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

@@ -132,15 +132,17 @@ 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 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(span, "remove it", "".to_string())
- .emit();
- continue;
+ 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(span, "remove it", "".to_string())
+ .emit();
+ continue;
+ }
  }
 
  // If we are not in Rust 2018 edition, then we don't make any further
@@ -192,6 +194,10 @@ 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> {
@@ -203,6 +209,7 @@ 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)]
- | ^^^^^^^^^^^^^^^^^^^^
-