546 lines
21 KiB
Rust
546 lines
21 KiB
Rust
//! See Rustc Guide chapters on [trait-resolution] and [trait-specialization] for more info on how
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//! this works.
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//!
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//! [trait-resolution]: https://rust-lang.github.io/rustc-guide/traits/resolution.html
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//! [trait-specialization]: https://rust-lang.github.io/rustc-guide/traits/specialization.html
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use crate::infer::{CombinedSnapshot, InferOk};
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use crate::traits::select::IntercrateAmbiguityCause;
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use crate::traits::IntercrateMode;
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use crate::traits::{self, Normalized, Obligation, ObligationCause, SelectionContext};
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use crate::ty::fold::TypeFoldable;
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use crate::ty::subst::Subst;
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use crate::ty::{self, Ty, TyCtxt};
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use rustc_hir::def_id::{DefId, LOCAL_CRATE};
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use rustc_span::symbol::sym;
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use rustc_span::DUMMY_SP;
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/// Whether we do the orphan check relative to this crate or
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/// to some remote crate.
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#[derive(Copy, Clone, Debug)]
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enum InCrate {
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Local,
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Remote,
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}
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#[derive(Debug, Copy, Clone)]
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pub enum Conflict {
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Upstream,
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Downstream { used_to_be_broken: bool },
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}
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pub struct OverlapResult<'tcx> {
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pub impl_header: ty::ImplHeader<'tcx>,
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pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
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/// `true` if the overlap might've been permitted before the shift
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/// to universes.
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pub involves_placeholder: bool,
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}
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pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
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err.note(&format!(
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"this behavior recently changed as a result of a bug fix; \
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see rust-lang/rust#56105 for details"
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));
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}
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/// If there are types that satisfy both impls, invokes `on_overlap`
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/// with a suitably-freshened `ImplHeader` with those types
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/// substituted. Otherwise, invokes `no_overlap`.
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pub fn overlapping_impls<F1, F2, R>(
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tcx: TyCtxt<'_>,
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impl1_def_id: DefId,
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impl2_def_id: DefId,
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intercrate_mode: IntercrateMode,
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on_overlap: F1,
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no_overlap: F2,
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) -> R
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where
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F1: FnOnce(OverlapResult<'_>) -> R,
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F2: FnOnce() -> R,
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{
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debug!(
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"overlapping_impls(\
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impl1_def_id={:?}, \
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impl2_def_id={:?},
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intercrate_mode={:?})",
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impl1_def_id, impl2_def_id, intercrate_mode
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);
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let overlaps = tcx.infer_ctxt().enter(|infcx| {
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let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
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overlap(selcx, impl1_def_id, impl2_def_id).is_some()
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});
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if !overlaps {
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return no_overlap();
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}
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// In the case where we detect an error, run the check again, but
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// this time tracking intercrate ambuiguity causes for better
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// diagnostics. (These take time and can lead to false errors.)
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tcx.infer_ctxt().enter(|infcx| {
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let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
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selcx.enable_tracking_intercrate_ambiguity_causes();
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on_overlap(overlap(selcx, impl1_def_id, impl2_def_id).unwrap())
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})
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}
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fn with_fresh_ty_vars<'cx, 'tcx>(
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selcx: &mut SelectionContext<'cx, 'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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impl_def_id: DefId,
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) -> ty::ImplHeader<'tcx> {
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let tcx = selcx.tcx();
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let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
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let header = ty::ImplHeader {
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impl_def_id,
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self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
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trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
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predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
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};
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let Normalized { value: mut header, obligations } =
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traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
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header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
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header
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}
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/// Can both impl `a` and impl `b` be satisfied by a common type (including
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/// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
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fn overlap<'cx, 'tcx>(
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selcx: &mut SelectionContext<'cx, 'tcx>,
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a_def_id: DefId,
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b_def_id: DefId,
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) -> Option<OverlapResult<'tcx>> {
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debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
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selcx.infcx().probe(|snapshot| overlap_within_probe(selcx, a_def_id, b_def_id, snapshot))
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}
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fn overlap_within_probe(
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selcx: &mut SelectionContext<'cx, 'tcx>,
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a_def_id: DefId,
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b_def_id: DefId,
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snapshot: &CombinedSnapshot<'_, 'tcx>,
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) -> Option<OverlapResult<'tcx>> {
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// For the purposes of this check, we don't bring any placeholder
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// types into scope; instead, we replace the generic types with
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// fresh type variables, and hence we do our evaluations in an
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// empty environment.
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let param_env = ty::ParamEnv::empty();
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let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
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let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
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debug!("overlap: a_impl_header={:?}", a_impl_header);
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debug!("overlap: b_impl_header={:?}", b_impl_header);
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// Do `a` and `b` unify? If not, no overlap.
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let obligations = match selcx
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.infcx()
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.at(&ObligationCause::dummy(), param_env)
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.eq_impl_headers(&a_impl_header, &b_impl_header)
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{
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Ok(InferOk { obligations, value: () }) => obligations,
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Err(_) => return None,
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};
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debug!("overlap: unification check succeeded");
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// Are any of the obligations unsatisfiable? If so, no overlap.
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let infcx = selcx.infcx();
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let opt_failing_obligation = a_impl_header
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.predicates
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.iter()
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.chain(&b_impl_header.predicates)
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.map(|p| infcx.resolve_vars_if_possible(p))
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.map(|p| Obligation {
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cause: ObligationCause::dummy(),
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param_env,
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recursion_depth: 0,
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predicate: p,
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})
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.chain(obligations)
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.find(|o| !selcx.predicate_may_hold_fatal(o));
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// FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
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// to the canonical trait query form, `infcx.predicate_may_hold`, once
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// the new system supports intercrate mode (which coherence needs).
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if let Some(failing_obligation) = opt_failing_obligation {
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debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
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return None;
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}
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let impl_header = selcx.infcx().resolve_vars_if_possible(&a_impl_header);
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let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
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debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
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let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
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Some(true) => true,
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_ => false,
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};
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Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
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}
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pub fn trait_ref_is_knowable<'tcx>(
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tcx: TyCtxt<'tcx>,
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trait_ref: ty::TraitRef<'tcx>,
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) -> Option<Conflict> {
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debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
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if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
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// A downstream or cousin crate is allowed to implement some
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// substitution of this trait-ref.
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// A trait can be implementable for a trait ref by both the current
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// crate and crates downstream of it. Older versions of rustc
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// were not aware of this, causing incoherence (issue #43355).
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let used_to_be_broken = orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
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if used_to_be_broken {
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debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
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}
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return Some(Conflict::Downstream { used_to_be_broken });
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}
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if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
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// This is a local or fundamental trait, so future-compatibility
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// is no concern. We know that downstream/cousin crates are not
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// allowed to implement a substitution of this trait ref, which
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// means impls could only come from dependencies of this crate,
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// which we already know about.
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return None;
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}
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// This is a remote non-fundamental trait, so if another crate
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// can be the "final owner" of a substitution of this trait-ref,
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// they are allowed to implement it future-compatibly.
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//
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// However, if we are a final owner, then nobody else can be,
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// and if we are an intermediate owner, then we don't care
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// about future-compatibility, which means that we're OK if
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// we are an owner.
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if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
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debug!("trait_ref_is_knowable: orphan check passed");
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return None;
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} else {
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debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
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return Some(Conflict::Upstream);
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}
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}
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pub fn trait_ref_is_local_or_fundamental<'tcx>(
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tcx: TyCtxt<'tcx>,
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trait_ref: ty::TraitRef<'tcx>,
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) -> bool {
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trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
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}
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pub enum OrphanCheckErr<'tcx> {
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NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
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UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
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}
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/// Checks the coherence orphan rules. `impl_def_id` should be the
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/// `DefId` of a trait impl. To pass, either the trait must be local, or else
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/// two conditions must be satisfied:
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///
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/// 1. All type parameters in `Self` must be "covered" by some local type constructor.
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/// 2. Some local type must appear in `Self`.
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pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
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debug!("orphan_check({:?})", impl_def_id);
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// We only except this routine to be invoked on implementations
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// of a trait, not inherent implementations.
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let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
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debug!("orphan_check: trait_ref={:?}", trait_ref);
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// If the *trait* is local to the crate, ok.
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if trait_ref.def_id.is_local() {
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debug!("trait {:?} is local to current crate", trait_ref.def_id);
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return Ok(());
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}
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orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
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}
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/// Checks whether a trait-ref is potentially implementable by a crate.
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///
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/// The current rule is that a trait-ref orphan checks in a crate C:
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///
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/// 1. Order the parameters in the trait-ref in subst order - Self first,
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/// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
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/// 2. Of these type parameters, there is at least one type parameter
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/// in which, walking the type as a tree, you can reach a type local
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/// to C where all types in-between are fundamental types. Call the
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/// first such parameter the "local key parameter".
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/// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
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/// going through `Box`, which is fundamental.
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/// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
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/// the same reason.
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/// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
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/// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
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/// the local type and the type parameter.
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/// 3. Every type parameter before the local key parameter is fully known in C.
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/// - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
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/// an unknown type.
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/// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
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/// occurs before `T`.
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/// 4. Every type in the local key parameter not known in C, going
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/// through the parameter's type tree, must appear only as a subtree of
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/// a type local to C, with only fundamental types between the type
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/// local to C and the local key parameter.
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/// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
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/// is bad, because the only local type with `T` as a subtree is
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/// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
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/// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
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/// the second occurrence of `T` is not a subtree of *any* local type.
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/// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
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/// `LocalType<Vec<T>>`, which is local and has no types between it and
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/// the type parameter.
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///
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/// The orphan rules actually serve several different purposes:
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///
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/// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
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/// every type local to one crate is unknown in the other) can't implement
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/// the same trait-ref. This follows because it can be seen that no such
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/// type can orphan-check in 2 such crates.
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///
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/// To check that a local impl follows the orphan rules, we check it in
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/// InCrate::Local mode, using type parameters for the "generic" types.
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///
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/// 2. They ground negative reasoning for coherence. If a user wants to
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/// write both a conditional blanket impl and a specific impl, we need to
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/// make sure they do not overlap. For example, if we write
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/// ```
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/// impl<T> IntoIterator for Vec<T>
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/// impl<T: Iterator> IntoIterator for T
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/// ```
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/// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
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/// We can observe that this holds in the current crate, but we need to make
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/// sure this will also hold in all unknown crates (both "independent" crates,
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/// which we need for link-safety, and also child crates, because we don't want
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/// child crates to get error for impl conflicts in a *dependency*).
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///
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/// For that, we only allow negative reasoning if, for every assignment to the
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/// inference variables, every unknown crate would get an orphan error if they
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/// try to implement this trait-ref. To check for this, we use InCrate::Remote
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/// mode. That is sound because we already know all the impls from known crates.
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///
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/// 3. For non-#[fundamental] traits, they guarantee that parent crates can
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/// add "non-blanket" impls without breaking negative reasoning in dependent
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/// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
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///
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/// For that, we only a allow crate to perform negative reasoning on
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/// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
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///
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/// Because we never perform negative reasoning generically (coherence does
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/// not involve type parameters), this can be interpreted as doing the full
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/// orphan check (using InCrate::Local mode), substituting non-local known
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/// types for all inference variables.
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///
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/// This allows for crates to future-compatibly add impls as long as they
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/// can't apply to types with a key parameter in a child crate - applying
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/// the rules, this basically means that every type parameter in the impl
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/// must appear behind a non-fundamental type (because this is not a
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/// type-system requirement, crate owners might also go for "semantic
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/// future-compatibility" involving things such as sealed traits, but
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/// the above requirement is sufficient, and is necessary in "open world"
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/// cases).
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///
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/// Note that this function is never called for types that have both type
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/// parameters and inference variables.
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fn orphan_check_trait_ref<'tcx>(
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tcx: TyCtxt<'tcx>,
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trait_ref: ty::TraitRef<'tcx>,
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in_crate: InCrate,
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) -> Result<(), OrphanCheckErr<'tcx>> {
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debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
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if trait_ref.needs_infer() && trait_ref.needs_subst() {
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bug!(
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"can't orphan check a trait ref with both params and inference variables {:?}",
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trait_ref
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);
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}
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// Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
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// if at least one of the following is true:
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//
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// - Trait is a local trait
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// (already checked in orphan_check prior to calling this function)
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// - All of
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// - At least one of the types T0..=Tn must be a local type.
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// Let Ti be the first such type.
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// - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
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//
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fn uncover_fundamental_ty<'tcx>(
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tcx: TyCtxt<'tcx>,
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ty: Ty<'tcx>,
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in_crate: InCrate,
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) -> Vec<Ty<'tcx>> {
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if fundamental_ty(ty) && ty_is_non_local(ty, in_crate).is_some() {
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ty.walk_shallow().flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)).collect()
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} else {
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vec![ty]
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}
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}
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let mut non_local_spans = vec![];
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for (i, input_ty) in
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trait_ref.input_types().flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)).enumerate()
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{
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debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
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let non_local_tys = ty_is_non_local(input_ty, in_crate);
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if non_local_tys.is_none() {
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debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
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return Ok(());
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} else if let ty::Param(_) = input_ty.kind {
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debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
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let local_type = trait_ref
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.input_types()
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.flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
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.filter(|ty| ty_is_non_local_constructor(ty, in_crate).is_none())
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.next();
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debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
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return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
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}
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if let Some(non_local_tys) = non_local_tys {
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for input_ty in non_local_tys {
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non_local_spans.push((input_ty, i == 0));
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}
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}
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}
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|
// If we exit above loop, never found a local type.
|
|
debug!("orphan_check_trait_ref: no local type");
|
|
Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
|
|
}
|
|
|
|
fn ty_is_non_local<'t>(ty: Ty<'t>, in_crate: InCrate) -> Option<Vec<Ty<'t>>> {
|
|
match ty_is_non_local_constructor(ty, in_crate) {
|
|
Some(ty) => {
|
|
if !fundamental_ty(ty) {
|
|
Some(vec![ty])
|
|
} else {
|
|
let tys: Vec<_> = ty
|
|
.walk_shallow()
|
|
.filter_map(|t| ty_is_non_local(t, in_crate))
|
|
.flat_map(|i| i)
|
|
.collect();
|
|
if tys.is_empty() { None } else { Some(tys) }
|
|
}
|
|
}
|
|
None => None,
|
|
}
|
|
}
|
|
|
|
fn fundamental_ty(ty: Ty<'_>) -> bool {
|
|
match ty.kind {
|
|
ty::Ref(..) => true,
|
|
ty::Adt(def, _) => def.is_fundamental(),
|
|
_ => false,
|
|
}
|
|
}
|
|
|
|
fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
|
|
match in_crate {
|
|
// The type is local to *this* crate - it will not be
|
|
// local in any other crate.
|
|
InCrate::Remote => false,
|
|
InCrate::Local => def_id.is_local(),
|
|
}
|
|
}
|
|
|
|
fn ty_is_non_local_constructor<'tcx>(ty: Ty<'tcx>, in_crate: InCrate) -> Option<Ty<'tcx>> {
|
|
debug!("ty_is_non_local_constructor({:?})", ty);
|
|
|
|
match ty.kind {
|
|
ty::Bool
|
|
| ty::Char
|
|
| ty::Int(..)
|
|
| ty::Uint(..)
|
|
| ty::Float(..)
|
|
| ty::Str
|
|
| ty::FnDef(..)
|
|
| ty::FnPtr(_)
|
|
| ty::Array(..)
|
|
| ty::Slice(..)
|
|
| ty::RawPtr(..)
|
|
| ty::Ref(..)
|
|
| ty::Never
|
|
| ty::Tuple(..)
|
|
| ty::Param(..)
|
|
| ty::Projection(..) => Some(ty),
|
|
|
|
ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
|
|
InCrate::Local => Some(ty),
|
|
// The inference variable might be unified with a local
|
|
// type in that remote crate.
|
|
InCrate::Remote => None,
|
|
},
|
|
|
|
ty::Adt(def, _) => {
|
|
if def_id_is_local(def.did, in_crate) {
|
|
None
|
|
} else {
|
|
Some(ty)
|
|
}
|
|
}
|
|
ty::Foreign(did) => {
|
|
if def_id_is_local(did, in_crate) {
|
|
None
|
|
} else {
|
|
Some(ty)
|
|
}
|
|
}
|
|
ty::Opaque(..) => {
|
|
// This merits some explanation.
|
|
// Normally, opaque types are not involed when performing
|
|
// coherence checking, since it is illegal to directly
|
|
// implement a trait on an opaque type. However, we might
|
|
// end up looking at an opaque type during coherence checking
|
|
// if an opaque type gets used within another type (e.g. as
|
|
// a type parameter). This requires us to decide whether or
|
|
// not an opaque type should be considered 'local' or not.
|
|
//
|
|
// We choose to treat all opaque types as non-local, even
|
|
// those that appear within the same crate. This seems
|
|
// somewhat suprising at first, but makes sense when
|
|
// you consider that opaque types are supposed to hide
|
|
// the underlying type *within the same crate*. When an
|
|
// opaque type is used from outside the module
|
|
// where it is declared, it should be impossible to observe
|
|
// anyything about it other than the traits that it implements.
|
|
//
|
|
// The alternative would be to look at the underlying type
|
|
// to determine whether or not the opaque type itself should
|
|
// be considered local. However, this could make it a breaking change
|
|
// to switch the underlying ('defining') type from a local type
|
|
// to a remote type. This would violate the rule that opaque
|
|
// types should be completely opaque apart from the traits
|
|
// that they implement, so we don't use this behavior.
|
|
Some(ty)
|
|
}
|
|
|
|
ty::Dynamic(ref tt, ..) => {
|
|
if let Some(principal) = tt.principal() {
|
|
if def_id_is_local(principal.def_id(), in_crate) { None } else { Some(ty) }
|
|
} else {
|
|
Some(ty)
|
|
}
|
|
}
|
|
|
|
ty::Error => None,
|
|
|
|
ty::UnnormalizedProjection(..)
|
|
| ty::Closure(..)
|
|
| ty::Generator(..)
|
|
| ty::GeneratorWitness(..) => bug!("ty_is_local invoked on unexpected type: {:?}", ty),
|
|
}
|
|
}
|