Rollup merge of #54603 - davidtwco:issue-54559, r=nikomatsakis

Add `crate::` to trait suggestions in Rust 2018.

Fixes #54559.

In the 2018 edition, when suggesting traits to import that implement a
given method that is being invoked, suggestions will now include the
`crate::` prefix if the suggested trait is local to the current crate.

r? @nikomatsakis

src/test/ui/rust-2018/auxiliary/trait-import-suggestions.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.
+
+pub trait Baz {
+    fn baz(&self) { }
+}
+
+impl Baz for u32 { }

src/test/ui/rust-2018/trait-import-suggestions.rs

@@ -0,0 +1,41 @@
+// 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
+// aux-build:trait-import-suggestions.rs
+// compile-flags:--extern trait-import-suggestions
+
+mod foo {
+    mod foobar {
+        pub(crate) trait Foobar {
+            fn foobar(&self) { }
+        }
+
+        impl Foobar for u32 { }
+    }
+
+    pub(crate) trait Bar {
+        fn bar(&self) { }
+    }
+
+    impl Bar for u32 { }
+
+    fn in_foo() {
+        let x: u32 = 22;
+        x.foobar();
+    }
+}
+
+fn main() {
+    let x: u32 = 22;
+    x.bar();
+    x.baz();
+    let y = u32::from_str("33");
+}

src/test/ui/rust-2018/trait-import-suggestions.stderr

@@ -0,0 +1,43 @@
+error[E0599]: no method named `foobar` found for type `u32` in the current scope
+  --> $DIR/trait-import-suggestions.rs:32:11
+   |
+LL |         x.foobar();
+   |           ^^^^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+   = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
+           `use crate::foo::foobar::Foobar;`
+
+error[E0599]: no method named `bar` found for type `u32` in the current scope
+  --> $DIR/trait-import-suggestions.rs:38:7
+   |
+LL |     x.bar();
+   |       ^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: the following trait is implemented but not in scope, perhaps add a `use` for it:
+   |
+LL | use crate::foo::Bar;
+   |
+
+error[E0599]: no method named `baz` found for type `u32` in the current scope
+  --> $DIR/trait-import-suggestions.rs:39:7
+   |
+LL |     x.baz();
+   |       ^^^
+
+error[E0599]: no function or associated item named `from_str` found for type `u32` in the current scope
+  --> $DIR/trait-import-suggestions.rs:40:13
+   |
+LL |     let y = u32::from_str("33");
+   |             ^^^^^^^^^^^^^ function or associated item not found in `u32`
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: the following trait is implemented but not in scope, perhaps add a `use` for it:
+   |
+LL | use std::str::FromStr;
+   |
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0599`.