Leak with box_syntax

Cargo.toml

[package]
name = "box_leak_maybe"
version = "0.1.0"
authors = ["Adam Gashlin <agashlin@mozilla.com>"]
edition = "2018"

[dependencies]
stats_alloc = "0.1.8"

main.rs

#![feature(box_syntax)]

extern crate stats_alloc;

use stats_alloc::{StatsAlloc, Region, INSTRUMENTED_SYSTEM};
use std::alloc::System;

#[global_allocator]
static GLOBAL: &StatsAlloc<System> = &INSTRUMENTED_SYSTEM;

use std::num::ParseFloatError;

fn f(s: &str, leaky: bool) -> Result<Box<f32>, ParseFloatError> {
    if leaky {
        Ok(box (s.parse::<f32>()?))
    } else {
        Ok(Box::new(s.parse::<f32>()?))
    }
}

fn test(s: &str, leaky: bool) {
    let mut v2 = Vec::new();
    for _i in 0..(1000*1000) {
        if let Ok(v) = f(s, leaky) {
            v2.push(v);
        }
    }
}

fn main() {
    {
        let reg = Region::new(&GLOBAL);
        dbg!(test("1.0", true));
        dbg!(reg.change());
    }

    {
        let reg = Region::new(&GLOBAL);
        dbg!(test("x", false));
        dbg!(reg.change());
    }

    {
        let reg = Region::new(&GLOBAL);
        dbg!(test("x", true));
        dbg!(reg.change());
    }
}

output.txt

$ cargo +nightly run --release
    Finished release [optimized] target(s) in 0.01s
     Running `target\release\box_leak_maybe.exe`
[src\main.rs:33] test("1.0", true) = ()
[src\main.rs:34] reg.change() = Stats {
    allocations: 1000007,
    deallocations: 1000001,
    reallocations: 21,
    bytes_allocated: 12388768,
    bytes_deallocated: 12388608,
    bytes_reallocated: 8388616,
}
[src\main.rs:39] test("x", false) = ()
[src\main.rs:40] reg.change() = Stats {
    allocations: 0,
    deallocations: 0,
    reallocations: 0,
    bytes_allocated: 0,
    bytes_deallocated: 0,
    bytes_reallocated: 0,
}
[src\main.rs:45] test("x", true) = ()
[src\main.rs:46] reg.change() = Stats {
    allocations: 1000000,
    deallocations: 0,
    reallocations: 0,
    bytes_allocated: 4000000,
    bytes_deallocated: 0,
    bytes_reallocated: 0,
}