rustc_hir_analysis/outlives/utils.rs
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use rustc_data_structures::fx::FxIndexMap;
use rustc_middle::ty::{self, GenericArg, GenericArgKind, Region, Ty, TyCtxt};
use rustc_middle::{bug, span_bug};
use rustc_span::Span;
use rustc_type_ir::outlives::{Component, push_outlives_components};
use smallvec::smallvec;
/// Tracks the `T: 'a` or `'a: 'a` predicates that we have inferred
/// must be added to the struct header.
pub(crate) type RequiredPredicates<'tcx> =
FxIndexMap<ty::OutlivesPredicate<'tcx, ty::GenericArg<'tcx>>, Span>;
/// Given a requirement `T: 'a` or `'b: 'a`, deduce the
/// outlives_component and add it to `required_predicates`
pub(crate) fn insert_outlives_predicate<'tcx>(
tcx: TyCtxt<'tcx>,
kind: GenericArg<'tcx>,
outlived_region: Region<'tcx>,
span: Span,
required_predicates: &mut RequiredPredicates<'tcx>,
) {
// If the `'a` region is bound within the field type itself, we
// don't want to propagate this constraint to the header.
if !is_free_region(outlived_region) {
return;
}
match kind.unpack() {
GenericArgKind::Type(ty) => {
// `T: 'outlived_region` for some type `T`
// But T could be a lot of things:
// e.g., if `T = &'b u32`, then `'b: 'outlived_region` is
// what we want to add.
//
// Or if within `struct Foo<U>` you had `T = Vec<U>`, then
// we would want to add `U: 'outlived_region`
let mut components = smallvec![];
push_outlives_components(tcx, ty, &mut components);
for component in components {
match component {
Component::Region(r) => {
// This would arise from something like:
//
// ```
// struct Foo<'a, 'b> {
// x: &'a &'b u32
// }
// ```
//
// Here `outlived_region = 'a` and `kind = &'b
// u32`. Decomposing `&'b u32` into
// components would yield `'b`, and we add the
// where clause that `'b: 'a`.
insert_outlives_predicate(
tcx,
r.into(),
outlived_region,
span,
required_predicates,
);
}
Component::Param(param_ty) => {
// param_ty: ty::ParamTy
// This would arise from something like:
//
// ```
// struct Foo<'a, U> {
// x: &'a Vec<U>
// }
// ```
//
// Here `outlived_region = 'a` and `kind =
// Vec<U>`. Decomposing `Vec<U>` into
// components would yield `U`, and we add the
// where clause that `U: 'a`.
let ty: Ty<'tcx> = param_ty.to_ty(tcx);
required_predicates
.entry(ty::OutlivesPredicate(ty.into(), outlived_region))
.or_insert(span);
}
Component::Placeholder(_) => {
span_bug!(span, "Should not deduce placeholder outlives component");
}
Component::Alias(alias_ty) => {
// This would either arise from something like:
//
// ```
// struct Foo<'a, T: Iterator> {
// x: &'a <T as Iterator>::Item
// }
// ```
//
// or:
//
// ```rust
// type Opaque<T> = impl Sized;
// fn defining<T>() -> Opaque<T> {}
// struct Ss<'a, T>(&'a Opaque<T>);
// ```
//
// Here we want to add an explicit `where <T as Iterator>::Item: 'a`
// or `Opaque<T>: 'a` depending on the alias kind.
let ty = alias_ty.to_ty(tcx);
required_predicates
.entry(ty::OutlivesPredicate(ty.into(), outlived_region))
.or_insert(span);
}
Component::EscapingAlias(_) => {
// As above, but the projection involves
// late-bound regions. Therefore, the WF
// requirement is not checked in type definition
// but at fn call site, so ignore it.
//
// ```
// struct Foo<'a, T: Iterator> {
// x: for<'b> fn(<&'b T as Iterator>::Item)
// // ^^^^^^^^^^^^^^^^^^^^^^^^^
// }
// ```
//
// Since `'b` is not in scope on `Foo`, can't
// do anything here, ignore it.
}
Component::UnresolvedInferenceVariable(_) => bug!("not using infcx"),
}
}
}
GenericArgKind::Lifetime(r) => {
if !is_free_region(r) {
return;
}
required_predicates.entry(ty::OutlivesPredicate(kind, outlived_region)).or_insert(span);
}
GenericArgKind::Const(_) => {
// Generic consts don't impose any constraints.
}
}
}
fn is_free_region(region: Region<'_>) -> bool {
// First, screen for regions that might appear in a type header.
match *region {
// These correspond to `T: 'a` relationships:
//
// struct Foo<'a, T> {
// field: &'a T, // this would generate a ReEarlyParam referencing `'a`
// }
//
// We care about these, so fall through.
ty::ReEarlyParam(_) => true,
// These correspond to `T: 'static` relationships which can be
// rather surprising.
//
// struct Foo<'a, T> {
// field: &'static T, // this would generate a ReStatic
// }
ty::ReStatic => false,
// Late-bound regions can appear in `fn` types:
//
// struct Foo<T> {
// field: for<'b> fn(&'b T) // e.g., 'b here
// }
//
// The type above might generate a `T: 'b` bound, but we can
// ignore it. We can't name this lifetime pn the struct header anyway.
ty::ReBound(..) => false,
ty::ReError(_) => false,
// These regions don't appear in types from type declarations:
ty::ReErased | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReLateParam(..) => {
bug!("unexpected region in outlives inference: {:?}", region);
}
}
}