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// Copyright 2013 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. use rustc::ty::outlives::Component; use rustc::ty::subst::{Kind, UnpackedKind}; use rustc::ty::{self, Region, RegionKind, Ty, TyCtxt}; use std::collections::BTreeSet; /// Tracks the `T: 'a` or `'a: 'a` predicates that we have inferred /// must be added to the struct header. pub type RequiredPredicates<'tcx> = BTreeSet<ty::OutlivesPredicate<Kind<'tcx>, ty::Region<'tcx>>>; /// Given a requirement `T: 'a` or `'b: 'a`, deduce the /// outlives_component and add it to `required_predicates` pub fn insert_outlives_predicate<'tcx>( tcx: TyCtxt<'_, 'tcx, 'tcx>, kind: Kind<'tcx>, outlived_region: Region<'tcx>, 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(tcx, outlived_region) { return; } match kind.unpack() { UnpackedKind::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` for component in tcx.outlives_components(ty) { 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, 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 .insert(ty::OutlivesPredicate(ty.into(), outlived_region)); } 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 .insert(ty::OutlivesPredicate(ty.into(), outlived_region)); } 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"), } } } UnpackedKind::Lifetime(r) => { if !is_free_region(tcx, r) { return; } required_predicates.insert(ty::OutlivesPredicate(kind, outlived_region)); } } } fn is_free_region<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, 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. RegionKind::ReEarlyBound(_) => true, // These correspond to `T: 'static` relationships which can be // rather surprising. We are therefore putting this behind a // feature flag: // // struct Foo<'a, T> { // field: &'static T, // this would generate a ReStatic // } RegionKind::ReStatic => { if tcx .sess .features_untracked() .infer_static_outlives_requirements { true } else { 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. RegionKind::ReLateBound(..) => false, // These regions don't appear in types from type declarations: RegionKind::ReEmpty | RegionKind::ReErased | RegionKind::ReClosureBound(..) | RegionKind::ReCanonical(..) | RegionKind::ReScope(..) | RegionKind::ReVar(..) | RegionKind::ReSkolemized(..) | RegionKind::ReFree(..) => { bug!("unexpected region in outlives inference: {:?}", region); } } }