/
mod.rs
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/
mod.rs
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// Copyright 2015 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.
// Logic and data structures related to impl specialization, explained in
// greater detail below.
//
// At the moment, this implementation support only the simple "chain" rule:
// If any two impls overlap, one must be a strict subset of the other.
//
// See traits/README.md for a bit more detail on how specialization
// fits together with the rest of the trait machinery.
use super::{SelectionContext, FulfillmentContext};
use super::util::impl_trait_ref_and_oblig;
use rustc_data_structures::fx::FxHashMap;
use hir::def_id::DefId;
use infer::{InferCtxt, InferOk};
use ty::subst::{Subst, Substs};
use traits::{self, Reveal, ObligationCause};
use ty::{self, TyCtxt, TypeFoldable};
use syntax_pos::DUMMY_SP;
use std::rc::Rc;
pub mod specialization_graph;
/// Information pertinent to an overlapping impl error.
pub struct OverlapError {
pub with_impl: DefId,
pub trait_desc: String,
pub self_desc: Option<String>,
}
/// Given a subst for the requested impl, translate it to a subst
/// appropriate for the actual item definition (whether it be in that impl,
/// a parent impl, or the trait).
/// When we have selected one impl, but are actually using item definitions from
/// a parent impl providing a default, we need a way to translate between the
/// type parameters of the two impls. Here the `source_impl` is the one we've
/// selected, and `source_substs` is a substitution of its generics.
/// And `target_node` is the impl/trait we're actually going to get the
/// definition from. The resulting substitution will map from `target_node`'s
/// generics to `source_impl`'s generics as instantiated by `source_subst`.
///
/// For example, consider the following scenario:
///
/// ```rust
/// trait Foo { ... }
/// impl<T, U> Foo for (T, U) { ... } // target impl
/// impl<V> Foo for (V, V) { ... } // source impl
/// ```
///
/// Suppose we have selected "source impl" with `V` instantiated with `u32`.
/// This function will produce a substitution with `T` and `U` both mapping to `u32`.
///
/// Where clauses add some trickiness here, because they can be used to "define"
/// an argument indirectly:
///
/// ```rust
/// impl<'a, I, T: 'a> Iterator for Cloned<I>
/// where I: Iterator<Item=&'a T>, T: Clone
/// ```
///
/// In a case like this, the substitution for `T` is determined indirectly,
/// through associated type projection. We deal with such cases by using
/// *fulfillment* to relate the two impls, requiring that all projections are
/// resolved.
pub fn translate_substs<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
source_impl: DefId,
source_substs: &'tcx Substs<'tcx>,
target_node: specialization_graph::Node)
-> &'tcx Substs<'tcx> {
let source_trait_ref = infcx.tcx
.impl_trait_ref(source_impl)
.unwrap()
.subst(infcx.tcx, &source_substs);
// translate the Self and TyParam parts of the substitution, since those
// vary across impls
let target_substs = match target_node {
specialization_graph::Node::Impl(target_impl) => {
// no need to translate if we're targetting the impl we started with
if source_impl == target_impl {
return source_substs;
}
fulfill_implication(infcx, source_trait_ref, target_impl).unwrap_or_else(|_| {
bug!("When translating substitutions for specialization, the expected \
specializaiton failed to hold")
})
}
specialization_graph::Node::Trait(..) => source_trait_ref.substs,
};
// directly inherent the method generics, since those do not vary across impls
source_substs.rebase_onto(infcx.tcx, source_impl, target_substs)
}
/// Given a selected impl described by `impl_data`, returns the
/// definition and substitions for the method with the name `name`
/// the kind `kind`, and trait method substitutions `substs`, in
/// that impl, a less specialized impl, or the trait default,
/// whichever applies.
pub fn find_associated_item<'a, 'tcx>(
tcx: TyCtxt<'a, 'tcx, 'tcx>,
item: &ty::AssociatedItem,
substs: &'tcx Substs<'tcx>,
impl_data: &super::VtableImplData<'tcx, ()>,
) -> (DefId, &'tcx Substs<'tcx>) {
assert!(!substs.needs_infer());
let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
let trait_def = tcx.trait_def(trait_def_id);
let ancestors = trait_def.ancestors(tcx, impl_data.impl_def_id);
match ancestors.defs(tcx, item.name, item.kind).next() {
Some(node_item) => {
let substs = tcx.infer_ctxt(Reveal::All).enter(|infcx| {
let substs = substs.rebase_onto(tcx, trait_def_id, impl_data.substs);
let substs = translate_substs(&infcx, impl_data.impl_def_id,
substs, node_item.node);
let substs = infcx.tcx.erase_regions(&substs);
tcx.lift(&substs).unwrap_or_else(|| {
bug!("find_method: translate_substs \
returned {:?} which contains inference types/regions",
substs);
})
});
(node_item.item.def_id, substs)
}
None => {
bug!("{:?} not found in {:?}", item, impl_data.impl_def_id)
}
}
}
/// Is impl1 a specialization of impl2?
///
/// Specialization is determined by the sets of types to which the impls apply;
/// impl1 specializes impl2 if it applies to a subset of the types impl2 applies
/// to.
pub fn specializes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
impl1_def_id: DefId,
impl2_def_id: DefId) -> bool {
debug!("specializes({:?}, {:?})", impl1_def_id, impl2_def_id);
if let Some(r) = tcx.specializes_cache.borrow().check(impl1_def_id, impl2_def_id) {
return r;
}
// The feature gate should prevent introducing new specializations, but not
// taking advantage of upstream ones.
if !tcx.sess.features.borrow().specialization &&
(impl1_def_id.is_local() || impl2_def_id.is_local()) {
return false;
}
// We determine whether there's a subset relationship by:
//
// - skolemizing impl1,
// - assuming the where clauses for impl1,
// - instantiating impl2 with fresh inference variables,
// - unifying,
// - attempting to prove the where clauses for impl2
//
// The last three steps are encapsulated in `fulfill_implication`.
//
// See RFC 1210 for more details and justification.
// Currently we do not allow e.g. a negative impl to specialize a positive one
if tcx.impl_polarity(impl1_def_id) != tcx.impl_polarity(impl2_def_id) {
return false;
}
// create a parameter environment corresponding to a (skolemized) instantiation of impl1
let penv = tcx.param_env(impl1_def_id);
let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();
// Create a infcx, taking the predicates of impl1 as assumptions:
let result = tcx.infer_ctxt(penv).enter(|infcx| {
// Normalize the trait reference. The WF rules ought to ensure
// that this always succeeds.
let impl1_trait_ref =
match traits::fully_normalize(&infcx, ObligationCause::dummy(), &impl1_trait_ref) {
Ok(impl1_trait_ref) => impl1_trait_ref,
Err(err) => {
bug!("failed to fully normalize {:?}: {:?}", impl1_trait_ref, err);
}
};
// Attempt to prove that impl2 applies, given all of the above.
fulfill_implication(&infcx, impl1_trait_ref, impl2_def_id).is_ok()
});
tcx.specializes_cache.borrow_mut().insert(impl1_def_id, impl2_def_id, result);
result
}
/// Attempt to fulfill all obligations of `target_impl` after unification with
/// `source_trait_ref`. If successful, returns a substitution for *all* the
/// generics of `target_impl`, including both those needed to unify with
/// `source_trait_ref` and those whose identity is determined via a where
/// clause in the impl.
fn fulfill_implication<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
source_trait_ref: ty::TraitRef<'tcx>,
target_impl: DefId)
-> Result<&'tcx Substs<'tcx>, ()> {
let selcx = &mut SelectionContext::new(&infcx);
let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
let (target_trait_ref, mut obligations) = impl_trait_ref_and_oblig(selcx,
target_impl,
target_substs);
// do the impls unify? If not, no specialization.
match infcx.eq_trait_refs(true,
&ObligationCause::dummy(),
source_trait_ref,
target_trait_ref) {
Ok(InferOk { obligations: o, .. }) => {
obligations.extend(o);
}
Err(_) => {
debug!("fulfill_implication: {:?} does not unify with {:?}",
source_trait_ref,
target_trait_ref);
return Err(());
}
}
// attempt to prove all of the predicates for impl2 given those for impl1
// (which are packed up in penv)
infcx.save_and_restore_in_snapshot_flag(|infcx| {
let mut fulfill_cx = FulfillmentContext::new();
for oblig in obligations.into_iter() {
fulfill_cx.register_predicate_obligation(&infcx, oblig);
}
match fulfill_cx.select_all_or_error(infcx) {
Err(errors) => {
// no dice!
debug!("fulfill_implication: for impls on {:?} and {:?}, \
could not fulfill: {:?} given {:?}",
source_trait_ref,
target_trait_ref,
errors,
infcx.param_env.caller_bounds);
Err(())
}
Ok(()) => {
debug!("fulfill_implication: an impl for {:?} specializes {:?}",
source_trait_ref,
target_trait_ref);
// Now resolve the *substitution* we built for the target earlier, replacing
// the inference variables inside with whatever we got from fulfillment.
Ok(infcx.resolve_type_vars_if_possible(&target_substs))
}
}
})
}
pub struct SpecializesCache {
map: FxHashMap<(DefId, DefId), bool>,
}
impl SpecializesCache {
pub fn new() -> Self {
SpecializesCache {
map: FxHashMap()
}
}
pub fn check(&self, a: DefId, b: DefId) -> Option<bool> {
self.map.get(&(a, b)).cloned()
}
pub fn insert(&mut self, a: DefId, b: DefId, result: bool) {
self.map.insert((a, b), result);
}
}
// Query provider for `specialization_graph_of`.
pub(super) fn specialization_graph_provider<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
trait_id: DefId)
-> Rc<specialization_graph::Graph> {
let mut sg = specialization_graph::Graph::new();
let mut trait_impls: Vec<DefId> = tcx.trait_impls_of(trait_id).iter().collect();
// The coherence checking implementation seems to rely on impls being
// iterated over (roughly) in definition order, so we are sorting by
// negated CrateNum (so remote definitions are visited first) and then
// by a flattend version of the DefIndex.
trait_impls.sort_unstable_by_key(|def_id| {
(-(def_id.krate.as_u32() as i64),
def_id.index.address_space().index(),
def_id.index.as_array_index())
});
for impl_def_id in trait_impls {
if impl_def_id.is_local() {
// This is where impl overlap checking happens:
let insert_result = sg.insert(tcx, impl_def_id);
// Report error if there was one.
if let Err(overlap) = insert_result {
let mut err = struct_span_err!(tcx.sess,
tcx.span_of_impl(impl_def_id).unwrap(),
E0119,
"conflicting implementations of trait `{}`{}:",
overlap.trait_desc,
overlap.self_desc.clone().map_or(String::new(),
|ty| {
format!(" for type `{}`", ty)
}));
match tcx.span_of_impl(overlap.with_impl) {
Ok(span) => {
err.span_label(span, format!("first implementation here"));
err.span_label(tcx.span_of_impl(impl_def_id).unwrap(),
format!("conflicting implementation{}",
overlap.self_desc
.map_or(String::new(),
|ty| format!(" for `{}`", ty))));
}
Err(cname) => {
err.note(&format!("conflicting implementation in crate `{}`", cname));
}
}
err.emit();
}
} else {
let parent = tcx.impl_parent(impl_def_id).unwrap_or(trait_id);
sg.record_impl_from_cstore(tcx, parent, impl_def_id)
}
}
Rc::new(sg)
}