Fix issues in suggesting importing extern crate paths

Author: chenyukang

compiler/rustc_resolve/src/diagnostics.rs

@@ -1290,10 +1290,20 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                     let mut path_segments = path_segments.clone();
                     path_segments.push(ast::PathSegment::from_ident(ident));
 
+                    let alias_import = if let NameBindingKind::Import { import, .. } =
+                        name_binding.kind
+                        && let ImportKind::ExternCrate { source: Some(_), .. } = import.kind
+                        && import.parent_scope.expansion == parent_scope.expansion
+                    {
+                        true
+                    } else {
+                        false
+                    };
+
                     let is_extern_crate_that_also_appears_in_prelude =
                         name_binding.is_extern_crate() && lookup_ident.span.at_least_rust_2018();
 
-                    if !is_extern_crate_that_also_appears_in_prelude {
+                    if !is_extern_crate_that_also_appears_in_prelude || alias_import {
                         // add the module to the lookup
                         if seen_modules.insert(module.def_id()) {
                             if via_import { &mut worklist_via_import } else { &mut worklist }

tests/ui/imports/auxiliary/issue-121168-extern.rs

@@ -0,0 +1 @@
+pub struct Foo<T>(pub core::ptr::NonNull<T>);

tests/ui/imports/issue-121168-2.rs

@@ -0,0 +1,12 @@
+//@ edition: 2018
+//@ compile-flags: --extern issue_121168_extern
+//@ aux-build: issue-121168-extern.rs
+
+extern crate issue_121168_extern as nice_crate_name;
+
+fn use_foo_from_another_crate_without_importing_it_first() {
+    //use nice_crate_name::Foo;
+    let _: Foo<i32> = todo!(); //~ ERROR cannot find type `Foo` in this scope
+}
+
+fn main() {}

tests/ui/imports/issue-121168-2.stderr

@@ -0,0 +1,16 @@
+error[E0412]: cannot find type `Foo` in this scope
+  --> $DIR/issue-121168-2.rs:9:12
+   |
+LL |     let _: Foo<i32> = todo!();
+   |            ^^^ not found in this scope
+   |
+help: consider importing one of these items
+   |
+LL + use crate::nice_crate_name::Foo;
+   |
+LL + use issue_121168_extern::Foo;
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0412`.

tests/ui/imports/issue-121168-3.rs

@@ -0,0 +1,12 @@
+//@ edition: 2015
+//@ compile-flags: --extern issue_121168_extern
+//@ aux-build: issue-121168-extern.rs
+
+extern crate issue_121168_extern as nice_crate_name;
+
+fn use_foo_from_another_crate_without_importing_it_first() {
+    //use nice_crate_name::Foo;
+    let _: Foo<i32> = todo!(); //~ ERROR cannot find type `Foo` in this scope
+}
+
+fn main() {}

tests/ui/imports/issue-121168-3.stderr

@@ -0,0 +1,14 @@
+error[E0412]: cannot find type `Foo` in this scope
+  --> $DIR/issue-121168-3.rs:9:12
+   |
+LL |     let _: Foo<i32> = todo!();
+   |            ^^^ not found in this scope
+   |
+help: consider importing this struct
+   |
+LL + use nice_crate_name::Foo;
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0412`.

tests/ui/imports/issue-121168.rs

@@ -0,0 +1,12 @@
+//@ edition: 2021
+//@ compile-flags: --extern issue_121168_extern
+//@ aux-build: issue-121168-extern.rs
+
+extern crate issue_121168_extern as nice_crate_name;
+
+fn use_foo_from_another_crate_without_importing_it_first() {
+    //use nice_crate_name::Foo;
+    let _: Foo<i32> = todo!(); //~ ERROR cannot find type `Foo` in this scope
+}
+
+fn main() {}

tests/ui/imports/issue-121168.stderr

@@ -0,0 +1,16 @@
+error[E0412]: cannot find type `Foo` in this scope
+  --> $DIR/issue-121168.rs:9:12
+   |
+LL |     let _: Foo<i32> = todo!();
+   |            ^^^ not found in this scope
+   |
+help: consider importing one of these items
+   |
+LL + use crate::nice_crate_name::Foo;
+   |
+LL + use issue_121168_extern::Foo;
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0412`.