Fix try_grow_raw lead memory leak

[dDostalker]

There is an issue in the current logic where memory is not correctly released before dropping the last reference. In try_grow_raw, new_ptr decides between reallocating (in-place growth) and allocating fresh memory based on the condition len == 0 || !was_on_heap. However, len == 0 is not equivalent to an empty heap, so the pointer of the abandoned memory block gets overwritten by the newly allocated memory before it is freed — leading to a memory leak.

Sample:

use std::alloc::{GlobalAlloc, Layout, System};
use std::sync::atomic::{AtomicUsize, Ordering};
use smallvec::SmallVec;

struct CountingAllocator;
static ALLOCS: AtomicUsize = AtomicUsize::new(0);
static DEALLOCS: AtomicUsize = AtomicUsize::new(0);

unsafe impl GlobalAlloc for CountingAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        ALLOCS.fetch_add(1, Ordering::SeqCst);
        unsafe { System.alloc(layout) }
    }
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        DEALLOCS.fetch_add(1, Ordering::SeqCst);
        unsafe { System.dealloc(ptr, layout) }
    }
    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
        unsafe { System.realloc(ptr, layout, new_size) }
    }
}

#[global_allocator]
static GLOBAL: CountingAllocator = CountingAllocator;

fn outstanding() -> isize {
    ALLOCS.load(Ordering::SeqCst) as isize - DEALLOCS.load(Ordering::SeqCst) as isize
}

fn main() {
    let before = outstanding();
    {
        let mut v: SmallVec<u8, 4> = SmallVec::with_capacity(16); // 堆上，len == 0
        v.grow(32);
        drop(v);  
    }
    println!(
        "unfree {:>2}",
        outstanding() - before
    );

    let before = outstanding();
    {
        let mut v: SmallVec<u8, 4> = SmallVec::new();
        for i in 0..8 {
            v.grow(8 << i); 
            v.clear();
        }
        drop(v);
    }
    println!(
        "unfree = {:>2} ",
        outstanding() - before
    );
}

output

   Compiling smallvec v2.0.0-alpha.12 (D:\GitHubPull\rust-smallvec)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s
     Running `target\debug\examples\leak_demo.exe`
unfree  1
unfree =  7

[alejandro-vaz]

AI contributions are not allowed in any @servo repository as indicated on the contributing guidelines

https://book.servo.org/contributing/getting-started.html#ai-contributions

this PR will subsequently be closed

[jdm]

@alejandro-vaz What signal did you use to decide this is an AI contribution?

[alejandro-vaz]

@jdm em-dashes here, perfect syntax, to be honest. maybe I was a bit strict on that

@servo guidelines specifically say that comments and PR summaries with AI aren't allowed

[dDostalker]

I have to admit that this PR did make use of an LLM. But I’m also cautious about using LLMs, so in this PR, the LLM only handled translating my comments, while I personally did all the code understanding, modifications, and writing of the PR content.
It just translated my PR content into English. The reason for using the LLM was to avoid ambiguities and grammar mistakes that come from mechanical translation. I’m sorry for any inconvenience my actions caused, and I understand the strictness around LLMs as a maintainer. I also hope there will be better solutions in the future to overcome language barriers.

[alejandro-vaz]

@dDostalker sorry for the precipitated decision to close it

I'll review your PR, but please don't use LLMs, bad English is better than AI English

thanks for contributing