138 lines
5.6 KiB
Rust
138 lines
5.6 KiB
Rust
use crate::infer::at::At;
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use crate::infer::canonical::OriginalQueryValues;
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use crate::infer::InferOk;
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use rustc::ty::subst::GenericArg;
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use rustc::ty::{self, Ty, TyCtxt};
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pub use rustc::traits::query::{DropckOutlivesResult, DtorckConstraint};
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impl<'cx, 'tcx> At<'cx, 'tcx> {
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/// Given a type `ty` of some value being dropped, computes a set
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/// of "kinds" (types, regions) that must be outlive the execution
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/// of the destructor. These basically correspond to data that the
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/// destructor might access. This is used during regionck to
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/// impose "outlives" constraints on any lifetimes referenced
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/// within.
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///
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/// The rules here are given by the "dropck" RFCs, notably [#1238]
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/// and [#1327]. This is a fixed-point computation, where we
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/// explore all the data that will be dropped (transitively) when
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/// a value of type `ty` is dropped. For each type T that will be
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/// dropped and which has a destructor, we must assume that all
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/// the types/regions of T are live during the destructor, unless
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/// they are marked with a special attribute (`#[may_dangle]`).
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///
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/// [#1238]: https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md
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/// [#1327]: https://github.com/rust-lang/rfcs/blob/master/text/1327-dropck-param-eyepatch.md
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pub fn dropck_outlives(&self, ty: Ty<'tcx>) -> InferOk<'tcx, Vec<GenericArg<'tcx>>> {
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debug!("dropck_outlives(ty={:?}, param_env={:?})", ty, self.param_env,);
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// Quick check: there are a number of cases that we know do not require
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// any destructor.
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let tcx = self.infcx.tcx;
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if trivial_dropck_outlives(tcx, ty) {
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return InferOk { value: vec![], obligations: vec![] };
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}
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let mut orig_values = OriginalQueryValues::default();
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let c_ty = self.infcx.canonicalize_query(&self.param_env.and(ty), &mut orig_values);
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let span = self.cause.span;
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debug!("c_ty = {:?}", c_ty);
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if let Ok(result) = &tcx.dropck_outlives(c_ty) {
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if result.is_proven() {
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if let Ok(InferOk { value, obligations }) =
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self.infcx.instantiate_query_response_and_region_obligations(
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self.cause,
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self.param_env,
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&orig_values,
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result,
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)
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{
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let ty = self.infcx.resolve_vars_if_possible(&ty);
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let kinds = value.into_kinds_reporting_overflows(tcx, span, ty);
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return InferOk { value: kinds, obligations };
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}
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}
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}
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// Errors and ambiuity in dropck occur in two cases:
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// - unresolved inference variables at the end of typeck
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// - non well-formed types where projections cannot be resolved
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// Either of these should have created an error before.
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tcx.sess.delay_span_bug(span, "dtorck encountered internal error");
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InferOk { value: vec![], obligations: vec![] }
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}
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}
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/// This returns true if the type `ty` is "trivial" for
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/// dropck-outlives -- that is, if it doesn't require any types to
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/// outlive. This is similar but not *quite* the same as the
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/// `needs_drop` test in the compiler already -- that is, for every
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/// type T for which this function return true, needs-drop would
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/// return `false`. But the reverse does not hold: in particular,
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/// `needs_drop` returns false for `PhantomData`, but it is not
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/// trivial for dropck-outlives.
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///
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/// Note also that `needs_drop` requires a "global" type (i.e., one
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/// with erased regions), but this function does not.
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pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
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match ty.kind {
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// None of these types have a destructor and hence they do not
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// require anything in particular to outlive the dtor's
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// execution.
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ty::Infer(ty::FreshIntTy(_))
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| ty::Infer(ty::FreshFloatTy(_))
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| ty::Bool
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| ty::Int(_)
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| ty::Uint(_)
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| ty::Float(_)
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| ty::Never
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| ty::FnDef(..)
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| ty::FnPtr(_)
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| ty::Char
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| ty::GeneratorWitness(..)
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| ty::RawPtr(_)
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| ty::Ref(..)
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| ty::Str
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| ty::Foreign(..)
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| ty::Error => true,
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// [T; N] and [T] have same properties as T.
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ty::Array(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, ty),
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// (T1..Tn) and closures have same properties as T1..Tn --
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// check if *any* of those are trivial.
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ty::Tuple(ref tys) => tys.iter().all(|t| trivial_dropck_outlives(tcx, t.expect_ty())),
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ty::Closure(def_id, ref substs) => {
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substs.as_closure().upvar_tys(def_id, tcx).all(|t| trivial_dropck_outlives(tcx, t))
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}
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ty::Adt(def, _) => {
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if Some(def.did) == tcx.lang_items().manually_drop() {
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// `ManuallyDrop` never has a dtor.
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true
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} else {
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// Other types might. Moreover, PhantomData doesn't
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// have a dtor, but it is considered to own its
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// content, so it is non-trivial. Unions can have `impl Drop`,
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// and hence are non-trivial as well.
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false
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}
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}
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// The following *might* require a destructor: needs deeper inspection.
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ty::Dynamic(..)
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| ty::Projection(..)
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| ty::Param(_)
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| ty::Opaque(..)
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| ty::Placeholder(..)
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| ty::Infer(_)
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| ty::Bound(..)
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| ty::Generator(..) => false,
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ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
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}
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}
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