Don't lint non-extern-prelude extern crate's in Rust 2018.

src/librustc/session/mod.rs

@@ -38,7 +38,7 @@ use syntax::parse;
 use syntax::parse::ParseSess;
 use syntax::{ast, source_map};
 use syntax::feature_gate::AttributeType;
-use syntax_pos::{MultiSpan, Span};
+use syntax_pos::{MultiSpan, Span, symbol::Symbol};
 use util::profiling::SelfProfiler;
 
 use rustc_target::spec::PanicStrategy;
@@ -168,6 +168,10 @@ pub struct Session {
 
     /// Cap lint level specified by a driver specifically.
     pub driver_lint_caps: FxHashMap<lint::LintId, lint::Level>,
+
+    /// All the crate names specified with `--extern`, and the builtin ones.
+    /// Starting with the Rust 2018 edition, absolute paths resolve in this set.
+    pub extern_prelude: FxHashSet<Symbol>,
 }
 
 pub struct PerfStats {
@@ -1126,6 +1130,18 @@ pub fn build_session_(
         CguReuseTracker::new_disabled()
     };
 
+
+    let mut extern_prelude: FxHashSet<Symbol> =
+        sopts.externs.iter().map(|kv| Symbol::intern(kv.0)).collect();
+
+    // HACK(eddyb) this ignores the `no_{core,std}` attributes.
+    // FIXME(eddyb) warn (somewhere) if core/std is used with `no_{core,std}`.
+    // if !attr::contains_name(&krate.attrs, "no_core") {
+    // if !attr::contains_name(&krate.attrs, "no_std") {
+    extern_prelude.insert(Symbol::intern("core"));
+    extern_prelude.insert(Symbol::intern("std"));
+    extern_prelude.insert(Symbol::intern("meta"));
+
     let sess = Session {
         target: target_cfg,
         host,
@@ -1201,6 +1217,7 @@ pub fn build_session_(
         has_global_allocator: Once::new(),
         has_panic_handler: Once::new(),
         driver_lint_caps: FxHashMap(),
+        extern_prelude,
     };
 
     validate_commandline_args_with_session_available(&sess);

src/librustc_resolve/lib.rs

@@ -1350,7 +1350,6 @@ pub struct Resolver<'a, 'b: 'a> {
     graph_root: Module<'a>,
 
     prelude: Option<Module<'a>>,
-    extern_prelude: FxHashSet<Name>,
 
     /// n.b. This is used only for better diagnostics, not name resolution itself.
     has_self: FxHashSet<DefId>,
@@ -1663,17 +1662,6 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         DefCollector::new(&mut definitions, Mark::root())
             .collect_root(crate_name, session.local_crate_disambiguator());
 
-        let mut extern_prelude: FxHashSet<Name> =
-            session.opts.externs.iter().map(|kv| Symbol::intern(kv.0)).collect();
-
-        // HACK(eddyb) this ignore the `no_{core,std}` attributes.
-        // FIXME(eddyb) warn (elsewhere) if core/std is used with `no_{core,std}`.
-        // if !attr::contains_name(&krate.attrs, "no_core") {
-        // if !attr::contains_name(&krate.attrs, "no_std") {
-        extern_prelude.insert(Symbol::intern("core"));
-        extern_prelude.insert(Symbol::intern("std"));
-        extern_prelude.insert(Symbol::intern("meta"));
-
         let mut invocations = FxHashMap();
         invocations.insert(Mark::root(),
                            arenas.alloc_invocation_data(InvocationData::root(graph_root)));
@@ -1692,7 +1680,6 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
             // AST.
             graph_root,
             prelude: None,
-            extern_prelude,
 
             has_self: FxHashSet(),
             field_names: FxHashMap(),
@@ -1963,7 +1950,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
 
         if !module.no_implicit_prelude {
             // `record_used` means that we don't try to load crates during speculative resolution
-            if record_used && ns == TypeNS && self.extern_prelude.contains(&ident.name) {
+            if record_used && ns == TypeNS && self.session.extern_prelude.contains(&ident.name) {
                 let crate_id = self.crate_loader.process_path_extern(ident.name, ident.span);
                 let crate_root = self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
                 self.populate_module_if_necessary(&crate_root);
@@ -3955,7 +3942,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                     } else {
                         // Items from the prelude
                         if !module.no_implicit_prelude {
-                            names.extend(self.extern_prelude.iter().cloned());
+                            names.extend(self.session.extern_prelude.iter().cloned());
                             if let Some(prelude) = self.prelude {
                                 add_module_candidates(prelude, &mut names);
                             }
@@ -4401,8 +4388,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         );
 
         if self.session.rust_2018() {
-            let extern_prelude_names = self.extern_prelude.clone();
-            for &name in extern_prelude_names.iter() {
+            for &name in &self.session.extern_prelude {
                 let ident = Ident::with_empty_ctxt(name);
                 match self.crate_loader.maybe_process_path_extern(name, ident.span) {
                     Some(crate_id) => {

src/librustc_resolve/macros.rs

@@ -682,7 +682,7 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                     result
                 }
                 WhereToResolve::ExternPrelude => {
-                    if use_prelude && self.extern_prelude.contains(&ident.name) {
+                    if use_prelude && self.session.extern_prelude.contains(&ident.name) {
                         let crate_id =
                             self.crate_loader.process_path_extern(ident.name, ident.span);
                         let crate_root =

src/librustc_resolve/resolve_imports.rs

@@ -199,7 +199,7 @@ impl<'a, 'crateloader> Resolver<'a, 'crateloader> {
                     if !(
                         ns == TypeNS &&
                         !ident.is_path_segment_keyword() &&
-                        self.extern_prelude.contains(&ident.name)
+                        self.session.extern_prelude.contains(&ident.name)
                     ) {
                         // ... unless the crate name is not in the `extern_prelude`.
                         return binding;
@@ -218,7 +218,7 @@ impl<'a, 'crateloader> Resolver<'a, 'crateloader> {
                 } else if
                     ns == TypeNS &&
                     !ident.is_path_segment_keyword() &&
-                    self.extern_prelude.contains(&ident.name)
+                    self.session.extern_prelude.contains(&ident.name)
                 {
                     let crate_id =
                         self.crate_loader.process_path_extern(ident.name, ident.span);
@@ -735,7 +735,7 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         let uniform_paths_feature = self.session.features_untracked().uniform_paths;
         for ((span, _, ns), results) in uniform_paths_canaries {
             let name = results.name;
-            let external_crate = if ns == TypeNS && self.extern_prelude.contains(&name) {
+            let external_crate = if ns == TypeNS && self.session.extern_prelude.contains(&name) {
                 let crate_id =
                     self.crate_loader.process_path_extern(name, span);
                 Some(Def::Mod(DefId { krate: crate_id, index: CRATE_DEF_INDEX }))

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 {