rustc_typeck: don't lint non-extern-prelude extern crate's in Rust 2018.

src/librustc_typeck/check_unused.rs

@@ -130,15 +130,13 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
     });
 
     for extern_crate in &crates_to_lint {
-        assert!(extern_crate.def_id.is_local());
+        let id = tcx.hir.as_local_node_id(extern_crate.def_id).unwrap();
+        let item = tcx.hir.expect_item(id);
 
         // If the crate is fully unused, we suggest removing it altogether.
         // We do this in any edition.
         if extern_crate.warn_if_unused {
             if let Some(&span) = unused_extern_crates.get(&extern_crate.def_id) {
-                assert_eq!(extern_crate.def_id.krate, LOCAL_CRATE);
-                let hir_id = tcx.hir.definitions().def_index_to_hir_id(extern_crate.def_id.index);
-                let id = tcx.hir.hir_to_node_id(hir_id);
                 let msg = "unused extern crate";
                 tcx.struct_span_lint_node(lint, id, span, msg)
                     .span_suggestion_short_with_applicability(
@@ -157,6 +155,13 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
             continue;
         }
 
+        // If the extern crate isn't in the extern prelude,
+        // there is no way it can be written as an `use`.
+        let orig_name = extern_crate.orig_name.unwrap_or(item.name);
+        if !tcx.sess.extern_prelude.contains(&orig_name) {
+            continue;
+        }
+
         // If the extern crate has any attributes, they may have funky
         // semantics we can't faithfully represent using `use` (most
         // notably `#[macro_use]`). Ignore it.
@@ -165,9 +170,6 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
         }
 
         // Otherwise, we can convert it into a `use` of some kind.
-        let hir_id = tcx.hir.definitions().def_index_to_hir_id(extern_crate.def_id.index);
-        let id = tcx.hir.hir_to_node_id(hir_id);
-        let item = tcx.hir.expect_item(id);
         let msg = "`extern crate` is not idiomatic in the new edition";
         let help = format!(
             "convert it to a `{}`",

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

@@ -20,33 +20,23 @@ extern crate alloc as x;
 //~^ ERROR unused extern crate
 //~| HELP remove
 
+extern crate proc_macro;
+
 #[macro_use]
 extern crate test;
 
 pub extern crate test as y;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
-//~| HELP convert it to a `pub use`
 
 pub extern crate libc;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
-//~| HELP convert it to a `pub use`
 
 pub(crate) extern crate libc as a;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
-//~| HELP convert it to a `pub(crate) use`
 
 crate extern crate libc as b;
-//~^ ERROR `extern crate` is not idiomatic in the new edition
-//~| HELP convert it to a `crate use`
 
 mod foo {
     pub(in crate::foo) extern crate libc as c;
-    //~^ ERROR `extern crate` is not idiomatic in the new edition
-    //~| HELP convert it to a `pub(in crate::foo) use`
 
     pub(super) extern crate libc as d;
-    //~^ ERROR `extern crate` is not idiomatic in the new edition
-    //~| HELP convert it to a `pub(super) use`
 
     extern crate alloc;
     //~^ ERROR unused extern crate
@@ -57,12 +47,8 @@ mod foo {
     //~| HELP remove
 
     pub extern crate test;
-    //~^ ERROR `extern crate` is not idiomatic in the new edition
-    //~| HELP convert it
 
     pub extern crate test as y;
-    //~^ ERROR `extern crate` is not idiomatic in the new edition
-    //~| HELP convert it
 
     mod bar {
         extern crate alloc;
@@ -74,8 +60,6 @@ mod foo {
         //~| HELP remove
 
         pub(in crate::foo::bar) extern crate libc as e;
-        //~^ ERROR `extern crate` is not idiomatic in the new edition
-        //~| HELP convert it to a `pub(in crate::foo::bar) use`
 
         fn dummy() {
             unsafe {
@@ -96,4 +80,6 @@ mod foo {
 fn main() {
     unsafe { a::getpid(); }
     unsafe { b::getpid(); }
+
+    proc_macro::TokenStream::new();
 }

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

@@ -16,83 +16,29 @@ error: unused extern crate
 LL | extern crate alloc as x;
    | ^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
 
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:26:1
-   |
-LL | pub extern crate test as y;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:30:1
-   |
-LL | pub extern crate libc;
-   | ^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:34:1
-   |
-LL | pub(crate) extern crate libc as a;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub(crate) use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:38:1
-   |
-LL | crate extern crate libc as b;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `crate use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:43:5
-   |
-LL |     pub(in crate::foo) extern crate libc as c;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub(in crate::foo) use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:47:5
-   |
-LL |     pub(super) extern crate libc as d;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub(super) use`
-
 error: unused extern crate
-  --> $DIR/unnecessary-extern-crate.rs:51:5
+  --> $DIR/unnecessary-extern-crate.rs:41:5
    |
 LL |     extern crate alloc;
    |     ^^^^^^^^^^^^^^^^^^^ help: remove it
 
 error: unused extern crate
-  --> $DIR/unnecessary-extern-crate.rs:55:5
+  --> $DIR/unnecessary-extern-crate.rs:45:5
    |
 LL |     extern crate alloc as x;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
 
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:59:5
-   |
-LL |     pub extern crate test;
-   |     ^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub use`
-
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:63:5
-   |
-LL |     pub extern crate test as y;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub use`
-
 error: unused extern crate
-  --> $DIR/unnecessary-extern-crate.rs:68:9
+  --> $DIR/unnecessary-extern-crate.rs:54:9
    |
 LL |         extern crate alloc;
    |         ^^^^^^^^^^^^^^^^^^^ help: remove it
 
 error: unused extern crate
-  --> $DIR/unnecessary-extern-crate.rs:72:9
+  --> $DIR/unnecessary-extern-crate.rs:58:9
    |
 LL |         extern crate alloc as x;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
 
-error: `extern crate` is not idiomatic in the new edition
-  --> $DIR/unnecessary-extern-crate.rs:76:9
-   |
-LL |         pub(in crate::foo::bar) extern crate libc as e;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `pub(in crate::foo::bar) use`
-
-error: aborting due to 15 previous errors
+error: aborting due to 6 previous errors
 

src/test/ui/rust-2018/remove-extern-crate.fixed

@@ -14,6 +14,7 @@
 // aux-build:remove-extern-crate.rs
 // compile-flags:--extern remove_extern_crate
 
+#![feature(alloc)]
 #![warn(rust_2018_idioms)]
 
 
@@ -22,11 +23,16 @@ use remove_extern_crate;
 #[macro_use]
 extern crate remove_extern_crate as something_else;
 
+// Shouldn't suggest changing to `use`, as the `alloc`
+// crate is not in the extern prelude - see #54381.
+extern crate alloc;
+
 fn main() {
     another_name::mem::drop(3);
     another::foo();
     remove_extern_crate::foo!();
     bar!();
+    alloc::vec![5];
 }
 
 mod another {

src/test/ui/rust-2018/remove-extern-crate.rs

@@ -14,6 +14,7 @@
 // aux-build:remove-extern-crate.rs
 // compile-flags:--extern remove_extern_crate
 
+#![feature(alloc)]
 #![warn(rust_2018_idioms)]
 
 extern crate core;
@@ -22,11 +23,16 @@ use remove_extern_crate;
 #[macro_use]
 extern crate remove_extern_crate as something_else;
 
+// Shouldn't suggest changing to `use`, as the `alloc`
+// crate is not in the extern prelude - see #54381.
+extern crate alloc;
+
 fn main() {
     another_name::mem::drop(3);
     another::foo();
     remove_extern_crate::foo!();
     bar!();
+    alloc::vec![5];
 }
 
 mod another {

src/test/ui/rust-2018/remove-extern-crate.stderr

@@ -1,24 +1,24 @@
 warning: unused extern crate
-  --> $DIR/remove-extern-crate.rs:19:1
+  --> $DIR/remove-extern-crate.rs:20:1
    |
 LL | extern crate core;
    | ^^^^^^^^^^^^^^^^^^ help: remove it
    |
 note: lint level defined here
-  --> $DIR/remove-extern-crate.rs:17:9
+  --> $DIR/remove-extern-crate.rs:18:9
    |
 LL | #![warn(rust_2018_idioms)]
    |         ^^^^^^^^^^^^^^^^
    = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]
 
 warning: `extern crate` is not idiomatic in the new edition
-  --> $DIR/remove-extern-crate.rs:20:1
+  --> $DIR/remove-extern-crate.rs:21:1
    |
 LL | extern crate core as another_name;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
 
 warning: `extern crate` is not idiomatic in the new edition
-  --> $DIR/remove-extern-crate.rs:33:5
+  --> $DIR/remove-extern-crate.rs:39:5
    |
 LL |     extern crate core;
    |     ^^^^^^^^^^^^^^^^^^ help: convert it to a `use`