Parenthesize break values containing leading label

[dtolnay]

The AST pretty printer previously produced invalid syntax in the case of break expressions with a value that begins with a loop or block label.

macro_rules! expr {
    ($e:expr) => {
        $e
    };
}

fn main() {
    loop {
        break expr!('a: loop { break 'a 1; } + 1);
    };
}

rustc -Zunpretty=expanded main.rs :

#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
macro_rules! expr { ($e:expr) => { $e }; }

fn main() { loop { break 'a: loop { break 'a 1; } + 1; }; }

The expanded code is not valid Rust syntax. Printing invalid syntax is bad because it blocks cargo expand from being able to format the output as Rust syntax using rustfmt.

error: parentheses are required around this expression to avoid confusion with a labeled break expression
 --> <anon>:9:26
  |
9 | fn main() { loop { break 'a: loop { break 'a 1; } + 1; }; }
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: wrap the expression in parentheses
  |
9 | fn main() { loop { break ('a: loop { break 'a 1; }) + 1; }; }
  |                          +                        +

This PR updates the AST pretty-printer to insert parentheses around the value of a break expression as required to avoid this edge case.

[rustbot]

r? @fmease

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[fmease]

Thanks! Sorry for the delay.
r=me with nit addressed

[fmease]

@bors rollup

[dtolnay]

@bors r=fmease

[bors]

📌 Commit 0698223 has been approved by fmease

It is now in the queue for this repository.