Rollup merge of #47726 - pietroalbini:fix-nested-empty-groups-span, r=petrochenkov

Fix spans in unused import lint for nested groups

This fixes an inconsistency for empty nested groups, and adds a test for all the possible cases of the lint.

```
warning: unused imports: `*`, `Foo`, `baz::{}`, `foobar::*`
  --> test.rs:16:11
   |
16 | use foo::{Foo, bar::{baz::{}, foobar::*}, *};
   |           ^^^        ^^^^^^^  ^^^^^^^^^   ^
   |
   = note: #[warn(unused_imports)] on by default

warning: unused import: `*`
  --> test.rs:17:24
   |
17 | use foo::bar::baz::{*, *};
   |                        ^

warning: unused import: `use foo::{};`
  --> test.rs:18:1
   |
18 | use foo::{};
   | ^^^^^^^^^^^^
```

cc #44494

Author: GuillaumeGomez

src/librustc_resolve/check_unused.rs

@@ -102,11 +102,18 @@ impl<'a, 'b> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b> {
         }
 
         if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
+            // If it's the parent group, cover the entire use item
+            let span = if nested {
+                use_tree.span
+            } else {
+                self.item_span
+            };
+
             if items.len() == 0 {
                 self.unused_imports
                     .entry(self.base_id)
                     .or_insert_with(NodeMap)
-                    .insert(id, self.item_span);
+                    .insert(id, span);
             }
         } else {
             let base_id = self.base_id;

src/test/ui/owl-import-generates-unused-import-lint.stderr

@@ -1,4 +1,4 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -9,13 +9,26 @@
 // except according to those terms.
 
 #![feature(use_nested_groups)]
+#![allow(dead_code)]
 #![deny(unused_imports)]
 
 mod foo {
-    pub enum Bar {}
+    pub mod bar {
+        pub mod baz {
+            pub struct Bar();
+        }
+        pub mod foobar {}
+    }
+
+    pub struct Foo();
 }
 
-use foo::{*, *}; //~ ERROR unused import: `*`
+use foo::{Foo, bar::{baz::{}, foobar::*}, *};
+    //~^ ERROR unused imports: `*`, `Foo`, `baz::{}`, `foobar::*`
+use foo::bar::baz::{*, *};
+    //~^ ERROR unused import: `*`
+use foo::{};
+    //~^ ERROR unused import: `use foo::{};`
 
 fn main() {
     let _: Bar;

src/test/ui/owl-import-generates-unused-import-lint.rs renamed to src/test/ui/use-nested-groups-unused-imports.rs

@@ -0,0 +1,26 @@
+error: unused imports: `*`, `Foo`, `baz::{}`, `foobar::*`
+  --> $DIR/use-nested-groups-unused-imports.rs:26:11
+   |
+26 | use foo::{Foo, bar::{baz::{}, foobar::*}, *};
+   |           ^^^        ^^^^^^^  ^^^^^^^^^   ^
+   |
+note: lint level defined here
+  --> $DIR/use-nested-groups-unused-imports.rs:13:9
+   |
+13 | #![deny(unused_imports)]
+   |         ^^^^^^^^^^^^^^
+
+error: unused import: `*`
+  --> $DIR/use-nested-groups-unused-imports.rs:28:24
+   |
+28 | use foo::bar::baz::{*, *};
+   |                        ^
+
+error: unused import: `use foo::{};`
+  --> $DIR/use-nested-groups-unused-imports.rs:30:1
+   |
+30 | use foo::{};
+   | ^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+