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Make Attribute an unsafe trait stating its re-park and glue coupling

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This commit is contained in:
Dennis Kobert
2026-09-08 13:14:59 +00:00
parent 682d3dafbd
commit 49daf2895e
1 changed files with 32 additions and 6 deletions
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38
node-graph/libraries/core-types/src/attribute.rs
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@@ -20,8 +20,24 @@ use std::sync::{LazyLock, Mutex};
/// Declares an attribute name: one marker per name, fixing the value type and
/// the name-specific default. Declare markers through the [`attribute!`]
/// macro, which also registers them into the [`ATTRIBUTE_REGISTRY`].
pub trait Attribute: 'static {
/// macro, which also registers them into the [`ATTRIBUTE_REGISTRY`] and emits
/// an impl meeting the obligations below.
///
/// # Safety
///
/// [`REPARK`](Self::REPARK) must be `Some` for every marker whose
/// [`Value<'e>`](Self::Value) can carry a borrow shorter than `'static`. The
/// plain-value arm of the census writer retypes the value [`from_stored`] hands
/// it from the stored form's lifetime to the field's, which is only a relabel
/// where the value borrows nothing; a re-parking marker instead writes a
/// reference into the arena it is given.
///
/// [`from_stored`](Self::from_stored) must accept exactly the erased form
/// [`read_erased`](Self::read_erased) produces, and [`read_erased`] must read
/// its `ptr` as this marker's own `Value`. The two are each other's inverse
/// across every persistence seam, so a marker that reads one type and stores
/// another writes a value of the wrong type into the field.
pub unsafe trait Attribute: 'static {
/// The name as it appears in documents and diagnostics.
const NAME: &'static str;
/// The value type every read and write of this name shares. The lifetime
@@ -267,7 +283,10 @@ macro_rules! attribute {
$(#[$meta])*
$vis struct $marker;
impl $crate::attribute::Attribute for $marker {
// SAFETY: the reference-valued arms emit `REPARK`, the plain arm's value
// type cannot name `'e`, and `read_erased` and `from_stored` are emitted
// as each other's inverse.
unsafe impl $crate::attribute::Attribute for $marker {
const NAME: &'static str = $name;
type Value<'e> = ::core::option::Option<&'e $value>;
@@ -306,7 +325,10 @@ macro_rules! attribute {
$(#[$meta])*
$vis struct $marker;
impl $crate::attribute::Attribute for $marker {
// SAFETY: the reference-valued arms emit `REPARK`, the plain arm's value
// type cannot name `'e`, and `read_erased` and `from_stored` are emitted
// as each other's inverse.
unsafe impl $crate::attribute::Attribute for $marker {
const NAME: &'static str = $name;
type Value<'e> = &'e $value;
$(
@@ -341,7 +363,10 @@ macro_rules! attribute {
$(#[$meta])*
$vis struct $marker;
impl $crate::attribute::Attribute for $marker {
// SAFETY: the reference-valued arms emit `REPARK`, the plain arm's value
// type cannot name `'e`, and `read_erased` and `from_stored` are emitted
// as each other's inverse.
unsafe impl $crate::attribute::Attribute for $marker {
const NAME: &'static str = $name;
type Value<'e> = $value;
$(
@@ -497,7 +522,8 @@ mod tests {
#[should_panic(expected = "two value types")]
fn a_second_marker_at_a_different_type_panics() {
struct Conflict;
impl Attribute for Conflict {
// SAFETY: `bool` borrows nothing, so the plain-value arm is the right one.
unsafe impl Attribute for Conflict {
const NAME: &'static str = "opacity";
type Value<'e> = bool;