resolve: Recover "did you mean" suggestions in imports

src/librustc_resolve/error_reporting.rs

@@ -11,46 +11,40 @@
 use {CrateLint, PathResult, Segment};
 use macros::ParentScope;
 
-use std::collections::BTreeSet;
-
 use syntax::ast::Ident;
-use syntax::symbol::{keywords, Symbol};
+use syntax::symbol::keywords;
 use syntax_pos::Span;
 
 use resolve_imports::ImportResolver;
+use std::cmp::Reverse;
 
 impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
     /// Add suggestions for a path that cannot be resolved.
     pub(crate) fn make_path_suggestion(
         &mut self,
         span: Span,
-        path: Vec<Segment>,
+        mut path: Vec<Segment>,
         parent_scope: &ParentScope<'b>,
     ) -> Option<(Vec<Segment>, Option<String>)> {
         debug!("make_path_suggestion: span={:?} path={:?}", span, path);
-        // If we don't have a path to suggest changes to, then return.
-        if path.is_empty() {
-            return None;
-        }
-
-        // Check whether a ident is a path segment that is not root.
-        let is_special = |ident: Ident| ident.is_path_segment_keyword() &&
-                                        ident.name != keywords::CrateRoot.name();
 
         match (path.get(0), path.get(1)) {
-            // Make suggestions that require at least two non-special path segments.
-            (Some(fst), Some(snd)) if !is_special(fst.ident) && !is_special(snd.ident) => {
-                debug!("make_path_suggestion: fst={:?} snd={:?}", fst, snd);
-
-                self.make_missing_self_suggestion(span, path.clone(), parent_scope)
-                    .or_else(|| self.make_missing_crate_suggestion(span, path.clone(),
-                                                                   parent_scope))
-                    .or_else(|| self.make_missing_super_suggestion(span, path.clone(),
-                                                                   parent_scope))
-                    .or_else(|| self.make_external_crate_suggestion(span, path, parent_scope))
-            },
-            _ => None,
+            // `{{root}}::ident::...` on both editions.
+            // On 2015 `{{root}}` is usually added implicitly.
+            (Some(fst), Some(snd)) if fst.name == keywords::CrateRoot.name() &&
+                                      !snd.is_path_segment_keyword() => {}
+            // `ident::...` on 2018
+            (Some(fst), _) if self.session.rust_2018() && !fst.is_path_segment_keyword() => {
+                // Insert a placeholder that's later replaced by `self`/`super`/etc.
+                path.insert(0, keywords::Invalid.ident());
+            }
+            _ => return None,
         }
+
+        self.make_missing_self_suggestion(span, path.clone(), parent_scope)
+            .or_else(|| self.make_missing_crate_suggestion(span, path.clone(), parent_scope))
+            .or_else(|| self.make_missing_super_suggestion(span, path.clone(), parent_scope))
+            .or_else(|| self.make_external_crate_suggestion(span, path, parent_scope))
     }
 
     /// Suggest a missing `self::` if that resolves to an correct module.
@@ -148,22 +142,20 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         mut path: Vec<Segment>,
         parent_scope: &ParentScope<'b>,
     ) -> Option<(Vec<Segment>, Option<String>)> {
-        // Need to clone else we can't call `resolve_path` without a borrow error. We also store
-        // into a `BTreeMap` so we can get consistent ordering (and therefore the same diagnostic)
-        // each time.
-        let external_crate_names: BTreeSet<Symbol> = self.resolver.extern_prelude
-            .iter().map(|(ident, _)| ident.name).collect();
+        if !self.session.rust_2018() {
+            return None;
+        }
 
-        // Insert a new path segment that we can replace.
-        let new_path_segment = path[0].clone();
-        path.insert(1, new_path_segment);
+        // Sort extern crate names in reverse order to get
+        // 1) some consistent ordering for emitted dignostics and
+        // 2) `std` suggestions before `core` suggestions.
+        let mut extern_crate_names =
+            self.resolver.extern_prelude.iter().map(|(ident, _)| ident.name).collect::<Vec<_>>();
+        extern_crate_names.sort_by_key(|name| Reverse(name.as_str()));
 
-        // Iterate in reverse so that we start with crates at the end of the alphabet. This means
-        // that we'll always get `std` before `core`.
-        for name in external_crate_names.iter().rev() {
-            // Replace the first after root (a placeholder we inserted) with a crate name
-            // and check if that is valid.
-            path[1].ident.name = *name;
+        for name in extern_crate_names.into_iter() {
+            // Replace first ident with a crate name and check if that is valid.
+            path[0].ident.name = name;
             let result = self.resolve_path(&path, None, parent_scope, false, span, CrateLint::No);
             debug!("make_external_crate_suggestion: name={:?} path={:?} result={:?}",
                     name, path, result);

src/test/ui/resolve_self_super_hint.rs

@@ -23,11 +23,11 @@ mod a {
         mod c {
             use alloc::HashMap;
             //~^ ERROR unresolved import `alloc` [E0432]
-            //~| Did you mean `std::alloc`?
+            //~| Did you mean `a::alloc`?
             mod d {
                 use alloc::HashMap;
                 //~^ ERROR unresolved import `alloc` [E0432]
-                //~| Did you mean `std::alloc`?
+                //~| Did you mean `a::alloc`?
             }
         }
     }

src/test/ui/resolve_self_super_hint.stderr

@@ -14,13 +14,13 @@ error[E0432]: unresolved import `alloc`
   --> $DIR/resolve_self_super_hint.rs:24:17
    |
 LL |             use alloc::HashMap;
-   |                 ^^^^^ Did you mean `std::alloc`?
+   |                 ^^^^^ Did you mean `a::alloc`?
 
 error[E0432]: unresolved import `alloc`
   --> $DIR/resolve_self_super_hint.rs:28:21
    |
 LL |                 use alloc::HashMap;
-   |                     ^^^^^ Did you mean `std::alloc`?
+   |                     ^^^^^ Did you mean `a::alloc`?
 
 error: aborting due to 4 previous errors
 

src/test/ui/rust-2018/local-path-suggestions-2018.rs

@@ -16,7 +16,7 @@ mod foo {
     pub type Bar = u32;
 }
 
-mod baz {
+mod bazz {
     use foo::Bar;
 
     fn baz() {

src/test/ui/rust-2018/local-path-suggestions-2018.stderr

@@ -6,18 +6,12 @@ LL |     use foo::Bar;
    |
    = note: `use` statements changed in Rust 2018; read more at <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html>
 
-error[E0432]: unresolved import `foo`
-  --> $DIR/local-path-suggestions-2018.rs:27:5
-   |
-LL | use foo::Bar;
-   |     ^^^ Did you mean `self::foo`?
-
 error[E0432]: unresolved import `foobar`
   --> $DIR/local-path-suggestions-2018.rs:29:5
    |
 LL | use foobar::Baz;
    |     ^^^^^^ Did you mean `baz::foobar`?
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0432`.