Parenthesize or-patterns in prefix pattern positions in pretty printer

[aytey]

The AST pretty printer was dropping parentheses around or-patterns when they appeared inside @ bindings, & references, or box patterns. For example:

• v @ (1 | 2 | 3) was printed as v @ 1 | 2 | 3
• &(1 | 2 | 3) was printed as &1 | 2 | 3
• box (1 | 2 | 3) was printed as box 1 | 2 | 3

Since | has the lowest precedence among pattern operators, all of these are parsed incorrectly without parentheses — e.g. v @ 1 | 2 | 3 becomes (v @ 1) | 2 | 3, binding v only to the first alternative.

This caused E0408 ("variable not bound in all patterns") when the expanded output was fed back to the compiler, affecting crates like html5ever and wgpu-core that use macros expanding to or-patterns after @.

The fix adds a print_pat_paren_if_or helper that wraps PatKind::Or subpatterns in parentheses, and uses it in the @, &, and box printing arms. This is similar in spirit to the existing FixupContext parenthesization approach used for expression printing.

Example

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

#![feature(prelude_import)]
#![no_std]
#![feature(box_patterns)]
extern crate std;
#[prelude_import]
use ::std::prelude::rust_2015::*;

//@ pretty-compare-only
//@ pretty-mode:expanded
//@ pp-exact:or-pattern-paren.pp

macro_rules! or_pat { ($($name:pat),+) => { $($name)|+ } }

fn check_at(x: Option<i32>) {
    match x {
        Some(v @ 1 | 2 | 3) =>


            {
            ::std::io::_print(format_args!("{0}\n", v));
        }
        _ => {}
    }
}
fn check_ref(x: &i32) { match x { &1 | 2 | 3 => {} _ => {} } }
fn check_box(x: Box<i32>) { match x { box 1 | 2 | 3 => {} _ => {} } }
fn main() { check_at(Some(2)); check_ref(&1); check_box(Box::new(1)); }

after (this branch):

#![feature(prelude_import)]
#![no_std]
#![feature(box_patterns)]
extern crate std;
#[prelude_import]
use ::std::prelude::rust_2015::*;

//@ pretty-compare-only
//@ pretty-mode:expanded
//@ pp-exact:or-pattern-paren.pp

macro_rules! or_pat { ($($name:pat),+) => { $($name)|+ } }

fn check_at(x: Option<i32>) {
    match x {
        Some(v @ (1 | 2 | 3)) =>


            {
            ::std::io::_print(format_args!("{0}\n", v));
        }
        _ => {}
    }
}
fn check_ref(x: &i32) { match x { &(1 | 2 | 3) => {} _ => {} } }
fn check_box(x: Box<i32>) { match x { box (1 | 2 | 3) => {} _ => {} } }
fn main() { check_at(Some(2)); check_ref(&1); check_box(Box::new(1)); }

Notice v @ 1 | 2 | 3 becomes v @ (1 | 2 | 3), &1 | 2 | 3 becomes &(1 | 2 | 3), and box 1 | 2 | 3 becomes box (1 | 2 | 3). Without parens, the or-pattern binds incorrectly — only the first alternative gets the @/&/box, causing E0408.

[rustbot]

r? @chenyukang

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• Owners of files modified in this PR: compiler
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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[chenyukang]

@bors r=chenyukang

[rust-bors]

📌 Commit 1a1a0d2 has been approved by chenyukang

It is now in the queue for this repository.