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
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0.0

/facet-core/src/impls_std/path.rs

use crate::Variance;
use crate::*;

unsafe impl Facet<'_> for std::path::PathBuf {
    const SHAPE: &'static Shape = &const {
        Shape {
            id: Shape::id_of::<Self>(),
            layout: Shape::layout_of::<Self>(),
            vtable: value_vtable!(std::path::PathBuf, |f, _opts| write!(
                f,
                "{}",
                Self::SHAPE.type_identifier
            )),
            ty: Type::User(UserType::Opaque),
            def: Def::Scalar,
            type_identifier: "PathBuf",
            type_params: &[],
            doc: &[],
            attributes: &[],
            type_tag: None,
            inner: None,
            proxy: None,
            // PathBuf has no lifetime parameters, so it's covariant
            variance: Variance::COVARIANT,
        }
    };
}

unsafe impl Facet<'_> for std::path::Path {
    const SHAPE: &'static Shape = &const {
        Shape {
            id: Shape::id_of::<Self>(),
            layout: Shape::UNSIZED_LAYOUT,
            vtable: value_vtable!(std::path::Path, |f, _opts| write!(
                f,
                "{}",
                Self::SHAPE.type_identifier
            )),
            ty: Type::User(UserType::Opaque),
            def: Def::Scalar,
            type_identifier: "Path",
            type_params: &[],
            doc: &[],
            attributes: &[],
            type_tag: None,
            inner: None,
            proxy: None,
            // Path has no lifetime parameters, so it's covariant
            variance: Variance::COVARIANT,
        }
    };
}

1	use crate::Variance;
2	use crate::*;
3
4	unsafe impl Facet<'_> for std::path::PathBuf {
5	const SHAPE: &'static Shape = &const {
6	Shape {
7	id: Shape::id_of::<Self>(),
8	layout: Shape::layout_of::<Self>(),
9	vtable: value_vtable!(std::path::PathBuf, \|f, _opts\| write!(	×
UNCOV 10	f,	×
11	"{}",
UNCOV 12	Self::SHAPE.type_identifier	×
13	)),
14	ty: Type::User(UserType::Opaque),
15	def: Def::Scalar,
16	type_identifier: "PathBuf",
17	type_params: &[],
18	doc: &[],
19	attributes: &[],
20	type_tag: None,
21	inner: None,
22	proxy: None,
23	// PathBuf has no lifetime parameters, so it's covariant
24	variance: Variance::COVARIANT,
25	}
26	};
27	}
28
29	unsafe impl Facet<'_> for std::path::Path {
30	const SHAPE: &'static Shape = &const {
31	Shape {
32	id: Shape::id_of::<Self>(),
33	layout: Shape::UNSIZED_LAYOUT,
UNCOV 34	vtable: value_vtable!(std::path::Path, \|f, _opts\| write!(	×
35	f,	×
36	"{}",
UNCOV 37	Self::SHAPE.type_identifier	×
38	)),
39	ty: Type::User(UserType::Opaque),
40	def: Def::Scalar,
41	type_identifier: "Path",
42	type_params: &[],
43	doc: &[],
44	attributes: &[],
45	type_tag: None,
46	inner: None,
47	proxy: None,
48	// Path has no lifetime parameters, so it's covariant
49	variance: Variance::COVARIANT,
50	}
51	};
52	}

facet-rs / facet / 20024290994

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