Rollup merge of #123344 - pietroalbini:pa-unused-imports, r=Nilstrieb

Remove braces when fixing a nested use tree into a single item

[Back in 2019](https://github.com/rust-lang/rust/pull/56645) I added rustfix support for the `unused_imports` lint, to automatically remove them when running `cargo fix`. For the most part this worked great, but when removing all but one childs of a nested use tree it turned `use foo::{Unused, Used}` into `use foo::{Used}`. This is slightly annoying, because it then requires you to run `rustfmt` to get `use foo::Used`.

This PR automatically removes braces and the surrouding whitespace when all but one child of a nested use tree are unused. To get it done I had to add the span of the nested use tree to the AST, and refactor a bit the code I wrote back then.

A thing I noticed is, there doesn't seem to be any `//@ run-rustfix` test for fixing the `unused_imports` lint. I created a test in `tests/suggestions` (is that the right directory?) that for now tests just what I added in the PR. I can followup in a separate PR to add more tests for fixing `unused_lints`.

This PR is best reviewed commit-by-commit.

1 files changed, 6 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 848277c4611..3ac50a6e4a8 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -336,7 +336,7 @@ impl<'a> Parser<'a> {
                 UseTreeKind::Glob => {
                     e.note("the wildcard token must be last on the path");
                 }
-                UseTreeKind::Nested(..) => {
+                UseTreeKind::Nested { .. } => {
                     e.note("glob-like brace syntax must be last on the path");
                 }
                 _ => (),
@@ -1056,7 +1056,11 @@ impl<'a> Parser<'a> {
         Ok(if self.eat(&token::BinOp(token::Star)) {
             UseTreeKind::Glob
         } else {
-            UseTreeKind::Nested(self.parse_use_tree_list()?)
+            let lo = self.token.span;
+            UseTreeKind::Nested {
+                items: self.parse_use_tree_list()?,
+                span: lo.to(self.prev_token.span),
+            }
         })
     }