resolve: Mark extern crate items as used in more cases

src/librustc_resolve/lib.rs

@@ -5089,6 +5089,9 @@ impl<'a> Resolver<'a> {
         }
         self.extern_prelude.get(&ident.modern()).cloned().and_then(|entry| {
             if let Some(binding) = entry.extern_crate_item {
+                if !speculative && entry.introduced_by_item {
+                    self.record_use(ident, TypeNS, binding, false);
+                }
                 Some(binding)
             } else {
                 let crate_id = if !speculative {

src/test/ui/imports/extern-crate-used.rs

@@ -0,0 +1,28 @@
+// Extern crate items are marked as used if they are used
+// through extern prelude entries introduced by them.
+
+// edition:2018
+
+#![deny(unused_extern_crates)]
+
+extern crate core as iso1; //~ ERROR `extern crate` is not idiomatic in the new edition
+extern crate core as iso2; //~ ERROR `extern crate` is not idiomatic in the new edition
+extern crate core as iso3; //~ ERROR `extern crate` is not idiomatic in the new edition
+extern crate core as iso4; //~ ERROR `extern crate` is not idiomatic in the new edition
+
+// Doesn't introduce its extern prelude entry, so it's still considered unused.
+extern crate core; //~ ERROR unused extern crate
+
+mod m {
+    use iso1::any as are_you_okay1;
+    use ::iso2::any as are_you_okay2;
+    type AreYouOkay1 = iso3::any::Any;
+    type AreYouOkay2 = ::iso4::any::Any;
+
+    use core::any as are_you_okay3;
+    use ::core::any as are_you_okay4;
+    type AreYouOkay3 = core::any::Any;
+    type AreYouOkay4 = ::core::any::Any;
+}
+
+fn main() {}

src/test/ui/imports/extern-crate-used.stderr

@@ -0,0 +1,38 @@
+error: `extern crate` is not idiomatic in the new edition
+  --> $DIR/extern-crate-used.rs:8:1
+   |
+LL | extern crate core as iso1; //~ ERROR `extern crate` is not idiomatic in the new edition
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
+   |
+note: lint level defined here
+  --> $DIR/extern-crate-used.rs:6:9
+   |
+LL | #![deny(unused_extern_crates)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: `extern crate` is not idiomatic in the new edition
+  --> $DIR/extern-crate-used.rs:9:1
+   |
+LL | extern crate core as iso2; //~ ERROR `extern crate` is not idiomatic in the new edition
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
+
+error: `extern crate` is not idiomatic in the new edition
+  --> $DIR/extern-crate-used.rs:10:1
+   |
+LL | extern crate core as iso3; //~ ERROR `extern crate` is not idiomatic in the new edition
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
+
+error: `extern crate` is not idiomatic in the new edition
+  --> $DIR/extern-crate-used.rs:11:1
+   |
+LL | extern crate core as iso4; //~ ERROR `extern crate` is not idiomatic in the new edition
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
+
+error: unused extern crate
+  --> $DIR/extern-crate-used.rs:14:1
+   |
+LL | extern crate core; //~ ERROR unused extern crate
+   | ^^^^^^^^^^^^^^^^^^ help: remove it
+
+error: aborting due to 5 previous errors
+