When building `alloc` with `no_global_oom_handling`, numerous disabled `impl`s for `Box`, `Vec`, `Rc`, `Arc` can be written by downstream code for local types

In https://github.com/rust-lang/rust/pull/157428/changes#r3620330696, @theemathas and I found that the Clone impl of Box is behind cfg(not(no_global_oom_handling)). This is weird, because Box is #[fundamental]. Downstream users can do this:

#![no_std]

use alloc::boxed::Box;

extern crate alloc;

#[derive(Clone)]
struct LocalType;

impl Clone for Box<LocalType> {
    fn clone(&self) -> Self {
        panic!("wat")
    }
}

This successfully builds with RUSTFLAGS="--cfg no_global_oom_handling -Cpanic=abort" cargo +nightly build -Z build-std=core,alloc.

Fullish list of affected impls under the orphan rule (using #\[cfg\(.*\)\]\n(\s*#\[.*\]\n)*\s*impl):

• Clone for Box<T, A>
• FromIterator<T> for Rc<[T]>/Arc<[T]>/Vec<T>/Box<[T]>
• From<Box<T, A>> for Rc<T, A>/Arc<T, A>
• From<T> for Rc<T>/Arc<T>/Box<T>
• Default for Box<T>
• Default for Pin<Box<T>>
• Extend<T> for Vec<T, A>
• Extend<&'a T> for Vec<T, A>
• Concat<T> for [T]
• Join<&T> for [T]
• From<E> for Box<dyn Error + 'a>
• From<E> for Box<dyn Error + Send + Sync + 'a>
• From<T> for ThinBox<T> (with thin_box feature)
• ToString for T

Review the cfg-gated impls for Box, Vec, Rc, Arc, Pin, ThinBox, and slices, using the linked pull request discussion and affected-impl list to define the scope. Reproduce the downstream example with the shown no_std cargo build command; done means the listed disabled impls no longer remain available for downstream local types under no_global_oom_handling.