Boa wrongly assumes that allocating never fails and writes to a null pointer

[y21]

Boa assumes that std::alloc::alloc never fails (returning a null pointer) even though it can.

boa/boa/src/string.rs

Lines 219 to 223 in 29cd909

	let inner = unsafe {
	let inner = alloc(layout).cast::<Self>();
	// Write the first part, the Inner.
	inner.write(Self {

Quoting the docs:

unsafe fn alloc(&self, layout: Layout) -> *mut u8
Allocate memory as described by the given layout.
Returns a pointer to newly-allocated memory, or null to indicate allocation failure.

If the allocation fails, then there will be a write on a null pointer on L223, which is considered UB:

• A null pointer is never valid, not even for accesses of size zero.

This also leads to violating the NonNull::new_unchecked invariant:

ptr must be non-null.

boa/boa/src/string.rs

Line 241 in 29cd909

unsafe { NonNull::new_unchecked(inner) }

To Reproduce

use boa::JsString;

fn main() {
    // choose a string length that's large enough to cause an allocation error when concatenated later but
    // not too large so the initial allocation still succeeds
    let string = "a".repeat(1000 * 1000 * 1000);
    let ve = vec![&string as &str; 1000];

    // this is almost certainly going to cause an allocation error unless you've got lots of RAM
    // if it does, then this will write to a null pointer 💥
    let _ = JsString::concat_array(&ve);
}

Alternatively, one can override the global allocator and return null for that specific allocation: link.
Implementing GlobalAlloc requires unsafe due to the nature of allocators being unsafe, but I'm fairly certain it's totally valid (albeit weird) for a crate to expose a safe API for an allocator like that.
Running the code under valgrind sums it up:

==22393== Process terminating with default action of signal 11 (SIGSEGV)
==22393==  Access not within mapped region at address 0x0
==22393==    at 0x110477: boa::string::JsString::concat_array
==22393==    by 0x11019F: boatest::main

[1]    22393 segmentation fault (core dumped)  valgrind ./target/release/boatest

Expected behavior
Handling null pointers, e.g. assert!(!inner.is_null()). Or better yet, calling std::alloc:handle_alloc_error as suggested in the docs

Build environment

• OS: Ubuntu
• Version: 20.04
• Target triple: x86_64-unknown-linux-gnu (installed)
• Rustc version: rustc 1.60.0-nightly (75d9a0ae2 2022-02-16)

[Razican]

Maybe this is addressed by #1659