rustc_trait_selection/traits/specialize/mod.rs
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//! 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 the [rustc dev guide] for a bit more detail on how specialization
//! fits together with the rest of the trait machinery.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html
pub mod specialization_graph;
use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::codes::*;
use rustc_errors::{Diag, EmissionGuarantee};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_infer::infer::DefineOpaqueTypes;
use rustc_middle::bug;
use rustc_middle::query::LocalCrate;
use rustc_middle::ty::print::PrintTraitRefExt as _;
use rustc_middle::ty::{
self, GenericArgsRef, ImplSubject, Ty, TyCtxt, TypeVisitableExt, TypingMode,
};
use rustc_session::lint::builtin::{COHERENCE_LEAK_CHECK, ORDER_DEPENDENT_TRAIT_OBJECTS};
use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, sym};
use specialization_graph::GraphExt;
use tracing::{debug, instrument};
use super::{SelectionContext, util};
use crate::error_reporting::traits::to_pretty_impl_header;
use crate::errors::NegativePositiveConflict;
use crate::infer::{InferCtxt, InferOk, TyCtxtInferExt};
use crate::traits::select::IntercrateAmbiguityCause;
use crate::traits::{FutureCompatOverlapErrorKind, ObligationCause, ObligationCtxt, coherence};
/// Information pertinent to an overlapping impl error.
#[derive(Debug)]
pub struct OverlapError<'tcx> {
pub with_impl: DefId,
pub trait_ref: ty::TraitRef<'tcx>,
pub self_ty: Option<Ty<'tcx>>,
pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
pub involves_placeholder: bool,
pub overflowing_predicates: Vec<ty::Predicate<'tcx>>,
}
/// Given the generic parameters for the requested impl, translate it to the generic parameters
/// 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_args` is its generic parameters.
/// And `target_node` is the impl/trait we're actually going to get the
/// definition from. The resulting instantiation will map from `target_node`'s
/// generics to `source_impl`'s generics as instantiated by `source_args`.
///
/// For example, consider the following scenario:
///
/// ```ignore (illustrative)
/// 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 an instantiation with `T` and `U` both mapping to `u32`.
///
/// where-clauses add some trickiness here, because they can be used to "define"
/// an argument indirectly:
///
/// ```ignore (illustrative)
/// impl<'a, I, T: 'a> Iterator for Cloned<I>
/// where I: Iterator<Item = &'a T>, T: Clone
/// ```
///
/// In a case like this, the instantiation 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_args<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
source_impl: DefId,
source_args: GenericArgsRef<'tcx>,
target_node: specialization_graph::Node,
) -> GenericArgsRef<'tcx> {
translate_args_with_cause(infcx, param_env, source_impl, source_args, target_node, |_, _| {
ObligationCause::dummy()
})
}
/// Like [translate_args], but obligations from the parent implementation
/// are registered with the provided `ObligationCause`.
///
/// This is for reporting *region* errors from those bounds. Type errors should
/// not happen because the specialization graph already checks for those, and
/// will result in an ICE.
pub fn translate_args_with_cause<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
source_impl: DefId,
source_args: GenericArgsRef<'tcx>,
target_node: specialization_graph::Node,
cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
) -> GenericArgsRef<'tcx> {
debug!(
"translate_args({:?}, {:?}, {:?}, {:?})",
param_env, source_impl, source_args, target_node
);
let source_trait_ref =
infcx.tcx.impl_trait_ref(source_impl).unwrap().instantiate(infcx.tcx, source_args);
// translate the Self and Param parts of the generic parameters, since those
// vary across impls
let target_args = match target_node {
specialization_graph::Node::Impl(target_impl) => {
// no need to translate if we're targeting the impl we started with
if source_impl == target_impl {
return source_args;
}
fulfill_implication(infcx, param_env, source_trait_ref, source_impl, target_impl, cause)
.unwrap_or_else(|()| {
bug!(
"When translating generic parameters from {source_impl:?} to \
{target_impl:?}, the expected specialization failed to hold"
)
})
}
specialization_graph::Node::Trait(..) => source_trait_ref.args,
};
// directly inherent the method generics, since those do not vary across impls
source_args.rebase_onto(infcx.tcx, source_impl, target_args)
}
pub(super) fn specialization_enabled_in(tcx: TyCtxt<'_>, _: LocalCrate) -> bool {
tcx.features().specialization() || tcx.features().min_specialization()
}
/// 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.
#[instrument(skip(tcx), level = "debug")]
pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId, DefId)) -> bool {
// We check that the specializing impl comes from a crate that has specialization enabled,
// or if the specializing impl is marked with `allow_internal_unstable`.
//
// We don't really care if the specialized impl (the parent) is in a crate that has
// specialization enabled, since it's not being specialized, and it's already been checked
// for coherence.
if !tcx.specialization_enabled_in(impl1_def_id.krate) {
let span = tcx.def_span(impl1_def_id);
if !span.allows_unstable(sym::specialization)
&& !span.allows_unstable(sym::min_specialization)
{
return false;
}
}
let impl1_trait_header = tcx.impl_trait_header(impl1_def_id).unwrap();
// We determine whether there's a subset relationship by:
//
// - replacing bound vars with placeholders in 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 impl1_trait_header.polarity != tcx.impl_polarity(impl2_def_id) {
return false;
}
// create a parameter environment corresponding to a (placeholder) instantiation of impl1
let penv = tcx.param_env(impl1_def_id);
// Create an infcx, taking the predicates of impl1 as assumptions:
let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
// Attempt to prove that impl2 applies, given all of the above.
fulfill_implication(
&infcx,
penv,
impl1_trait_header.trait_ref.instantiate_identity(),
impl1_def_id,
impl2_def_id,
|_, _| ObligationCause::dummy(),
)
.is_ok()
}
/// Attempt to fulfill all obligations of `target_impl` after unification with
/// `source_trait_ref`. If successful, returns the generic parameters 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<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
source_trait_ref: ty::TraitRef<'tcx>,
source_impl: DefId,
target_impl: DefId,
error_cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
) -> Result<GenericArgsRef<'tcx>, ()> {
debug!(
"fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
param_env, source_trait_ref, target_impl
);
let ocx = ObligationCtxt::new(infcx);
let source_trait_ref = ocx.normalize(&ObligationCause::dummy(), param_env, source_trait_ref);
if !ocx.select_all_or_error().is_empty() {
infcx.dcx().span_delayed_bug(
infcx.tcx.def_span(source_impl),
format!("failed to fully normalize {source_trait_ref}"),
);
}
let source_trait_ref = infcx.resolve_vars_if_possible(source_trait_ref);
let source_trait = ImplSubject::Trait(source_trait_ref);
let selcx = SelectionContext::new(infcx);
let target_args = infcx.fresh_args_for_item(DUMMY_SP, target_impl);
let (target_trait, obligations) =
util::impl_subject_and_oblig(&selcx, param_env, target_impl, target_args, error_cause);
// do the impls unify? If not, no specialization.
let Ok(InferOk { obligations: more_obligations, .. }) = infcx
.at(&ObligationCause::dummy(), param_env)
// Ok to use `Yes`, as all the generic params are already replaced by inference variables,
// which will match the opaque type no matter if it is defining or not.
// Any concrete type that would match the opaque would already be handled by coherence rules,
// and thus either be ok to match here and already have errored, or it won't match, in which
// case there is no issue anyway.
.eq(DefineOpaqueTypes::Yes, source_trait, target_trait)
else {
debug!("fulfill_implication: {:?} does not unify with {:?}", source_trait, target_trait);
return Err(());
};
// attempt to prove all of the predicates for impl2 given those for impl1
// (which are packed up in penv)
ocx.register_obligations(obligations.chain(more_obligations));
let errors = ocx.select_all_or_error();
if !errors.is_empty() {
// no dice!
debug!(
"fulfill_implication: for impls on {:?} and {:?}, \
could not fulfill: {:?} given {:?}",
source_trait,
target_trait,
errors,
param_env.caller_bounds()
);
return Err(());
}
debug!("fulfill_implication: an impl for {:?} specializes {:?}", source_trait, target_trait);
// Now resolve the *generic parameters* we built for the target earlier, replacing
// the inference variables inside with whatever we got from fulfillment.
Ok(infcx.resolve_vars_if_possible(target_args))
}
/// Query provider for `specialization_graph_of`.
pub(super) fn specialization_graph_provider(
tcx: TyCtxt<'_>,
trait_id: DefId,
) -> Result<&'_ specialization_graph::Graph, ErrorGuaranteed> {
let mut sg = specialization_graph::Graph::new();
let overlap_mode = specialization_graph::OverlapMode::get(tcx, trait_id);
let mut trait_impls: Vec<_> = tcx.all_impls(trait_id).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 flattened version of the `DefIndex`.
trait_impls
.sort_unstable_by_key(|def_id| (-(def_id.krate.as_u32() as i64), def_id.index.index()));
let mut errored = Ok(());
for impl_def_id in trait_impls {
if let Some(impl_def_id) = impl_def_id.as_local() {
// This is where impl overlap checking happens:
let insert_result = sg.insert(tcx, impl_def_id.to_def_id(), overlap_mode);
// Report error if there was one.
let (overlap, used_to_be_allowed) = match insert_result {
Err(overlap) => (Some(overlap), None),
Ok(Some(overlap)) => (Some(overlap.error), Some(overlap.kind)),
Ok(None) => (None, None),
};
if let Some(overlap) = overlap {
errored = errored.and(report_overlap_conflict(
tcx,
overlap,
impl_def_id,
used_to_be_allowed,
));
}
} else {
let parent = tcx.impl_parent(impl_def_id).unwrap_or(trait_id);
sg.record_impl_from_cstore(tcx, parent, impl_def_id)
}
}
errored?;
Ok(tcx.arena.alloc(sg))
}
// This function is only used when
// encountering errors and inlining
// it negatively impacts perf.
#[cold]
#[inline(never)]
fn report_overlap_conflict<'tcx>(
tcx: TyCtxt<'tcx>,
overlap: OverlapError<'tcx>,
impl_def_id: LocalDefId,
used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
) -> Result<(), ErrorGuaranteed> {
let impl_polarity = tcx.impl_polarity(impl_def_id.to_def_id());
let other_polarity = tcx.impl_polarity(overlap.with_impl);
match (impl_polarity, other_polarity) {
(ty::ImplPolarity::Negative, ty::ImplPolarity::Positive) => {
Err(report_negative_positive_conflict(
tcx,
&overlap,
impl_def_id,
impl_def_id.to_def_id(),
overlap.with_impl,
))
}
(ty::ImplPolarity::Positive, ty::ImplPolarity::Negative) => {
Err(report_negative_positive_conflict(
tcx,
&overlap,
impl_def_id,
overlap.with_impl,
impl_def_id.to_def_id(),
))
}
_ => report_conflicting_impls(tcx, overlap, impl_def_id, used_to_be_allowed),
}
}
fn report_negative_positive_conflict<'tcx>(
tcx: TyCtxt<'tcx>,
overlap: &OverlapError<'tcx>,
local_impl_def_id: LocalDefId,
negative_impl_def_id: DefId,
positive_impl_def_id: DefId,
) -> ErrorGuaranteed {
tcx.dcx()
.create_err(NegativePositiveConflict {
impl_span: tcx.def_span(local_impl_def_id),
trait_desc: overlap.trait_ref,
self_ty: overlap.self_ty,
negative_impl_span: tcx.span_of_impl(negative_impl_def_id),
positive_impl_span: tcx.span_of_impl(positive_impl_def_id),
})
.emit()
}
fn report_conflicting_impls<'tcx>(
tcx: TyCtxt<'tcx>,
overlap: OverlapError<'tcx>,
impl_def_id: LocalDefId,
used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
) -> Result<(), ErrorGuaranteed> {
let impl_span = tcx.def_span(impl_def_id);
// Work to be done after we've built the Diag. We have to define it now
// because the lint emit methods don't return back the Diag that's passed
// in.
fn decorate<'tcx, G: EmissionGuarantee>(
tcx: TyCtxt<'tcx>,
overlap: &OverlapError<'tcx>,
impl_span: Span,
err: &mut Diag<'_, G>,
) {
match tcx.span_of_impl(overlap.with_impl) {
Ok(span) => {
err.span_label(span, "first implementation here");
err.span_label(
impl_span,
format!(
"conflicting implementation{}",
overlap.self_ty.map_or_else(String::new, |ty| format!(" for `{ty}`"))
),
);
}
Err(cname) => {
let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
Some(s) => {
format!("conflicting implementation in crate `{cname}`:\n- {s}")
}
None => format!("conflicting implementation in crate `{cname}`"),
};
err.note(msg);
}
}
for cause in &overlap.intercrate_ambiguity_causes {
cause.add_intercrate_ambiguity_hint(err);
}
if overlap.involves_placeholder {
coherence::add_placeholder_note(err);
}
if !overlap.overflowing_predicates.is_empty() {
coherence::suggest_increasing_recursion_limit(
tcx,
err,
&overlap.overflowing_predicates,
);
}
}
let msg = || {
format!(
"conflicting implementations of trait `{}`{}{}",
overlap.trait_ref.print_trait_sugared(),
overlap.self_ty.map_or_else(String::new, |ty| format!(" for type `{ty}`")),
match used_to_be_allowed {
Some(FutureCompatOverlapErrorKind::OrderDepTraitObjects) => ": (E0119)",
_ => "",
}
)
};
// Don't report overlap errors if the header references error
if let Err(err) = (overlap.trait_ref, overlap.self_ty).error_reported() {
return Err(err);
}
match used_to_be_allowed {
None => {
let reported = if overlap.with_impl.is_local()
|| tcx.ensure().orphan_check_impl(impl_def_id).is_ok()
{
let mut err = tcx.dcx().struct_span_err(impl_span, msg());
err.code(E0119);
decorate(tcx, &overlap, impl_span, &mut err);
err.emit()
} else {
tcx.dcx().span_delayed_bug(impl_span, "impl should have failed the orphan check")
};
Err(reported)
}
Some(kind) => {
let lint = match kind {
FutureCompatOverlapErrorKind::OrderDepTraitObjects => ORDER_DEPENDENT_TRAIT_OBJECTS,
FutureCompatOverlapErrorKind::LeakCheck => COHERENCE_LEAK_CHECK,
};
tcx.node_span_lint(lint, tcx.local_def_id_to_hir_id(impl_def_id), impl_span, |err| {
err.primary_message(msg());
decorate(tcx, &overlap, impl_span, err);
});
Ok(())
}
}
}