465 lines
18 KiB
Rust
465 lines
18 KiB
Rust
//! Logic and data structures related to impl specialization, explained in
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//! greater detail below.
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//!
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//! At the moment, this implementation support only the simple "chain" rule:
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//! If any two impls overlap, one must be a strict subset of the other.
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//!
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//! See the [rustc guide] for a bit more detail on how specialization
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//! fits together with the rest of the trait machinery.
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//!
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//! [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/specialization.html
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pub mod specialization_graph;
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use crate::infer::{InferCtxt, InferOk};
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use crate::lint;
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use crate::traits::select::IntercrateAmbiguityCause;
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use crate::traits::{self, coherence, FutureCompatOverlapErrorKind, ObligationCause, TraitEngine};
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use crate::ty::subst::{InternalSubsts, Subst, SubstsRef};
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use crate::ty::{self, TyCtxt, TypeFoldable};
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use errors::struct_span_err;
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use rustc_data_structures::fx::FxHashSet;
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use rustc_hir::def_id::DefId;
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use rustc_span::DUMMY_SP;
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use super::util::impl_trait_ref_and_oblig;
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use super::{FulfillmentContext, SelectionContext};
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use rustc_error_codes::*;
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/// Information pertinent to an overlapping impl error.
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#[derive(Debug)]
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pub struct OverlapError {
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pub with_impl: DefId,
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pub trait_desc: String,
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pub self_desc: Option<String>,
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pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
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pub involves_placeholder: bool,
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}
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/// Given a subst for the requested impl, translate it to a subst
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/// appropriate for the actual item definition (whether it be in that impl,
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/// a parent impl, or the trait).
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///
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/// When we have selected one impl, but are actually using item definitions from
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/// a parent impl providing a default, we need a way to translate between the
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/// type parameters of the two impls. Here the `source_impl` is the one we've
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/// selected, and `source_substs` is a substitution of its generics.
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/// And `target_node` is the impl/trait we're actually going to get the
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/// definition from. The resulting substitution will map from `target_node`'s
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/// generics to `source_impl`'s generics as instantiated by `source_subst`.
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///
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/// For example, consider the following scenario:
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///
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/// ```rust
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/// trait Foo { ... }
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/// impl<T, U> Foo for (T, U) { ... } // target impl
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/// impl<V> Foo for (V, V) { ... } // source impl
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/// ```
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///
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/// Suppose we have selected "source impl" with `V` instantiated with `u32`.
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/// This function will produce a substitution with `T` and `U` both mapping to `u32`.
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///
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/// where-clauses add some trickiness here, because they can be used to "define"
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/// an argument indirectly:
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///
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/// ```rust
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/// impl<'a, I, T: 'a> Iterator for Cloned<I>
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/// where I: Iterator<Item = &'a T>, T: Clone
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/// ```
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///
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/// In a case like this, the substitution for `T` is determined indirectly,
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/// through associated type projection. We deal with such cases by using
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/// *fulfillment* to relate the two impls, requiring that all projections are
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/// resolved.
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pub fn translate_substs<'a, 'tcx>(
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infcx: &InferCtxt<'a, 'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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source_impl: DefId,
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source_substs: SubstsRef<'tcx>,
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target_node: specialization_graph::Node,
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) -> SubstsRef<'tcx> {
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debug!(
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"translate_substs({:?}, {:?}, {:?}, {:?})",
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param_env, source_impl, source_substs, target_node
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);
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let source_trait_ref =
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infcx.tcx.impl_trait_ref(source_impl).unwrap().subst(infcx.tcx, &source_substs);
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// translate the Self and Param parts of the substitution, since those
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// vary across impls
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let target_substs = match target_node {
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specialization_graph::Node::Impl(target_impl) => {
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// no need to translate if we're targeting the impl we started with
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if source_impl == target_impl {
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return source_substs;
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}
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fulfill_implication(infcx, param_env, source_trait_ref, target_impl).unwrap_or_else(
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|_| {
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bug!(
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"When translating substitutions for specialization, the expected \
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specialization failed to hold"
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)
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},
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)
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}
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specialization_graph::Node::Trait(..) => source_trait_ref.substs,
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};
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// directly inherent the method generics, since those do not vary across impls
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source_substs.rebase_onto(infcx.tcx, source_impl, target_substs)
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}
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/// Given a selected impl described by `impl_data`, returns the
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/// definition and substitutions for the method with the name `name`
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/// the kind `kind`, and trait method substitutions `substs`, in
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/// that impl, a less specialized impl, or the trait default,
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/// whichever applies.
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pub fn find_associated_item<'tcx>(
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tcx: TyCtxt<'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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item: &ty::AssocItem,
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substs: SubstsRef<'tcx>,
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impl_data: &super::VtableImplData<'tcx, ()>,
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) -> (DefId, SubstsRef<'tcx>) {
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debug!("find_associated_item({:?}, {:?}, {:?}, {:?})", param_env, item, substs, impl_data);
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assert!(!substs.needs_infer());
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let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
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let trait_def = tcx.trait_def(trait_def_id);
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let ancestors = trait_def.ancestors(tcx, impl_data.impl_def_id);
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match ancestors.leaf_def(tcx, item.ident, item.kind) {
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Some(node_item) => {
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let substs = tcx.infer_ctxt().enter(|infcx| {
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let param_env = param_env.with_reveal_all();
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let substs = substs.rebase_onto(tcx, trait_def_id, impl_data.substs);
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let substs = translate_substs(
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&infcx,
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param_env,
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impl_data.impl_def_id,
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substs,
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node_item.node,
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);
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infcx.tcx.erase_regions(&substs)
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});
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(node_item.item.def_id, substs)
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}
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None => bug!("{:?} not found in {:?}", item, impl_data.impl_def_id),
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}
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}
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/// Is `impl1` a specialization of `impl2`?
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///
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/// Specialization is determined by the sets of types to which the impls apply;
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/// `impl1` specializes `impl2` if it applies to a subset of the types `impl2` applies
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/// to.
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pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId, DefId)) -> bool {
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debug!("specializes({:?}, {:?})", impl1_def_id, impl2_def_id);
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// The feature gate should prevent introducing new specializations, but not
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// taking advantage of upstream ones.
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if !tcx.features().specialization && (impl1_def_id.is_local() || impl2_def_id.is_local()) {
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return false;
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}
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// We determine whether there's a subset relationship by:
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//
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// - skolemizing impl1,
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// - assuming the where clauses for impl1,
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// - instantiating impl2 with fresh inference variables,
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// - unifying,
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// - attempting to prove the where clauses for impl2
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//
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// The last three steps are encapsulated in `fulfill_implication`.
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//
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// See RFC 1210 for more details and justification.
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// Currently we do not allow e.g., a negative impl to specialize a positive one
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if tcx.impl_polarity(impl1_def_id) != tcx.impl_polarity(impl2_def_id) {
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return false;
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}
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// create a parameter environment corresponding to a (placeholder) instantiation of impl1
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let penv = tcx.param_env(impl1_def_id);
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let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();
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// Create a infcx, taking the predicates of impl1 as assumptions:
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tcx.infer_ctxt().enter(|infcx| {
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// Normalize the trait reference. The WF rules ought to ensure
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// that this always succeeds.
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let impl1_trait_ref = match traits::fully_normalize(
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&infcx,
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FulfillmentContext::new(),
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ObligationCause::dummy(),
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penv,
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&impl1_trait_ref,
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) {
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Ok(impl1_trait_ref) => impl1_trait_ref,
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Err(err) => {
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bug!("failed to fully normalize {:?}: {:?}", impl1_trait_ref, err);
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}
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};
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// Attempt to prove that impl2 applies, given all of the above.
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fulfill_implication(&infcx, penv, impl1_trait_ref, impl2_def_id).is_ok()
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})
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}
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/// Attempt to fulfill all obligations of `target_impl` after unification with
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/// `source_trait_ref`. If successful, returns a substitution for *all* the
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/// generics of `target_impl`, including both those needed to unify with
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/// `source_trait_ref` and those whose identity is determined via a where
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/// clause in the impl.
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fn fulfill_implication<'a, 'tcx>(
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infcx: &InferCtxt<'a, 'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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source_trait_ref: ty::TraitRef<'tcx>,
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target_impl: DefId,
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) -> Result<SubstsRef<'tcx>, ()> {
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debug!(
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"fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
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param_env, source_trait_ref, target_impl
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);
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let selcx = &mut SelectionContext::new(&infcx);
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let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
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let (target_trait_ref, mut obligations) =
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impl_trait_ref_and_oblig(selcx, param_env, target_impl, target_substs);
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debug!(
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"fulfill_implication: target_trait_ref={:?}, obligations={:?}",
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target_trait_ref, obligations
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);
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// do the impls unify? If not, no specialization.
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match infcx.at(&ObligationCause::dummy(), param_env).eq(source_trait_ref, target_trait_ref) {
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Ok(InferOk { obligations: o, .. }) => {
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obligations.extend(o);
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}
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Err(_) => {
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debug!(
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"fulfill_implication: {:?} does not unify with {:?}",
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source_trait_ref, target_trait_ref
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);
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return Err(());
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}
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}
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// attempt to prove all of the predicates for impl2 given those for impl1
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// (which are packed up in penv)
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infcx.save_and_restore_in_snapshot_flag(|infcx| {
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// If we came from `translate_substs`, we already know that the
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// predicates for our impl hold (after all, we know that a more
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// specialized impl holds, so our impl must hold too), and
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// we only want to process the projections to determine the
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// the types in our substs using RFC 447, so we can safely
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// ignore region obligations, which allows us to avoid threading
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// a node-id to assign them with.
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//
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// If we came from specialization graph construction, then
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// we already make a mockery out of the region system, so
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// why not ignore them a bit earlier?
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let mut fulfill_cx = FulfillmentContext::new_ignoring_regions();
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for oblig in obligations.into_iter() {
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fulfill_cx.register_predicate_obligation(&infcx, oblig);
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}
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match fulfill_cx.select_all_or_error(infcx) {
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Err(errors) => {
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// no dice!
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debug!(
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"fulfill_implication: for impls on {:?} and {:?}, \
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could not fulfill: {:?} given {:?}",
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source_trait_ref, target_trait_ref, errors, param_env.caller_bounds
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);
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Err(())
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}
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Ok(()) => {
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debug!(
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"fulfill_implication: an impl for {:?} specializes {:?}",
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source_trait_ref, target_trait_ref
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);
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// Now resolve the *substitution* we built for the target earlier, replacing
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// the inference variables inside with whatever we got from fulfillment.
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Ok(infcx.resolve_vars_if_possible(&target_substs))
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}
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}
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})
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}
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// Query provider for `specialization_graph_of`.
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pub(super) fn specialization_graph_provider(
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tcx: TyCtxt<'_>,
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trait_id: DefId,
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) -> &specialization_graph::Graph {
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let mut sg = specialization_graph::Graph::new();
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let mut trait_impls = tcx.all_impls(trait_id);
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// The coherence checking implementation seems to rely on impls being
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// iterated over (roughly) in definition order, so we are sorting by
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// negated `CrateNum` (so remote definitions are visited first) and then
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// by a flattened version of the `DefIndex`.
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trait_impls
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.sort_unstable_by_key(|def_id| (-(def_id.krate.as_u32() as i64), def_id.index.index()));
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for impl_def_id in trait_impls {
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if impl_def_id.is_local() {
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// This is where impl overlap checking happens:
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let insert_result = sg.insert(tcx, impl_def_id);
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// Report error if there was one.
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let (overlap, used_to_be_allowed) = match insert_result {
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Err(overlap) => (Some(overlap), None),
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Ok(Some(overlap)) => (Some(overlap.error), Some(overlap.kind)),
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Ok(None) => (None, None),
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};
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if let Some(overlap) = overlap {
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let msg = format!(
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"conflicting implementations of trait `{}`{}:{}",
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overlap.trait_desc,
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overlap
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.self_desc
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.clone()
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.map_or(String::new(), |ty| { format!(" for type `{}`", ty) }),
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match used_to_be_allowed {
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Some(FutureCompatOverlapErrorKind::Issue33140) => " (E0119)",
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_ => "",
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}
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);
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let impl_span =
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tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
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let mut err = match used_to_be_allowed {
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Some(FutureCompatOverlapErrorKind::Issue43355) | None => {
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struct_span_err!(tcx.sess, impl_span, E0119, "{}", msg)
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}
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Some(kind) => {
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let lint = match kind {
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FutureCompatOverlapErrorKind::Issue43355 => {
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unreachable!("converted to hard error above")
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}
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FutureCompatOverlapErrorKind::Issue33140 => {
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lint::builtin::ORDER_DEPENDENT_TRAIT_OBJECTS
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}
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};
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tcx.struct_span_lint_hir(
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lint,
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tcx.hir().as_local_hir_id(impl_def_id).unwrap(),
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impl_span,
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&msg,
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)
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}
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};
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match tcx.span_of_impl(overlap.with_impl) {
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Ok(span) => {
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err.span_label(
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tcx.sess.source_map().def_span(span),
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"first implementation here".to_string(),
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);
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err.span_label(
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impl_span,
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format!(
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"conflicting implementation{}",
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overlap
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.self_desc
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.map_or(String::new(), |ty| format!(" for `{}`", ty))
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),
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);
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}
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Err(cname) => {
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let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
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Some(s) => {
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format!("conflicting implementation in crate `{}`:\n- {}", cname, s)
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}
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None => format!("conflicting implementation in crate `{}`", cname),
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};
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err.note(&msg);
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}
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}
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for cause in &overlap.intercrate_ambiguity_causes {
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cause.add_intercrate_ambiguity_hint(&mut err);
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}
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if overlap.involves_placeholder {
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coherence::add_placeholder_note(&mut err);
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}
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err.emit();
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}
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} else {
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let parent = tcx.impl_parent(impl_def_id).unwrap_or(trait_id);
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sg.record_impl_from_cstore(tcx, parent, impl_def_id)
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}
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}
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tcx.arena.alloc(sg)
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}
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/// Recovers the "impl X for Y" signature from `impl_def_id` and returns it as a
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/// string.
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fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Option<String> {
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use std::fmt::Write;
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let trait_ref = if let Some(tr) = tcx.impl_trait_ref(impl_def_id) {
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tr
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} else {
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return None;
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};
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let mut w = "impl".to_owned();
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let substs = InternalSubsts::identity_for_item(tcx, impl_def_id);
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// FIXME: Currently only handles ?Sized.
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// Needs to support ?Move and ?DynSized when they are implemented.
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let mut types_without_default_bounds = FxHashSet::default();
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let sized_trait = tcx.lang_items().sized_trait();
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if !substs.is_noop() {
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types_without_default_bounds.extend(substs.types());
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w.push('<');
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w.push_str(
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&substs
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.iter()
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.map(|k| k.to_string())
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.filter(|k| k != "'_")
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.collect::<Vec<_>>()
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.join(", "),
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);
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w.push('>');
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}
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write!(w, " {} for {}", trait_ref.print_only_trait_path(), tcx.type_of(impl_def_id)).unwrap();
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// The predicates will contain default bounds like `T: Sized`. We need to
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// remove these bounds, and add `T: ?Sized` to any untouched type parameters.
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let predicates = tcx.predicates_of(impl_def_id).predicates;
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let mut pretty_predicates =
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Vec::with_capacity(predicates.len() + types_without_default_bounds.len());
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for (p, _) in predicates {
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if let Some(poly_trait_ref) = p.to_opt_poly_trait_ref() {
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if Some(poly_trait_ref.def_id()) == sized_trait {
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types_without_default_bounds.remove(poly_trait_ref.self_ty());
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continue;
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}
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}
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pretty_predicates.push(p.to_string());
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}
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pretty_predicates
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.extend(types_without_default_bounds.iter().map(|ty| format!("{}: ?Sized", ty)));
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if !pretty_predicates.is_empty() {
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write!(w, "\n where {}", pretty_predicates.join(", ")).unwrap();
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}
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w.push(';');
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Some(w)
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}
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