Auto merge of #50260 - Manishearth:no-extern-crate, r=nikomatsakis

idiom lints for removing `extern crate`

Based off of https://github.com/rust-lang/rust/pull/49789

This contains two lints:

 - One that suggests replacing pub extern crates with pub use, and removing non-pub extern crates entirely
 - One that suggests rewriting `use modulename::...::cratename::foo` as `cratename::foo`

The latter is a bit tricky to emit suggestions for; for one this involves splicing spans (never a good idea), and it also won't be able to correctly
handle `use module::{cratename, foo}` and use-trees. I'm not sure how to proceed here. Currently it doesn't suggest anything at all.

Perhaps we can go the other way and suggest removal of all extern crates _except_ those used through modules (stash node ids somewhere) and suggest replacing those with `<visibility> use`?

r? @nikomatsakis

fixes https://github.com/rust-lang/rust/issues/48719

src/test/ui-fulldeps/unnecessary-extern-crate.rs

@@ -0,0 +1,55 @@
+// Copyright 2015 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.
+
+#![deny(unnecessary_extern_crate)]
+#![feature(alloc, test, libc)]
+
+extern crate alloc;
+//~^ ERROR `extern crate` is unnecessary in the new edition
+//~| HELP remove
+extern crate alloc as x;
+//~^ ERROR `extern crate` is unnecessary in the new edition
+//~| HELP use `use`
+
+#[macro_use]
+extern crate test;
+pub extern crate test as y;
+//~^ ERROR `extern crate` is unnecessary in the new edition
+//~| HELP use `pub use`
+pub extern crate libc;
+//~^ ERROR `extern crate` is unnecessary in the new edition
+//~| HELP use `pub use`
+
+
+mod foo {
+    extern crate alloc;
+    //~^ ERROR `extern crate` is unnecessary in the new edition
+    //~| HELP use `use`
+    extern crate alloc as x;
+    //~^ ERROR `extern crate` is unnecessary in the new edition
+    //~| HELP use `use`
+    pub extern crate test;
+    //~^ ERROR `extern crate` is unnecessary in the new edition
+    //~| HELP use `pub use`
+    pub extern crate test as y;
+    //~^ ERROR `extern crate` is unnecessary in the new edition
+    //~| HELP use `pub use`
+    mod bar {
+        extern crate alloc;
+        //~^ ERROR `extern crate` is unnecessary in the new edition
+        //~| HELP use `use`
+        extern crate alloc as x;
+        //~^ ERROR `extern crate` is unnecessary in the new edition
+        //~| HELP use `use`
+    }
+}
+
+
+fn main() {}

src/test/ui-fulldeps/unnecessary-extern-crate.stderr

@@ -0,0 +1,68 @@
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:14:1
+   |
+LL | extern crate alloc;
+   | ^^^^^^^^^^^^^^^^^^^ help: remove it
+   |
+note: lint level defined here
+  --> $DIR/unnecessary-extern-crate.rs:11:9
+   |
+LL | #![deny(unnecessary_extern_crate)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:17:1
+   |
+LL | extern crate alloc as x;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ help: use `use`: `use alloc as x`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:23:1
+   |
+LL | pub extern crate test as y;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `pub use`: `pub use test as y`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:26:1
+   |
+LL | pub extern crate libc;
+   | ^^^^^^^^^^^^^^^^^^^^^^ help: use `pub use`: `pub use libc`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:32:5
+   |
+LL |     extern crate alloc;
+   |     ^^^^^^^^^^^^^^^^^^^ help: use `use`: `use alloc`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:35:5
+   |
+LL |     extern crate alloc as x;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: use `use`: `use alloc as x`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:38:5
+   |
+LL |     pub extern crate test;
+   |     ^^^^^^^^^^^^^^^^^^^^^^ help: use `pub use`: `pub use test`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:41:5
+   |
+LL |     pub extern crate test as y;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `pub use`: `pub use test as y`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:45:9
+   |
+LL |         extern crate alloc;
+   |         ^^^^^^^^^^^^^^^^^^^ help: use `use`: `use alloc`
+
+error: `extern crate` is unnecessary in the new edition
+  --> $DIR/unnecessary-extern-crate.rs:48:9
+   |
+LL |         extern crate alloc as x;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: use `use`: `use alloc as x`
+
+error: aborting due to 10 previous errors
+