BTreeMap uses Global allocator in it's PhantomData even when instantiated with custom Allocator

[shamatar]

I'm referring to this particular line

rust/library/alloc/src/collections/btree/map.rs

Line 183 in d3d145e

_marker: PhantomData<crate::boxed::Box<(K, V)>>,

Even though in no_std environments Global as a structure exists and is defined, and it only shows up in PhantomData, the use of the structure is most likely undefined if #[global_allocator] attribute is never used. Yes, there will be no calls to Global as it's only a phantom, and execution will not be affected, but just seems ill-defined at least.

For completeness: compilation of code that uses alloc::collection::BTreeMap with custom allocator under #![feature(btreemap_alloc)] is not affected, and there is no sign of runtime bugs.

[ChayimFriedman2]

Does this matter? It holds it directly:

rust/library/alloc/src/collections/btree/map.rs

Line 181 in d3d145e

pub(super) alloc: ManuallyDrop<A>,

[shamatar]

My issue is the opposite - it should be

_marker: PhantomData<crate::boxed::Box<(K, V), A>>,

in the PhantomData too, but not implicit

_marker: PhantomData<crate::boxed::Box<(K, V), Global>>,

[ChayimFriedman2]

I understand, but it already includes A for dropck, so why should it include it in the PhantomData too?

[shamatar]

To avoid inclusion of Global that has no logical relation to the BTreeMap instance with custom allocator

[shamatar]

If it's accepted as a bug I can quickly fix it, at least it passes tests