Add suggestions for unresolved imports.

This commit adds suggestions for unresolved imports in the cases where
there could be a missing `crate::`, `super::`, `self::` or a missing
external crate name before an import.

src/test/ui/resolve_self_super_hint.rs

@@ -19,15 +19,15 @@ mod a {
     mod b {
         use alloc::HashMap;
         //~^ ERROR unresolved import `alloc` [E0432]
-        //~| Did you mean `a::alloc`?
+        //~| Did you mean `super::alloc`?
         mod c {
             use alloc::HashMap;
             //~^ ERROR unresolved import `alloc` [E0432]
-            //~| Did you mean `a::alloc`?
+            //~| Did you mean `std::alloc`?
             mod d {
                 use alloc::HashMap;
                 //~^ ERROR unresolved import `alloc` [E0432]
-                //~| Did you mean `a::alloc`?
+                //~| Did you mean `std::alloc`?
             }
         }
     }

src/test/ui/resolve_self_super_hint.stderr

@@ -8,19 +8,19 @@ error[E0432]: unresolved import `alloc`
   --> $DIR/resolve_self_super_hint.rs:20:13
    |
 LL |         use alloc::HashMap;
-   |             ^^^^^ Did you mean `a::alloc`?
+   |             ^^^^^ Did you mean `super::alloc`?
 
 error[E0432]: unresolved import `alloc`
   --> $DIR/resolve_self_super_hint.rs:24:17
    |
 LL |             use alloc::HashMap;
-   |                 ^^^^^ Did you mean `a::alloc`?
+   |                 ^^^^^ Did you mean `std::alloc`?
 
 error[E0432]: unresolved import `alloc`
   --> $DIR/resolve_self_super_hint.rs:28:21
    |
 LL |                 use alloc::HashMap;
-   |                     ^^^^^ Did you mean `a::alloc`?
+   |                     ^^^^^ Did you mean `std::alloc`?
 
 error: aborting due to 4 previous errors
 

src/test/ui/rust-2018/auxiliary/baz.rs

@@ -0,0 +1,15 @@
+// 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.
+
+// This file is used as part of the local-path-suggestions.rs test.
+
+pub mod foobar {
+    pub struct Baz;
+}

src/test/ui/rust-2018/issue-54006.stderr

@@ -2,7 +2,7 @@ error[E0432]: unresolved import `alloc`
   --> $DIR/issue-54006.rs:16:5
    |
 LL | use alloc::vec;
-   |     ^^^^^ Could not find `alloc` in `{{root}}`
+   |     ^^^^^ Did you mean `std::alloc`?
 
 error: cannot determine resolution for the macro `vec`
   --> $DIR/issue-54006.rs:20:18

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

@@ -0,0 +1,31 @@
+// 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.
+
+// aux-build:baz.rs
+// compile-flags:--extern baz
+// edition:2018
+
+mod foo {
+    type Bar = u32;
+}
+
+mod baz {
+    use foo::Bar;
+
+    fn baz() {
+        let x: Bar = 22;
+    }
+}
+
+use foo::Bar;
+
+use foobar::Baz;
+
+fn main() { }

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

@@ -0,0 +1,21 @@
+error[E0432]: unresolved import `foo`
+  --> $DIR/local-path-suggestions.rs:20:9
+   |
+LL |     use foo::Bar;
+   |         ^^^ Did you mean `crate::foo`?
+
+error[E0432]: unresolved import `foo`
+  --> $DIR/local-path-suggestions.rs:27:5
+   |
+LL | use foo::Bar;
+   |     ^^^ Did you mean `self::foo`?
+
+error[E0432]: unresolved import `foobar`
+  --> $DIR/local-path-suggestions.rs:29:5
+   |
+LL | use foobar::Baz;
+   |     ^^^^^^ Did you mean `baz::foobar`?
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0432`.