20024290994

Committed 08 Dec 2025 10:08AM UTC coverage: 58.613% (+0.01%) from 58.6%

Build # 20024290994

Build Type

push

github

Committed by

fasterthanlime

Commit Message

feat: compute variance automatically from field types

This implements automatic variance computation for derived types:

- Change `Shape.variance` from `fn() -> Variance` to `fn(&'static Shape) -> Variance`
  to allow variance functions to walk type structure without capturing generics

- Add `Shape::computed_variance()` that walks struct fields, enum variants,
  and inner shapes (for Box, Vec, Option, etc.) to compute variance

- Use thread-local cycle detection to prevent stack overflow with recursive types.
  When a cycle is detected, returns `Covariant` (identity for `combine`)

- Update derive macros to use `.variance(Shape::computed_variance)`
  instead of generating per-type variance computation code

- Update `Box<T>` to use computed variance (depends on T's variance)
  instead of hardcoded `Invariant`

- Add variance tests for recursive types

Closes #1171

Run Details

44 of 60 new or added lines in 3 files covered. (73.33%)

487 existing lines in 23 files now uncovered.

24730 of 42192 relevant lines covered (58.61%)

544.95 hits per line

Source File
Press 'n' to go to next uncovered line, 'b' for previous

46.0

/facet-core/src/impls_alloc/vec.rs

use core::ptr::NonNull;

use crate::shape_util::vtable_for_list;
use crate::*;

use alloc::boxed::Box;
use alloc::vec::Vec;

type VecIterator<'mem, T> = core::slice::Iter<'mem, T>;

unsafe impl<'a, T> Facet<'a> for Vec<T>
where
    T: Facet<'a>,
{
    const SHAPE: &'static Shape = &const {
        Shape {
            id: Shape::id_of::<Self>(),
            layout: Shape::layout_of::<Self>(),
            vtable: ValueVTable {
                type_name: |f, opts| {
                    if let Some(opts) = opts.for_children() {
                        write!(f, "{}<", Self::SHAPE.type_identifier)?;
                        T::SHAPE.vtable.type_name()(f, opts)?;
                        write!(f, ">")
                    } else {
                        write!(f, "{}<…>", Self::SHAPE.type_identifier)
                    }
                },
                default_in_place: Some(|target| unsafe { target.put(Self::default()) }),
                ..vtable_for_list::<T, Self>()
            },
            ty: Type::User(UserType::Opaque),
            def: Def::List(ListDef::new(
                &const {
                    ListVTable {
                        init_in_place_with_capacity: Some(|data, capacity| unsafe {
                            data.put(Self::with_capacity(capacity))
                        }),
                        push: Some(|ptr, item| unsafe {
                            let vec = ptr.as_mut::<Self>();
                            let item = item.read::<T>();
                            (*vec).push(item);
                        }),
                        len: |ptr| unsafe {
                            let vec = ptr.get::<Self>();
                            vec.len()
                        },
                        get: |ptr, index| unsafe {
                            let vec = ptr.get::<Self>();
                            let item = vec.get(index)?;
                            Some(PtrConst::new(NonNull::from(item)))
                        },
                        get_mut: Some(|ptr, index| unsafe {
                            let vec = ptr.as_mut::<Self>();
                            let item = vec.get_mut(index)?;
                            Some(PtrMut::new(NonNull::from(item)))
                        }),
                        as_ptr: Some(|ptr| unsafe {
                            let vec = ptr.get::<Self>();
                            PtrConst::new(NonNull::new_unchecked(vec.as_ptr() as *mut T))
                        }),
                        as_mut_ptr: Some(|ptr| unsafe {
                            let vec = ptr.as_mut::<Self>();
                            PtrMut::new(NonNull::new_unchecked(vec.as_mut_ptr()))
                        }),
                        iter_vtable: {
                            IterVTable {
                                init_with_value: Some(|ptr| unsafe {
                                    let vec = ptr.get::<Self>();
                                    let iter: VecIterator<T> = vec.iter();
                                    let iter_state = Box::new(iter);
                                    PtrMut::new(NonNull::new_unchecked(
                                        Box::into_raw(iter_state) as *mut u8
                                    ))
                                }),
                                next: |iter_ptr| unsafe {
                                    let state = iter_ptr.as_mut::<VecIterator<'_, T>>();
                                    state
                                        .next()
                                        .map(|value| PtrConst::new(NonNull::from(value)))
                                },
                                next_back: Some(|iter_ptr| unsafe {
                                    let state = iter_ptr.as_mut::<VecIterator<'_, T>>();
                                    state
                                        .next_back()
                                        .map(|value| PtrConst::new(NonNull::from(value)))
                                }),
                                size_hint: None,
                                dealloc: |iter_ptr| unsafe {
                                    drop(Box::from_raw(iter_ptr.as_ptr::<VecIterator<'_, T>>()
                                        as *mut VecIterator<'_, T>));
                                },
                            }
                        },
                    }
                },
                T::SHAPE,
            )),
            type_identifier: "Vec",
            type_params: &[TypeParam {
                name: "T",
                shape: T::SHAPE,
            }],
            doc: &[],
            attributes: &[],
            type_tag: None,
            // Vec uses Def::List, not inner, for element access
            inner: None,
            proxy: None,
            // Vec<T> is covariant in T, computed from the element type
            variance: Shape::computed_variance,
        }
    };
}

1	use core::ptr::NonNull;
2
3	use crate::shape_util::vtable_for_list;
4	use crate::*;
5
6	use alloc::boxed::Box;
7	use alloc::vec::Vec;
8
9	type VecIterator<'mem, T> = core::slice::Iter<'mem, T>;
10
11	unsafe impl<'a, T> Facet<'a> for Vec<T>
12	where
13	T: Facet<'a>,
14	{
15	const SHAPE: &'static Shape = &const {
16	Shape {
17	id: Shape::id_of::<Self>(),
18	layout: Shape::layout_of::<Self>(),
19	vtable: ValueVTable {
20	type_name: \|f, opts\| {	206✔
21	if let Some(opts) = opts.for_children() {	206✔
22	write!(f, "{}<", Self::SHAPE.type_identifier)?;	206✔
23	T::SHAPE.vtable.type_name()(f, opts)?;	206✔
24	write!(f, ">")	206✔
25	} else {
UNCOV 26	write!(f, "{}<…>", Self::SHAPE.type_identifier)	×
27	}
28	},	206✔
29	default_in_place: Some(\|target\| unsafe { target.put(Self::default()) }),	2✔
30	..vtable_for_list::<T, Self>()
31	},
32	ty: Type::User(UserType::Opaque),
33	def: Def::List(ListDef::new(
34	&const {
35	ListVTable {
36	init_in_place_with_capacity: Some(\|data, capacity\| unsafe {
37	data.put(Self::with_capacity(capacity))	441✔
38	}),	441✔
39	push: Some(\|ptr, item\| unsafe {
40	let vec = ptr.as_mut::<Self>();	12,827✔
41	let item = item.read::<T>();	12,827✔
42	(*vec).push(item);	12,827✔
43	}),	12,827✔
44	len: \|ptr\| unsafe {
45	let vec = ptr.get::<Self>();	1,216✔
46	vec.len()	1,216✔
47	},	1,216✔
48	get: \|ptr, index\| unsafe {
49	let vec = ptr.get::<Self>();	26✔
50	let item = vec.get(index)?;	26✔
51	Some(PtrConst::new(NonNull::from(item)))	24✔
52	},	26✔
53	get_mut: Some(\|ptr, index\| unsafe {
54	let vec = ptr.as_mut::<Self>();	×
55	let item = vec.get_mut(index)?;	×
56	Some(PtrMut::new(NonNull::from(item)))	×
UNCOV 57	}),	×
58	as_ptr: Some(\|ptr\| unsafe {
59	let vec = ptr.get::<Self>();	746✔
60	PtrConst::new(NonNull::new_unchecked(vec.as_ptr() as *mut T))	746✔
61	}),	746✔
62	as_mut_ptr: Some(\|ptr\| unsafe {
63	let vec = ptr.as_mut::<Self>();	×
64	PtrMut::new(NonNull::new_unchecked(vec.as_mut_ptr()))	×
UNCOV 65	}),	×
66	iter_vtable: {
67	IterVTable {
68	init_with_value: Some(\|ptr\| unsafe {
69	let vec = ptr.get::<Self>();	×
70	let iter: VecIterator<T> = vec.iter();	×
71	let iter_state = Box::new(iter);	×
72	PtrMut::new(NonNull::new_unchecked(	×
UNCOV 73	Box::into_raw(iter_state) as *mut u8	×
74	))
UNCOV 75	}),	×
76	next: \|iter_ptr\| unsafe {
77	let state = iter_ptr.as_mut::<VecIterator<'_, T>>();	×
78	state	×
79	.next()	×
80	.map(\|value\| PtrConst::new(NonNull::from(value)))	×
UNCOV 81	},	×
82	next_back: Some(\|iter_ptr\| unsafe {
83	let state = iter_ptr.as_mut::<VecIterator<'_, T>>();	×
84	state	×
85	.next_back()	×
86	.map(\|value\| PtrConst::new(NonNull::from(value)))	×
UNCOV 87	}),	×
88	size_hint: None,
89	dealloc: \|iter_ptr\| unsafe {
90	drop(Box::from_raw(iter_ptr.as_ptr::<VecIterator<'_, T>>()	×
91	as *mut VecIterator<'_, T>));	×
UNCOV 92	},	×
93	}
94	},
95	}
96	},
97	T::SHAPE,
98	)),
99	type_identifier: "Vec",
100	type_params: &[TypeParam {
101	name: "T",
102	shape: T::SHAPE,
103	}],
104	doc: &[],
105	attributes: &[],
106	type_tag: None,
107	// Vec uses Def::List, not inner, for element access
108	inner: None,
109	proxy: None,
110	// Vec<T> is covariant in T, computed from the element type
111	variance: Shape::computed_variance,
112	}
113	};
114	}

facet-rs / facet / 20024290994

Source File Press 'n' to go to next uncovered line, 'b' for previous

Source File
Press 'n' to go to next uncovered line, 'b' for previous