Preserve braces around self in use tree pretty printing

The AST pretty printer strips braces from single-item `use` sub-groups,
simplifying `use foo::{Bar}` to `use foo::Bar`. However, when the single
item is `self`, this produces `use foo::self` which is not valid Rust
(E0429) — the grammar requires `use foo::{self}`.

This affects both `stringify!` and `rustc -Zunpretty=expanded`, causing
`cargo expand` output to be unparseable when a crate uses `use
path::{self}` imports (a common pattern in the ecosystem).

The fix checks whether the single nested item's path starts with `self`
before stripping braces. If so, the braces are preserved.

Signed-off-by: Andrew V. Teylu <andrew.teylu@vector.com>

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -881,7 +881,13 @@ fn print_use_tree(&mut self, tree: &ast::UseTree) {
                 }
                 if items.is_empty() {
                     self.word("{}");
-                } else if let [(item, _)] = items.as_slice() {
+                } else if let [(item, _)] = items.as_slice()
+                    && !item
+                        .prefix
+                        .segments
+                        .first()
+                        .is_some_and(|seg| seg.ident.name == rustc_span::symbol::kw::SelfLower)
+                {
                     self.print_use_tree(item);
                 } else {
                     let cb = self.cbox(INDENT_UNIT);

tests/pretty/use-self-braces.rs

@@ -0,0 +1,10 @@
+//@ pp-exact
+//@ edition:2021
+
+#![allow(unused_imports)]
+
+// Braces around `self` must be preserved, because `use foo::self` is not valid Rust.
+use std::io::{self};
+use std::fmt::{self, Debug};
+
+fn main() {}