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//! # Minimal Specialization
//!
//! This module contains the checks for sound specialization used when the
//! `min_specialization` feature is enabled. This requires that the impl is
//! *always applicable*.
//!
//! If `impl1` specializes `impl2` then `impl1` is always applicable if we know
//! that all the bounds of `impl2` are satisfied, and all of the bounds of
//! `impl1` are satisfied for some choice of lifetimes then we know that
//! `impl1` applies for any choice of lifetimes.
//!
//! ## Basic approach
//!
//! To enforce this requirement on specializations we take the following
//! approach:
//!
//! 1. Match up the args for `impl2` so that the implemented trait and
//! self-type match those for `impl1`.
//! 2. Check for any direct use of `'static` in the args of `impl2`.
//! 3. Check that all of the generic parameters of `impl1` occur at most once
//! in the *unconstrained* args for `impl2`. A parameter is constrained if
//! its value is completely determined by an associated type projection
//! predicate.
//! 4. Check that all predicates on `impl1` either exist on `impl2` (after
//! matching args), or are well-formed predicates for the trait's type
//! arguments.
//!
//! ## Example
//!
//! Suppose we have the following always applicable impl:
//!
//! ```ignore (illustrative)
//! impl<T> SpecExtend<T> for std::vec::IntoIter<T> { /* specialized impl */ }
//! impl<T, I: Iterator<Item=T>> SpecExtend<T> for I { /* default impl */ }
//! ```
//!
//! We get that the subst for `impl2` are `[T, std::vec::IntoIter<T>]`. `T` is
//! constrained to be `<I as Iterator>::Item`, so we check only
//! `std::vec::IntoIter<T>` for repeated parameters, which it doesn't have. The
//! predicates of `impl1` are only `T: Sized`, which is also a predicate of
//! `impl2`. So this specialization is sound.
//!
//! ## Extensions
//!
//! Unfortunately not all specializations in the standard library are allowed
//! by this. So there are two extensions to these rules that allow specializing
//! on some traits: that is, using them as bounds on the specializing impl,
//! even when they don't occur in the base impl.
//!
//! ### rustc_specialization_trait
//!
//! If a trait is always applicable, then it's sound to specialize on it. We
//! check trait is always applicable in the same way as impls, except that step
//! 4 is now "all predicates on `impl1` are always applicable". We require that
//! `specialization` or `min_specialization` is enabled to implement these
//! traits.
//!
//! ### rustc_unsafe_specialization_marker
//!
//! There are also some specialization on traits with no methods, including the
//! stable `FusedIterator` trait. We allow marking marker traits with an
//! unstable attribute that means we ignore them in point 3 of the checks
//! above. This is unsound, in the sense that the specialized impl may be used
//! when it doesn't apply, but we allow it in the short term since it can't
//! cause use after frees with purely safe code in the same way as specializing
//! on traits with methods can.
use crate::errors::SubstsOnOverriddenImpl;
use crate::{constrained_generic_params as cgp, errors};
use rustc_data_structures::fx::FxHashSet;
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_infer::infer::outlives::env::OutlivesEnvironment;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::specialization_graph::Node;
use rustc_middle::ty::trait_def::TraitSpecializationKind;
use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
use rustc_middle::ty::{GenericArg, GenericArgs, GenericArgsRef};
use rustc_span::{ErrorGuaranteed, Span};
use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
use rustc_trait_selection::traits::{self, translate_args_with_cause, wf, ObligationCtxt};
pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
if let Some(node) = parent_specialization_node(tcx, impl_def_id) {
check_always_applicable(tcx, impl_def_id, node);
}
}
fn parent_specialization_node(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId) -> Option<Node> {
let trait_ref = tcx.impl_trait_ref(impl1_def_id)?;
let trait_def = tcx.trait_def(trait_ref.skip_binder().def_id);
let impl2_node = trait_def.ancestors(tcx, impl1_def_id.to_def_id()).ok()?.nth(1)?;
let always_applicable_trait =
matches!(trait_def.specialization_kind, TraitSpecializationKind::AlwaysApplicable);
if impl2_node.is_from_trait() && !always_applicable_trait {
// Implementing a normal trait isn't a specialization.
return None;
}
if trait_def.is_marker {
// Overlapping marker implementations are not really specializations.
return None;
}
Some(impl2_node)
}
/// Check that `impl1` is a sound specialization
#[instrument(level = "debug", skip(tcx))]
fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node) {
let span = tcx.def_span(impl1_def_id);
check_has_items(tcx, impl1_def_id, impl2_node, span);
if let Ok((impl1_args, impl2_args)) = get_impl_args(tcx, impl1_def_id, impl2_node) {
let impl2_def_id = impl2_node.def_id();
debug!(?impl2_def_id, ?impl2_args);
let parent_args = if impl2_node.is_from_trait() {
impl2_args.to_vec()
} else {
unconstrained_parent_impl_args(tcx, impl2_def_id, impl2_args)
};
check_constness(tcx, impl1_def_id, impl2_node, span);
check_static_lifetimes(tcx, &parent_args, span);
check_duplicate_params(tcx, impl1_args, &parent_args, span);
check_predicates(tcx, impl1_def_id, impl1_args, impl2_node, impl2_args, span);
}
}
fn check_has_items(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node, span: Span) {
if let Node::Impl(impl2_id) = impl2_node && tcx.associated_item_def_ids(impl1_def_id).is_empty() {
let base_impl_span = tcx.def_span(impl2_id);
tcx.sess.emit_err(errors::EmptySpecialization { span, base_impl_span });
}
}
/// Check that the specializing impl `impl1` is at least as const as the base
/// impl `impl2`
fn check_constness(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node, span: Span) {
if impl2_node.is_from_trait() {
// This isn't a specialization
return;
}
let impl1_constness = tcx.constness(impl1_def_id.to_def_id());
let impl2_constness = tcx.constness(impl2_node.def_id());
if let hir::Constness::Const = impl2_constness {
if let hir::Constness::NotConst = impl1_constness {
tcx.sess.emit_err(errors::ConstSpecialize { span });
}
}
}
/// Given a specializing impl `impl1`, and the base impl `impl2`, returns two
/// substitutions `(S1, S2)` that equate their trait references. The returned
/// types are expressed in terms of the generics of `impl1`.
///
/// Example
///
/// ```ignore (illustrative)
/// impl<A, B> Foo<A> for B { /* impl2 */ }
/// impl<C> Foo<Vec<C>> for C { /* impl1 */ }
/// ```
///
/// Would return `S1 = [C]` and `S2 = [Vec<C>, C]`.
fn get_impl_args(
tcx: TyCtxt<'_>,
impl1_def_id: LocalDefId,
impl2_node: Node,
) -> Result<(GenericArgsRef<'_>, GenericArgsRef<'_>), ErrorGuaranteed> {
let infcx = &tcx.infer_ctxt().build();
let ocx = ObligationCtxt::new(infcx);
let param_env = tcx.param_env(impl1_def_id);
let impl1_span = tcx.def_span(impl1_def_id);
let assumed_wf_types = ocx.assumed_wf_types_and_report_errors(param_env, impl1_def_id)?;
let impl1_args = GenericArgs::identity_for_item(tcx, impl1_def_id);
let impl2_args = translate_args_with_cause(
infcx,
param_env,
impl1_def_id.to_def_id(),
impl1_args,
impl2_node,
|_, span| {
traits::ObligationCause::new(
impl1_span,
impl1_def_id,
traits::ObligationCauseCode::BindingObligation(impl2_node.def_id(), span),
)
},
);
let errors = ocx.select_all_or_error();
if !errors.is_empty() {
let guar = ocx.infcx.err_ctxt().report_fulfillment_errors(&errors);
return Err(guar);
}
let implied_bounds = infcx.implied_bounds_tys(param_env, impl1_def_id, assumed_wf_types);
let outlives_env = OutlivesEnvironment::with_bounds(param_env, implied_bounds);
let _ = ocx.resolve_regions_and_report_errors(impl1_def_id, &outlives_env);
let Ok(impl2_args) = infcx.fully_resolve(impl2_args) else {
let span = tcx.def_span(impl1_def_id);
let guar = tcx.sess.emit_err(SubstsOnOverriddenImpl { span });
return Err(guar);
};
Ok((impl1_args, impl2_args))
}
/// Returns a list of all of the unconstrained subst of the given impl.
///
/// For example given the impl:
///
/// impl<'a, T, I> ... where &'a I: IntoIterator<Item=&'a T>
///
/// This would return the args corresponding to `['a, I]`, because knowing
/// `'a` and `I` determines the value of `T`.
fn unconstrained_parent_impl_args<'tcx>(
tcx: TyCtxt<'tcx>,
impl_def_id: DefId,
impl_args: GenericArgsRef<'tcx>,
) -> Vec<GenericArg<'tcx>> {
let impl_generic_predicates = tcx.predicates_of(impl_def_id);
let mut unconstrained_parameters = FxHashSet::default();
let mut constrained_params = FxHashSet::default();
let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).map(ty::EarlyBinder::instantiate_identity);
// Unfortunately the functions in `constrained_generic_parameters` don't do
// what we want here. We want only a list of constrained parameters while
// the functions in `cgp` add the constrained parameters to a list of
// unconstrained parameters.
for (clause, _) in impl_generic_predicates.predicates.iter() {
if let ty::ClauseKind::Projection(proj) = clause.kind().skip_binder() {
let projection_ty = proj.projection_ty;
let projected_ty = proj.term;
let unbound_trait_ref = projection_ty.trait_ref(tcx);
if Some(unbound_trait_ref) == impl_trait_ref {
continue;
}
unconstrained_parameters.extend(cgp::parameters_for(&projection_ty, true));
for param in cgp::parameters_for(&projected_ty, false) {
if !unconstrained_parameters.contains(¶m) {
constrained_params.insert(param.0);
}
}
unconstrained_parameters.extend(cgp::parameters_for(&projected_ty, true));
}
}
impl_args
.iter()
.enumerate()
.filter(|&(idx, _)| !constrained_params.contains(&(idx as u32)))
.map(|(_, arg)| arg)
.collect()
}
/// Check that parameters of the derived impl don't occur more than once in the
/// equated args of the base impl.
///
/// For example forbid the following:
///
/// ```ignore (illustrative)
/// impl<A> Tr for A { }
/// impl<B> Tr for (B, B) { }
/// ```
///
/// Note that only consider the unconstrained parameters of the base impl:
///
/// ```ignore (illustrative)
/// impl<S, I: IntoIterator<Item = S>> Tr<S> for I { }
/// impl<T> Tr<T> for Vec<T> { }
/// ```
///
/// The args for the parent impl here are `[T, Vec<T>]`, which repeats `T`,
/// but `S` is constrained in the parent impl, so `parent_args` is only
/// `[Vec<T>]`. This means we allow this impl.
fn check_duplicate_params<'tcx>(
tcx: TyCtxt<'tcx>,
impl1_args: GenericArgsRef<'tcx>,
parent_args: &Vec<GenericArg<'tcx>>,
span: Span,
) {
let mut base_params = cgp::parameters_for(parent_args, true);
base_params.sort_by_key(|param| param.0);
if let (_, [duplicate, ..]) = base_params.partition_dedup() {
let param = impl1_args[duplicate.0 as usize];
tcx.sess
.struct_span_err(span, format!("specializing impl repeats parameter `{param}`"))
.emit();
}
}
/// Check that `'static` lifetimes are not introduced by the specializing impl.
///
/// For example forbid the following:
///
/// ```ignore (illustrative)
/// impl<A> Tr for A { }
/// impl Tr for &'static i32 { }
/// ```
fn check_static_lifetimes<'tcx>(
tcx: TyCtxt<'tcx>,
parent_args: &Vec<GenericArg<'tcx>>,
span: Span,
) {
if tcx.any_free_region_meets(parent_args, |r| r.is_static()) {
tcx.sess.emit_err(errors::StaticSpecialize { span });
}
}
/// Check whether predicates on the specializing impl (`impl1`) are allowed.
///
/// Each predicate `P` must be one of:
///
/// * Global (not reference any parameters).
/// * A `T: Tr` predicate where `Tr` is an always-applicable trait.
/// * Present on the base impl `impl2`.
/// * This check is done using the `trait_predicates_eq` function below.
/// * A well-formed predicate of a type argument of the trait being implemented,
/// including the `Self`-type.
#[instrument(level = "debug", skip(tcx))]
fn check_predicates<'tcx>(
tcx: TyCtxt<'tcx>,
impl1_def_id: LocalDefId,
impl1_args: GenericArgsRef<'tcx>,
impl2_node: Node,
impl2_args: GenericArgsRef<'tcx>,
span: Span,
) {
let impl1_predicates: Vec<_> = traits::elaborate(
tcx,
tcx.predicates_of(impl1_def_id).instantiate(tcx, impl1_args).into_iter(),
)
.collect();
let mut impl2_predicates = if impl2_node.is_from_trait() {
// Always applicable traits have to be always applicable without any
// assumptions.
Vec::new()
} else {
traits::elaborate(
tcx,
tcx.predicates_of(impl2_node.def_id())
.instantiate(tcx, impl2_args)
.into_iter()
.map(|(c, _s)| c.as_predicate()),
)
.collect()
};
debug!(?impl1_predicates, ?impl2_predicates);
// Since impls of always applicable traits don't get to assume anything, we
// can also assume their supertraits apply.
//
// For example, we allow:
//
// #[rustc_specialization_trait]
// trait AlwaysApplicable: Debug { }
//
// impl<T> Tr for T { }
// impl<T: AlwaysApplicable> Tr for T { }
//
// Specializing on `AlwaysApplicable` allows also specializing on `Debug`
// which is sound because we forbid impls like the following
//
// impl<D: Debug> AlwaysApplicable for D { }
let always_applicable_traits = impl1_predicates
.iter()
.copied()
.filter(|&(clause, _span)| {
matches!(
trait_specialization_kind(tcx, clause),
Some(TraitSpecializationKind::AlwaysApplicable)
)
})
.map(|(c, _span)| c.as_predicate());
// Include the well-formed predicates of the type parameters of the impl.
for arg in tcx.impl_trait_ref(impl1_def_id).unwrap().instantiate_identity().args {
let infcx = &tcx.infer_ctxt().build();
let obligations =
wf::obligations(infcx, tcx.param_env(impl1_def_id), impl1_def_id, 0, arg, span)
.unwrap();
assert!(!obligations.has_infer());
impl2_predicates
.extend(traits::elaborate(tcx, obligations).map(|obligation| obligation.predicate))
}
impl2_predicates.extend(traits::elaborate(tcx, always_applicable_traits));
for (clause, span) in impl1_predicates {
if !impl2_predicates
.iter()
.any(|pred2| trait_predicates_eq(tcx, clause.as_predicate(), *pred2, span))
{
check_specialization_on(tcx, clause, span)
}
}
}
/// Checks if some predicate on the specializing impl (`predicate1`) is the same
/// as some predicate on the base impl (`predicate2`).
///
/// This basically just checks syntactic equivalence, but is a little more
/// forgiving since we want to equate `T: Tr` with `T: ~const Tr` so this can work:
///
/// ```ignore (illustrative)
/// #[rustc_specialization_trait]
/// trait Specialize { }
///
/// impl<T: Bound> Tr for T { }
/// impl<T: ~const Bound + Specialize> const Tr for T { }
/// ```
///
/// However, we *don't* want to allow the reverse, i.e., when the bound on the
/// specializing impl is not as const as the bound on the base impl:
///
/// ```ignore (illustrative)
/// impl<T: ~const Bound> const Tr for T { }
/// impl<T: Bound + Specialize> const Tr for T { } // should be T: ~const Bound
/// ```
///
/// So we make that check in this function and try to raise a helpful error message.
fn trait_predicates_eq<'tcx>(
_tcx: TyCtxt<'tcx>,
predicate1: ty::Predicate<'tcx>,
predicate2: ty::Predicate<'tcx>,
_span: Span,
) -> bool {
// FIXME(effects)
predicate1 == predicate2
}
#[instrument(level = "debug", skip(tcx))]
fn check_specialization_on<'tcx>(tcx: TyCtxt<'tcx>, clause: ty::Clause<'tcx>, span: Span) {
match clause.kind().skip_binder() {
// Global predicates are either always true or always false, so we
// are fine to specialize on.
_ if clause.is_global() => (),
// We allow specializing on explicitly marked traits with no associated
// items.
ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity: _ }) => {
if !matches!(
trait_specialization_kind(tcx, clause),
Some(TraitSpecializationKind::Marker)
) {
tcx.sess
.struct_span_err(
span,
format!(
"cannot specialize on trait `{}`",
tcx.def_path_str(trait_ref.def_id),
),
)
.emit();
}
}
ty::ClauseKind::Projection(ty::ProjectionPredicate { projection_ty, term }) => {
tcx.sess
.struct_span_err(
span,
format!("cannot specialize on associated type `{projection_ty} == {term}`",),
)
.emit();
}
ty::ClauseKind::ConstArgHasType(..) => {
// FIXME(min_specialization), FIXME(const_generics):
// It probably isn't right to allow _every_ `ConstArgHasType` but I am somewhat unsure
// about the actual rules that would be sound. Can't just always error here because otherwise
// std/core doesn't even compile as they have `const N: usize` in some specializing impls.
//
// While we do not support constructs like `<T, const N: T>` there is probably no risk of
// soundness bugs, but when we support generic const parameter types this will need to be
// revisited.
}
_ => {
tcx.sess
.struct_span_err(span, format!("cannot specialize on predicate `{clause}`"))
.emit();
}
}
}
fn trait_specialization_kind<'tcx>(
tcx: TyCtxt<'tcx>,
clause: ty::Clause<'tcx>,
) -> Option<TraitSpecializationKind> {
match clause.kind().skip_binder() {
ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity: _ }) => {
Some(tcx.trait_def(trait_ref.def_id).specialization_kind)
}
ty::ClauseKind::RegionOutlives(_)
| ty::ClauseKind::TypeOutlives(_)
| ty::ClauseKind::Projection(_)
| ty::ClauseKind::ConstArgHasType(..)
| ty::ClauseKind::WellFormed(_)
| ty::ClauseKind::ConstEvaluatable(..) => None,
}
}