Unsupport box_syntax feature

[neonaot]

Environment

• IntelliJ Rust plugin version: 0.4.191.5270-231-nightly
• Rust toolchain version: 1.70.0-nightly (22f247c6f 2023-03-13) x86_64-apple-darwin
• IDE name and version: CLion 2023.1 Beta (CL-231.8109.80)
• Operating system: macOS 12.5.1
• Macro expansion: enabled
• Additional experimental features: org.rust.cargo.emulate.terminal

Problem description

See rust-lang/rust#108471

We suggest adding this feature for let b = box 5; and we use (support) it in type inference.

It will be helpful to have a quick to transform let a = box 5 to let a = Box::new(5).

Steps to reproduce

#![feature(box_syntax)]

fn main() {
    let b = box 5;
}