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rust: pin-init: internal: pin_data: allow lifetime to be shortened per field drop order

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Add the outlive relations per field drop order. This allows a single
lifetime to be used when a field potentially borrow from two different
fields, by allowing the longer-living field lifetime to be shortened to a
shorter-living field lifetime.

Acked-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20261008-dev-selfref-v2-12-e280b3c8fba5@garyguo.net/
Signed-off-by: Gary Guo <gary@garyguo.net>
This commit is contained in:
Gary Guo 2026-10-01 14:37:21 +01:00
parent 8f6607d1e5
commit f53736dd80
1 changed files with 40 additions and 42 deletions
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82
rust/pin-init/internal/src/pin_data.rs
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@ -107,6 +107,7 @@ fn cmp(&self, other: &Self) -> std::cmp::Ordering {
}
}
#[expect(unused)]
struct FieldInfo {
field: Field,
member: Member,
@ -124,8 +125,8 @@ struct StructInfo {
field_idx_map: BTreeMap<Ident, usize>,
is_tuple_struct: bool,
self_referential: bool,
/// Field lifetime generics.
field_lts: Generics,
/// Field lifetime generics with outlive chain.
field_lts_outlive_chain: Generics,
}
pub(crate) fn expand_with_cfg(
@ -346,16 +347,20 @@ fn expand(
// Create a lifetime parameter for each field.
let borrowed_fields: Vec<_> = fields.iter().filter_map(|f| f.borrowed.as_ref()).collect();
let mut field_lts = Generics {
let mut field_lts_outlive_chain = Generics {
lt_token: None,
params: borrowed_fields
.iter()
.map(|borrowed| {
.zip(std::iter::once(None).chain(borrowed_fields.iter().map(Some)))
.map(|(borrowed, prev)| {
GenericParam::Lifetime(LifetimeParam {
attrs: Vec::new(),
lifetime: borrowed.lifetime.clone(),
colon_token: None,
bounds: Default::default(),
bounds: prev
.iter()
.map(|borrowed| borrowed.lifetime.clone())
.collect(),
})
})
.collect(),
@ -363,36 +368,26 @@ fn expand(
where_clause: None,
};
// Insert necessary bounds to make types well-formed.
for field in fields.iter() {
let Some(borrowed) = &field.borrowed else {
continue;
};
let field_lt = &borrowed.lifetime;
// For each borrowed field that borrows from other fields, we need to insert outlive bounds.
for capture in &field.captures {
let lt = &capture.lifetime;
field_lts
.make_where_clause()
.predicates
.push(parse_quote!(#lt: #field_lt));
}
// For each borrowed field that references a generic, we also need to insert their outlive
// bounds so they can refer to generics.
for lt in field.generic_lt_captures.iter() {
field_lts
.make_where_clause()
.predicates
.push(parse_quote!(#lt: #field_lt));
}
for ty in field.generic_ty_captures.iter() {
field_lts
.make_where_clause()
.predicates
.push(parse_quote!(#ty: #field_lt));
if let Some(last_borrowed_field) = borrowed_fields.last() {
let field_lt = &last_borrowed_field.lifetime;
for param in struct_.generics.params.iter() {
match param {
GenericParam::Lifetime(param) => {
let lt = &param.lifetime;
field_lts_outlive_chain
.make_where_clause()
.predicates
.push(parse_quote!(#lt: #field_lt));
}
GenericParam::Type(param) => {
let ty = &param.ident;
field_lts_outlive_chain
.make_where_clause()
.predicates
.push(parse_quote!(#ty: #field_lt));
}
GenericParam::Const(_) => (),
}
}
}
@ -406,7 +401,7 @@ fn expand(
fields,
field_idx_map,
is_tuple_struct,
field_lts,
field_lts_outlive_chain,
};
for field in &info.fields {
@ -703,7 +698,7 @@ fn generate_drop_order_check(dcx: &mut DiagCtxt, info: &StructInfo) -> TokenStre
// with known lifetime bounds as bounds on the function, and asks Rust to *prove* that the types
// are wellformed, given the bounds that we understand.
let generics_with_field_lt = CombinedGenerics(vec![&info.field_lts, generics]);
let generics_with_field_lt = CombinedGenerics(vec![&info.field_lts_outlive_chain, generics]);
let (_, ty_generics, _) = generics.split_for_impl();
let (impl_generics_with_field_lt, _, whr_with_field_lt) =
@ -842,10 +837,13 @@ fn generate_projections(info: &StructInfo) -> TokenStream {
// Wrap in `CombinedGenerics` because it's ty generics will always output `<>`, so it can be
// used with `for`.
let field_lts = CombinedGenerics(vec![&info.field_lts]);
let field_lts = CombinedGenerics(vec![&info.field_lts_outlive_chain]);
let generics_with_this_lt = CombinedGenerics(vec![&this_lt_generics, generics]);
let generics_with_this_field_lt =
CombinedGenerics(vec![&this_lt_generics, &info.field_lts, generics]);
let generics_with_this_field_lt = CombinedGenerics(vec![
&this_lt_generics,
&info.field_lts_outlive_chain,
generics,
]);
let (impl_generics, ty_generics, whr) = generics.split_for_impl();
let (_, field_lt_ty_generics, _) = field_lts.split_for_impl();
let (_, ty_generics_with_this_lt, _) = generics_with_this_lt.split_for_impl();
@ -1176,8 +1174,8 @@ fn generate_the_pin_data(info: &StructInfo) -> TokenStream {
// Wrap in `CombinedGenerics` because it's ty generics will always output `<>`, so it can be
// used with `for`.
let field_lts = CombinedGenerics(vec![&info.field_lts]);
let generics_with_field_lt = CombinedGenerics(vec![&info.field_lts, generics]);
let field_lts = CombinedGenerics(vec![&info.field_lts_outlive_chain]);
let generics_with_field_lt = CombinedGenerics(vec![&info.field_lts_outlive_chain, generics]);
let (impl_generics, ty_generics, whr) = generics.split_for_impl();
let (_, field_lt_ty_generics, _) = field_lts.split_for_impl();