.

r? @petrochenkov cc @aturon @Centril @joshtriplett

Author: bors

Diff for: src/librustc_resolve/build_reduced_graph.rs

@@ -194,27 +194,11 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         // ergonomically unacceptable.
         let emit_uniform_paths_canary =
             !uniform_paths_canary_emitted &&
-            uniform_paths &&
+            self.session.rust_2018() &&
             starts_with_non_keyword;
         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,

Diff for: 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<
@@ -663,20 +663,12 @@ 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);
                         }
                     }
                 });
@@ -716,18 +708,39 @@ 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;
                 }
@@ -737,25 +750,34 @@ 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);
-                    err.span_label(span,
-                        format!("can refer to `self::{}`", name));
+                    if uniform_paths_feature {
+                        err.span_label(result.span,
+                            format!("can refer to `self::{}`", name));
+                    } else {
+                        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));
-                err.note("relative `use` paths enabled by `#![feature(uniform_paths)]`");
+                if uniform_paths_feature {
+                    err.note("relative `use` paths enabled by `#![feature(uniform_paths)]`");
+                } else {
+                    err.note("in the future, `#![feature(uniform_paths)]` may become the default");
+                }
                 err.emit();
             });
         }
@@ -968,11 +990,15 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
             _ => unreachable!(),
         };
 
+        // Do not record uses from canaries, to avoid interfering with other
+        // diagnostics or suggestions that rely on some items not being used.
+        let record_used = !directive.is_uniform_paths_canary;
+
         let mut all_ns_err = true;
         self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
             if let Ok(binding) = result[ns].get() {
                 all_ns_err = false;
-                if this.record_use(ident, ns, binding) {
+                if record_used && this.record_use(ident, ns, binding) {
                     if let ModuleOrUniformRoot::Module(module) = module {
                         this.resolution(module, ident, ns).borrow_mut().binding =
                             Some(this.dummy_binding);
@@ -984,7 +1010,7 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         if all_ns_err {
             let mut all_ns_failed = true;
             self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
-                match this.resolve_ident_in_module(module, ident, ns, true, span) {
+                match this.resolve_ident_in_module(module, ident, ns, record_used, span) {
                     Ok(_) => all_ns_failed = false,
                     _ => {}
                 }

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros-nested.rs

@@ -0,0 +1,29 @@
+// 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
+
+// This test is similar to `ambiguity-macros.rs`, but nested in a module.
+
+mod foo {
+    pub use std::io;
+    //~^ ERROR `std` import is ambiguous
+
+    macro_rules! m {
+        () => {
+            mod std {
+                pub struct io;
+            }
+        }
+    }
+    m!();
+}
+
+fn main() {}

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros-nested.stderr

@@ -0,0 +1,16 @@
+error: `std` import is ambiguous
+  --> $DIR/ambiguity-macros-nested.rs:16:13
+   |
+LL |       pub use std::io;
+   |               ^^^ can refer to external crate `::std`
+...
+LL | /             mod std {
+LL | |                 pub struct io;
+LL | |             }
+   | |_____________- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros.rs

@@ -0,0 +1,27 @@
+// 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
+
+// This test is similar to `ambiguity.rs`, but with macros defining local items.
+
+use std::io;
+//~^ ERROR `std` import is ambiguous
+
+macro_rules! m {
+    () => {
+        mod std {
+            pub struct io;
+        }
+    }
+}
+m!();
+
+fn main() {}

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-macros.stderr

@@ -0,0 +1,16 @@
+error: `std` import is ambiguous
+  --> $DIR/ambiguity-macros.rs:15:5
+   |
+LL |   use std::io;
+   |       ^^^ can refer to external crate `::std`
+...
+LL | /         mod std {
+LL | |             pub struct io;
+LL | |         }
+   | |_________- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-nested.rs

@@ -0,0 +1,24 @@
+// 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
+
+// This test is similar to `ambiguity.rs`, but nested in a module.
+
+mod foo {
+    pub use std::io;
+    //~^ ERROR `std` import is ambiguous
+
+    mod std {
+        pub struct io;
+    }
+}
+
+fn main() {}

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity-nested.stderr

@@ -0,0 +1,16 @@
+error: `std` import is ambiguous
+  --> $DIR/ambiguity-nested.rs:16:13
+   |
+LL |       pub use std::io;
+   |               ^^^ can refer to external crate `::std`
+...
+LL | /     mod std {
+LL | |         pub struct io;
+LL | |     }
+   | |_____- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity.rs

@@ -0,0 +1,20 @@
+// 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
+
+use std::io;
+//~^ ERROR `std` import is ambiguous
+
+mod std {
+    pub struct io;
+}
+
+fn main() {}

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/ambiguity.stderr

@@ -0,0 +1,16 @@
+error: `std` import is ambiguous
+  --> $DIR/ambiguity.rs:13:5
+   |
+LL |   use std::io;
+   |       ^^^ can refer to external crate `::std`
+...
+LL | / mod std {
+LL | |     pub struct io;
+LL | | }
+   | |_- may refer to `self::std` in the future
+   |
+   = help: write `::std` or `self::std` explicitly instead
+   = note: in the future, `#![feature(uniform_paths)]` may become the default
+
+error: aborting due to previous error
+

Diff for: src/test/ui/rust-2018/uniform-paths-forward-compat/block-scoped-shadow.rs

@@ -0,0 +1,21 @@
+// 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
+
+struct std;
+
+fn main() {
+    fn std() {}
+    enum std {}
+    use std as foo;
+    //~^ ERROR `std` import is ambiguous
+    //~| ERROR `std` import is ambiguous
+}