Don't emit "unused extern crate" warnings for extern crate foo as _;

When importing a crate and renaming it to an underscore-prefixed name,
suppress "unused extern crate" warnings (but not idiom lints).

Author: joshtriplett

src/librustc_typeck/check_unused.rs

@@ -133,6 +133,7 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
 
         // 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);
@@ -147,6 +148,7 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
                     .emit();
                 continue;
             }
+        }
 
         // If we are not in Rust 2018 edition, then we don't make any further
         // suggestions.
@@ -202,6 +204,10 @@ struct ExternCrateToLint {
     /// crate_name`), and -- perhaps surprisingly -- this stores the
     /// *original* name (`item.name` will contain the new name)
     orig_name: Option<ast::Name>,
+
+    /// if `false`, the original name started with `_`, so we shouldn't lint
+    /// about it going unused (but we should still emit idiom lints).
+    warn_if_unused: bool,
 }
 
 impl<'a, 'tcx, 'v> ItemLikeVisitor<'v> for CollectExternCrateVisitor<'a, 'tcx> {
@@ -213,6 +219,7 @@ impl<'a, 'tcx, 'v> ItemLikeVisitor<'v> for CollectExternCrateVisitor<'a, 'tcx> {
                     def_id: extern_crate_def_id,
                     span: item.span,
                     orig_name,
+                    warn_if_unused: !item.name.as_str().starts_with('_'),
                 }
             );
         }

src/test/ui/rfc-2166-underscore-imports/basic.rs

@@ -30,7 +30,7 @@ mod m {
 mod unused {
     use m::Tr1 as _; //~ WARN unused import
     use S as _; //~ WARN unused import
-    extern crate core as _; //~ WARN unused extern crate
+    extern crate core as _; // OK
 }
 
 mod outer {

src/test/ui/rfc-2166-underscore-imports/basic.stderr

@@ -16,15 +16,3 @@ warning: unused import: `S as _`
 LL |     use S as _; //~ WARN unused import
    |         ^^^^^^
 
-warning: unused extern crate
-  --> $DIR/basic.rs:33:5
-   |
-LL |     extern crate core as _; //~ WARN unused extern crate
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
-   |
-note: lint level defined here
-  --> $DIR/basic.rs:14:25
-   |
-LL | #![warn(unused_imports, unused_extern_crates)]
-   |                         ^^^^^^^^^^^^^^^^^^^^
-