Auto merge of #52204 - zackmdavis:and_the_crate_of_the_missing_module, r=oli-obk

correct import suggestions for edition 2018

For #52202.

Author: bors

Diff for: src/librustc_resolve/lib.rs

@@ -4125,7 +4125,18 @@ impl<'a> Resolver<'a> {
                 if ident.name == lookup_name && ns == namespace {
                     if filter_fn(name_binding.def()) {
                         // create the path
-                        let mut segms = path_segments.clone();
+                        let mut segms = if self.session.rust_2018() && !in_module_is_extern {
+                            // crate-local absolute paths start with `crate::` in edition 2018
+                            // FIXME: may also be stabilized for Rust 2015 (Issues #45477, #44660)
+                            let mut full_segms = vec![
+                                ast::PathSegment::from_ident(keywords::Crate.ident())
+                            ];
+                            full_segms.extend(path_segments.clone());
+                            full_segms
+                        } else {
+                            path_segments.clone()
+                        };
+
                         segms.push(ast::PathSegment::from_ident(ident));
                         let path = Path {
                             span: name_binding.span,

Diff for: src/test/ui/rust-2018/issue-52202-use-suggestions.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// compile-flags: --edition 2018
+
+// The local `use` suggestion should start with `crate::` (but the
+// standard-library suggestions should not, obviously).
+
+mod plumbing {
+    pub struct Drain;
+}
+
+fn main() {
+    let _d = Drain {};
+    //~^ ERROR cannot find struct, variant or union type `Drain` in this scope
+}

Diff for: src/test/ui/rust-2018/issue-52202-use-suggestions.stderr

@@ -0,0 +1,20 @@
+error[E0422]: cannot find struct, variant or union type `Drain` in this scope
+  --> $DIR/issue-52202-use-suggestions.rs:21:14
+   |
+LL |     let _d = Drain {};
+   |              ^^^^^ not found in this scope
+help: possible candidates are found in other modules, you can import them into scope
+   |
+LL | use crate::plumbing::Drain;
+   |
+LL | use std::collections::binary_heap::Drain;
+   |
+LL | use std::collections::hash_map::Drain;
+   |
+LL | use std::collections::hash_set::Drain;
+   |
+and 3 other candidates
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0422`.