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Diffstat (limited to 'compiler/rustc_typeck/src/outlives/utils.rs')
| -rw-r--r-- | compiler/rustc_typeck/src/outlives/utils.rs | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/compiler/rustc_typeck/src/outlives/utils.rs b/compiler/rustc_typeck/src/outlives/utils.rs new file mode 100644 index 000000000..b718ca942 --- /dev/null +++ b/compiler/rustc_typeck/src/outlives/utils.rs @@ -0,0 +1,175 @@ +use rustc_infer::infer::outlives::components::{push_outlives_components, Component}; +use rustc_middle::ty::subst::{GenericArg, GenericArgKind}; +use rustc_middle::ty::{self, Region, Ty, TyCtxt}; +use rustc_span::Span; +use smallvec::smallvec; +use std::collections::BTreeMap; + +/// Tracks the `T: 'a` or `'a: 'a` predicates that we have inferred +/// must be added to the struct header. +pub(crate) type RequiredPredicates<'tcx> = + BTreeMap<ty::OutlivesPredicate<GenericArg<'tcx>, ty::Region<'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::Projection(proj_ty) => { + // This would arise from something like: + // + // ``` + // struct Foo<'a, T: Iterator> { + // x: &'a <T as Iterator>::Item + // } + // ``` + // + // Here we want to add an explicit `where <T as Iterator>::Item: 'a`. + let ty: Ty<'tcx> = tcx.mk_projection(proj_ty.item_def_id, proj_ty.substs); + required_predicates + .entry(ty::OutlivesPredicate(ty.into(), outlived_region)) + .or_insert(span); + } + + Component::EscapingProjection(_) => { + // 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 ReEarlyBound referencing `'a` + // } + // + // We care about these, so fall through. + ty::ReEarlyBound(_) => 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 put it on the struct header anyway. + ty::ReLateBound(..) => false, + + // This can appear in `where Self: ` bounds (#64855): + // + // struct Bar<T>(<Self as Foo>::Type) where Self: ; + // struct Baz<'a>(&'a Self) where Self: ; + ty::ReEmpty(_) => false, + + // These regions don't appear in types from type declarations: + ty::ReErased | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReFree(..) => { + bug!("unexpected region in outlives inference: {:?}", region); + } + } +} |