Preserve braces around self in use tree pretty printing

[aytey]

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.

Example

before (rustc 1.96.0-nightly (3b1b0ef4d 2026-03-11)):

#![feature(prelude_import)]
//@ pp-exact
//@ edition:2021

#![allow(unused_imports)]
extern crate std;
#[prelude_import]
use std::prelude::rust_2021::*;

// 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() {}

after (this branch):

#![feature(prelude_import)]
//@ pp-exact
//@ edition:2021

#![allow(unused_imports)]
extern crate std;
#[prelude_import]
use std::prelude::rust_2021::*;

// 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() {}

Notice the use std::io::self (invalid, E0429) in the before becomes use std::io::{self} in the after.

[rustbot]

r? @davidtwco

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[Kivooeo]

r? me @bors r+ rollup

[rust-bors]

📌 Commit 082cdf7 has been approved by Kivooeo

It is now in the queue for this repository.