Remove useless filter in unused extern crate check.

compiler/rustc_hir_analysis/src/check_unused.rs

@@ -56,25 +56,6 @@ fn unused_crates_lint(tcx: TyCtxt<'_>) {
     let unused_extern_crates: FxHashMap<LocalDefId, Span> = tcx
         .maybe_unused_extern_crates(())
         .iter()
-        .filter(|&&(def_id, _)| {
-            // The `def_id` here actually was calculated during resolution (at least
-            // at the time of this writing) and is being shipped to us via a side
-            // channel of the tcx. There may have been extra expansion phases,
-            // however, which ended up removing the `def_id` *after* expansion.
-            //
-            // As a result we need to verify that `def_id` is indeed still valid for
-            // our AST and actually present in the HIR map. If it's not there then
-            // there's safely nothing to warn about, and otherwise we carry on with
-            // our execution.
-            //
-            // Note that if we carry through to the `extern_mod_stmt_cnum` query
-            // below it'll cause a panic because `def_id` is actually bogus at this
-            // point in time otherwise.
-            if tcx.hir().find(tcx.hir().local_def_id_to_hir_id(def_id)).is_none() {
-                return false;
-            }
-            true
-        })
         .filter(|&&(def_id, _)| {
             tcx.extern_mod_stmt_cnum(def_id).map_or(true, |cnum| {
                 !tcx.is_compiler_builtins(cnum)

src/test/ui/attributes/unused-item-in-attr.rs

@@ -0,0 +1,6 @@
+#[w = { extern crate alloc; }]
+//~^ ERROR unexpected expression: `{
+//~| ERROR cannot find attribute `w` in this scope
+fn f() {}
+
+fn main() {}

src/test/ui/attributes/unused-item-in-attr.stderr

@@ -0,0 +1,16 @@
+error: unexpected expression: `{
+           extern crate alloc;
+       }`
+  --> $DIR/unused-item-in-attr.rs:1:7
+   |
+LL | #[w = { extern crate alloc; }]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: cannot find attribute `w` in this scope
+  --> $DIR/unused-item-in-attr.rs:1:3
+   |
+LL | #[w = { extern crate alloc; }]
+   |   ^
+
+error: aborting due to 2 previous errors
+