resolve: Do not skip extern prelude during speculative resolution

Author: petrochenkov

src/librustc_resolve/lib.rs

@@ -1977,9 +1977,15 @@ 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) {
-                let crate_id = self.crate_loader.process_path_extern(ident.name, ident.span);
+            if ns == TypeNS && self.extern_prelude.contains(&ident.name) {
+                let crate_id = if record_used {
+                    self.crate_loader.process_path_extern(ident.name, ident.span)
+                } else if let Some(crate_id) =
+                        self.crate_loader.maybe_process_path_extern(ident.name, ident.span) {
+                    crate_id
+                } else {
+                    return None;
+                };
                 let crate_root = self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
                 self.populate_module_if_necessary(&crate_root);
 

src/test/ui/impl-trait/auxiliary/extra-item.rs

@@ -0,0 +1 @@
+pub trait MyTrait {}

src/test/ui/impl-trait/extra-item.rs

@@ -0,0 +1,10 @@
+// aux-build:extra-item.rs
+// compile-flags:--extern extra_item
+
+struct S;
+
+impl extra_item::MyTrait for S {
+    fn extra() {} //~ ERROR method `extra` is not a member of trait `extra_item::MyTrait`
+}
+
+fn main() {}

src/test/ui/impl-trait/extra-item.stderr

@@ -0,0 +1,9 @@
+error[E0407]: method `extra` is not a member of trait `extra_item::MyTrait`
+  --> $DIR/extra-item.rs:7:5
+   |
+LL |     fn extra() {} //~ ERROR method `extra` is not a member of trait `extra_item::MyTrait`
+   |     ^^^^^^^^^^^^^ not a member of trait `extra_item::MyTrait`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0407`.

src/test/ui/run-pass/extern/extern-prelude-no-speculative.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 // run-pass
-// compile-flags: --extern LooksLikeExternCrate=/path/to/nowhere
+// compile-flags: --extern LooksLikeExternCrate
 
 mod m {
     pub struct LooksLikeExternCrate;