729 lines
30 KiB
Rust
729 lines
30 KiB
Rust
//! "Object safety" refers to the ability for a trait to be converted
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//! to an object. In general, traits may only be converted to an
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//! object if all of their methods meet certain criteria. In particular,
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//! they must:
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//!
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//! - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version
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//! that doesn't contain the vtable;
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//! - not reference the erased type `Self` except for in this receiver;
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//! - not have generic type parameters.
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use super::elaborate_predicates;
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use crate::hir;
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use crate::hir::def_id::DefId;
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use crate::lint;
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use crate::traits::{self, Obligation, ObligationCause};
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use crate::ty::{self, Ty, TyCtxt, TypeFoldable, Predicate, ToPredicate};
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use crate::ty::subst::{Subst, InternalSubsts};
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use std::borrow::Cow;
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use std::iter::{self};
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use syntax::ast::{self};
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use syntax::symbol::InternedString;
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use syntax_pos::{Span, DUMMY_SP};
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#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
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pub enum ObjectSafetyViolation {
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/// `Self: Sized` declared on the trait.
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SizedSelf,
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/// Supertrait reference references `Self` an in illegal location
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/// (e.g., `trait Foo : Bar<Self>`).
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SupertraitSelf,
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/// Method has something illegal.
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Method(ast::Name, MethodViolationCode, Span),
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/// Associated const.
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AssocConst(ast::Name, Span),
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}
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impl ObjectSafetyViolation {
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pub fn error_msg(&self) -> Cow<'static, str> {
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match *self {
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ObjectSafetyViolation::SizedSelf =>
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"the trait cannot require that `Self : Sized`".into(),
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ObjectSafetyViolation::SupertraitSelf =>
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"the trait cannot use `Self` as a type parameter \
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in the supertraits or where-clauses".into(),
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ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod, _) =>
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format!("associated function `{}` has no `self` parameter", name).into(),
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ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelf, _) => format!(
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"method `{}` references the `Self` type in its parameters or return type",
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name,
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).into(),
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ObjectSafetyViolation::Method(
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name,
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MethodViolationCode::WhereClauseReferencesSelf,
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_,
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) => format!("method `{}` references the `Self` type in where clauses", name).into(),
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ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) =>
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format!("method `{}` has generic type parameters", name).into(),
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ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) =>
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format!("method `{}`'s `self` parameter cannot be dispatched on", name).into(),
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ObjectSafetyViolation::AssocConst(name, _) =>
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format!("the trait cannot contain associated consts like `{}`", name).into(),
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}
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}
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pub fn span(&self) -> Option<Span> {
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// When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
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// diagnostics use a `note` instead of a `span_label`.
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match *self {
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ObjectSafetyViolation::AssocConst(_, span) |
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ObjectSafetyViolation::Method(_, _, span) if span != DUMMY_SP => Some(span),
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_ => None,
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}
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}
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}
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/// Reasons a method might not be object-safe.
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#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
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pub enum MethodViolationCode {
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/// e.g., `fn foo()`
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StaticMethod,
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/// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self`
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ReferencesSelf,
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/// e.g., `fn foo(&self) where Self: Clone`
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WhereClauseReferencesSelf,
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/// e.g., `fn foo<A>()`
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Generic,
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/// the method's receiver (`self` argument) can't be dispatched on
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UndispatchableReceiver,
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}
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impl<'tcx> TyCtxt<'tcx> {
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/// Returns the object safety violations that affect
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/// astconv -- currently, `Self` in supertraits. This is needed
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/// because `object_safety_violations` can't be used during
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/// type collection.
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pub fn astconv_object_safety_violations(
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self,
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trait_def_id: DefId,
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) -> Vec<ObjectSafetyViolation> {
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debug_assert!(self.generics_of(trait_def_id).has_self);
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let violations = traits::supertrait_def_ids(self, trait_def_id)
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.filter(|&def_id| self.predicates_reference_self(def_id, true))
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.map(|_| ObjectSafetyViolation::SupertraitSelf)
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.collect();
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debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}",
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trait_def_id,
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violations);
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violations
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}
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pub fn object_safety_violations(self, trait_def_id: DefId)
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-> Vec<ObjectSafetyViolation>
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{
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debug_assert!(self.generics_of(trait_def_id).has_self);
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debug!("object_safety_violations: {:?}", trait_def_id);
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traits::supertrait_def_ids(self, trait_def_id)
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.flat_map(|def_id| self.object_safety_violations_for_trait(def_id))
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.collect()
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}
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/// We say a method is *vtable safe* if it can be invoked on a trait
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/// object. Note that object-safe traits can have some
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/// non-vtable-safe methods, so long as they require `Self: Sized` or
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/// otherwise ensure that they cannot be used when `Self = Trait`.
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pub fn is_vtable_safe_method(self, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
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debug_assert!(self.generics_of(trait_def_id).has_self);
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debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
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// Any method that has a `Self: Sized` bound cannot be called.
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if self.generics_require_sized_self(method.def_id) {
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return false;
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}
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match self.virtual_call_violation_for_method(trait_def_id, method) {
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None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
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Some(_) => false,
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}
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}
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fn object_safety_violations_for_trait(self, trait_def_id: DefId) -> Vec<ObjectSafetyViolation> {
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// Check methods for violations.
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let mut violations: Vec<_> = self.associated_items(trait_def_id)
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.filter(|item| item.kind == ty::AssocKind::Method)
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.filter_map(|item|
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self.object_safety_violation_for_method(trait_def_id, &item).map(|code| {
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ObjectSafetyViolation::Method(item.ident.name, code, item.ident.span)
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})
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).filter(|violation| {
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if let ObjectSafetyViolation::Method(
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_,
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MethodViolationCode::WhereClauseReferencesSelf,
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span,
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) = violation {
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// Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
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// It's also hard to get a use site span, so we use the method definition span.
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self.lint_node_note(
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lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY,
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hir::CRATE_HIR_ID,
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*span,
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&format!("the trait `{}` cannot be made into an object",
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self.def_path_str(trait_def_id)),
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&violation.error_msg());
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false
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} else {
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true
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}
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}).collect();
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// Check the trait itself.
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if self.trait_has_sized_self(trait_def_id) {
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violations.push(ObjectSafetyViolation::SizedSelf);
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}
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if self.predicates_reference_self(trait_def_id, false) {
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violations.push(ObjectSafetyViolation::SupertraitSelf);
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}
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violations.extend(self.associated_items(trait_def_id)
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.filter(|item| item.kind == ty::AssocKind::Const)
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.map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)));
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debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
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trait_def_id,
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violations);
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violations
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}
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fn predicates_reference_self(
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self,
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trait_def_id: DefId,
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supertraits_only: bool,
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) -> bool {
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let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(self, trait_def_id));
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let predicates = if supertraits_only {
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self.super_predicates_of(trait_def_id)
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} else {
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self.predicates_of(trait_def_id)
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};
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let self_ty = self.types.self_param;
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let has_self_ty = |t: Ty<'tcx>| t.walk().any(|t| t == self_ty);
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predicates
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.predicates
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.iter()
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.map(|(predicate, _)| predicate.subst_supertrait(self, &trait_ref))
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.any(|predicate| {
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match predicate {
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ty::Predicate::Trait(ref data) => {
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// In the case of a trait predicate, we can skip the "self" type.
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data.skip_binder().input_types().skip(1).any(has_self_ty)
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}
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ty::Predicate::Projection(ref data) => {
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// And similarly for projections. This should be redundant with
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// the previous check because any projection should have a
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// matching `Trait` predicate with the same inputs, but we do
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// the check to be safe.
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//
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// Note that we *do* allow projection *outputs* to contain
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// `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
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// we just require the user to specify *both* outputs
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// in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
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//
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// This is ALT2 in issue #56288, see that for discussion of the
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// possible alternatives.
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data.skip_binder()
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.projection_ty
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.trait_ref(self)
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.input_types()
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.skip(1)
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.any(has_self_ty)
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}
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ty::Predicate::WellFormed(..) |
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ty::Predicate::ObjectSafe(..) |
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ty::Predicate::TypeOutlives(..) |
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ty::Predicate::RegionOutlives(..) |
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ty::Predicate::ClosureKind(..) |
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ty::Predicate::Subtype(..) |
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ty::Predicate::ConstEvaluatable(..) => {
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false
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}
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}
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})
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}
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fn trait_has_sized_self(self, trait_def_id: DefId) -> bool {
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self.generics_require_sized_self(trait_def_id)
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}
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fn generics_require_sized_self(self, def_id: DefId) -> bool {
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let sized_def_id = match self.lang_items().sized_trait() {
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Some(def_id) => def_id,
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None => { return false; /* No Sized trait, can't require it! */ }
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};
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// Search for a predicate like `Self : Sized` amongst the trait bounds.
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let predicates = self.predicates_of(def_id);
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let predicates = predicates.instantiate_identity(self).predicates;
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elaborate_predicates(self, predicates)
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.any(|predicate| match predicate {
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ty::Predicate::Trait(ref trait_pred) => {
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trait_pred.def_id() == sized_def_id
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&& trait_pred.skip_binder().self_ty().is_param(0)
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}
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ty::Predicate::Projection(..) |
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ty::Predicate::Subtype(..) |
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ty::Predicate::RegionOutlives(..) |
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ty::Predicate::WellFormed(..) |
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ty::Predicate::ObjectSafe(..) |
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ty::Predicate::ClosureKind(..) |
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ty::Predicate::TypeOutlives(..) |
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ty::Predicate::ConstEvaluatable(..) => {
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false
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}
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}
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)
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}
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/// Returns `Some(_)` if this method makes the containing trait not object safe.
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fn object_safety_violation_for_method(
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self,
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trait_def_id: DefId,
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method: &ty::AssocItem,
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) -> Option<MethodViolationCode> {
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debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
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// Any method that has a `Self : Sized` requisite is otherwise
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// exempt from the regulations.
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if self.generics_require_sized_self(method.def_id) {
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return None;
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}
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self.virtual_call_violation_for_method(trait_def_id, method)
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}
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/// Returns `Some(_)` if this method cannot be called on a trait
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/// object; this does not necessarily imply that the enclosing trait
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/// is not object safe, because the method might have a where clause
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/// `Self:Sized`.
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fn virtual_call_violation_for_method(
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self,
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trait_def_id: DefId,
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method: &ty::AssocItem,
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) -> Option<MethodViolationCode> {
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// The method's first parameter must be named `self`
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if !method.method_has_self_argument {
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return Some(MethodViolationCode::StaticMethod);
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}
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let sig = self.fn_sig(method.def_id);
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for input_ty in &sig.skip_binder().inputs()[1..] {
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if self.contains_illegal_self_type_reference(trait_def_id, input_ty) {
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return Some(MethodViolationCode::ReferencesSelf);
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}
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}
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if self.contains_illegal_self_type_reference(trait_def_id, sig.output().skip_binder()) {
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return Some(MethodViolationCode::ReferencesSelf);
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}
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// We can't monomorphize things like `fn foo<A>(...)`.
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let own_counts = self.generics_of(method.def_id).own_counts();
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if own_counts.types + own_counts.consts != 0 {
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return Some(MethodViolationCode::Generic);
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}
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if self.predicates_of(method.def_id).predicates.iter()
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// A trait object can't claim to live more than the concrete type,
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// so outlives predicates will always hold.
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.cloned()
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.filter(|(p, _)| p.to_opt_type_outlives().is_none())
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.collect::<Vec<_>>()
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// Do a shallow visit so that `contains_illegal_self_type_reference`
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// may apply it's custom visiting.
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.visit_tys_shallow(|t| {
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self.contains_illegal_self_type_reference(trait_def_id, t)
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}) {
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return Some(MethodViolationCode::WhereClauseReferencesSelf);
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}
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let receiver_ty = self.liberate_late_bound_regions(
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method.def_id,
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&sig.map_bound(|sig| sig.inputs()[0]),
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);
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// Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
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// However, this is already considered object-safe. We allow it as a special case here.
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// FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
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// `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
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if receiver_ty != self.types.self_param {
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if !self.receiver_is_dispatchable(method, receiver_ty) {
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return Some(MethodViolationCode::UndispatchableReceiver);
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} else {
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// Do sanity check to make sure the receiver actually has the layout of a pointer.
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use crate::ty::layout::Abi;
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let param_env = self.param_env(method.def_id);
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let abi_of_ty = |ty: Ty<'tcx>| -> &Abi {
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match self.layout_of(param_env.and(ty)) {
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Ok(layout) => &layout.abi,
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Err(err) => bug!(
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"error: {}\n while computing layout for type {:?}", err, ty
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)
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}
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};
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// e.g., `Rc<()>`
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let unit_receiver_ty = self.receiver_for_self_ty(
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receiver_ty, self.mk_unit(), method.def_id
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);
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match abi_of_ty(unit_receiver_ty) {
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&Abi::Scalar(..) => (),
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abi => {
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self.sess.delay_span_bug(
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self.def_span(method.def_id),
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&format!(
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"receiver when `Self = ()` should have a Scalar ABI; found {:?}",
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abi
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),
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);
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}
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}
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let trait_object_ty = self.object_ty_for_trait(
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trait_def_id, self.mk_region(ty::ReStatic)
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);
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// e.g., `Rc<dyn Trait>`
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let trait_object_receiver = self.receiver_for_self_ty(
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receiver_ty, trait_object_ty, method.def_id
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);
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match abi_of_ty(trait_object_receiver) {
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&Abi::ScalarPair(..) => (),
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abi => {
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self.sess.delay_span_bug(
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self.def_span(method.def_id),
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&format!(
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"receiver when `Self = {}` should have a ScalarPair ABI; \
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found {:?}",
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trait_object_ty, abi
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),
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);
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}
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}
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}
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}
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None
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}
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|
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/// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
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/// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
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fn receiver_for_self_ty(
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self,
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receiver_ty: Ty<'tcx>,
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self_ty: Ty<'tcx>,
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method_def_id: DefId,
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) -> Ty<'tcx> {
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debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
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let substs = InternalSubsts::for_item(self, method_def_id, |param, _| {
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if param.index == 0 {
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self_ty.into()
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} else {
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self.mk_param_from_def(param)
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}
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});
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let result = receiver_ty.subst(self, substs);
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debug!("receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
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receiver_ty, self_ty, method_def_id, result);
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result
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}
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|
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/// Creates the object type for the current trait. For example,
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/// if the current trait is `Deref`, then this will be
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/// `dyn Deref<Target = Self::Target> + 'static`.
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fn object_ty_for_trait(self, trait_def_id: DefId, lifetime: ty::Region<'tcx>) -> Ty<'tcx> {
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debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
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let trait_ref = ty::TraitRef::identity(self, trait_def_id);
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let trait_predicate = ty::ExistentialPredicate::Trait(
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ty::ExistentialTraitRef::erase_self_ty(self, trait_ref)
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);
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let mut associated_types = traits::supertraits(self, ty::Binder::dummy(trait_ref))
|
|
.flat_map(|super_trait_ref| {
|
|
self.associated_items(super_trait_ref.def_id())
|
|
.map(move |item| (super_trait_ref, item))
|
|
})
|
|
.filter(|(_, item)| item.kind == ty::AssocKind::Type)
|
|
.collect::<Vec<_>>();
|
|
|
|
// existential predicates need to be in a specific order
|
|
associated_types.sort_by_cached_key(|(_, item)| self.def_path_hash(item.def_id));
|
|
|
|
let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
|
|
// We *can* get bound lifetimes here in cases like
|
|
// `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
|
|
//
|
|
// binder moved to (*)...
|
|
let super_trait_ref = super_trait_ref.skip_binder();
|
|
ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
|
|
ty: self.mk_projection(item.def_id, super_trait_ref.substs),
|
|
item_def_id: item.def_id,
|
|
substs: super_trait_ref.substs,
|
|
})
|
|
});
|
|
|
|
let existential_predicates = self.mk_existential_predicates(
|
|
iter::once(trait_predicate).chain(projection_predicates)
|
|
);
|
|
|
|
let object_ty = self.mk_dynamic(
|
|
// (*) ... binder re-introduced here
|
|
ty::Binder::bind(existential_predicates),
|
|
lifetime,
|
|
);
|
|
|
|
debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
|
|
|
|
object_ty
|
|
}
|
|
|
|
/// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
|
|
/// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
|
|
/// in the following way:
|
|
/// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
|
|
/// - require the following bound:
|
|
///
|
|
/// ```
|
|
/// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
|
|
/// ```
|
|
///
|
|
/// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
|
|
/// (substitution notation).
|
|
///
|
|
/// Some examples of receiver types and their required obligation:
|
|
/// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
|
|
/// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
|
|
/// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
|
|
///
|
|
/// The only case where the receiver is not dispatchable, but is still a valid receiver
|
|
/// type (just not object-safe), is when there is more than one level of pointer indirection.
|
|
/// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
|
|
/// is no way, or at least no inexpensive way, to coerce the receiver from the version where
|
|
/// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
|
|
/// contained by the trait object, because the object that needs to be coerced is behind
|
|
/// a pointer.
|
|
///
|
|
/// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
|
|
/// in a new check that `Trait` is object safe, creating a cycle. So instead, we fudge a little
|
|
/// by introducing a new type parameter `U` such that `Self: Unsize<U>` and `U: Trait + ?Sized`,
|
|
/// and use `U` in place of `dyn Trait`. Written as a chalk-style query:
|
|
///
|
|
/// forall (U: Trait + ?Sized) {
|
|
/// if (Self: Unsize<U>) {
|
|
/// Receiver: DispatchFromDyn<Receiver[Self => U]>
|
|
/// }
|
|
/// }
|
|
///
|
|
/// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
|
|
/// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
|
|
/// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
|
|
//
|
|
// FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
|
|
// fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
|
|
// `self: Wrapper<Self>`.
|
|
#[allow(dead_code)]
|
|
fn receiver_is_dispatchable(
|
|
self,
|
|
method: &ty::AssocItem,
|
|
receiver_ty: Ty<'tcx>,
|
|
) -> bool {
|
|
debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
|
|
|
|
let traits = (self.lang_items().unsize_trait(),
|
|
self.lang_items().dispatch_from_dyn_trait());
|
|
let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
|
|
(u, cu)
|
|
} else {
|
|
debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
|
|
return false;
|
|
};
|
|
|
|
// the type `U` in the query
|
|
// use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
|
|
// FIXME(mikeyhew) this is a total hack, and we should replace it when real forall queries
|
|
// are implemented
|
|
let unsized_self_ty: Ty<'tcx> = self.mk_ty_param(
|
|
::std::u32::MAX,
|
|
InternedString::intern("RustaceansAreAwesome"),
|
|
);
|
|
|
|
// `Receiver[Self => U]`
|
|
let unsized_receiver_ty = self.receiver_for_self_ty(
|
|
receiver_ty, unsized_self_ty, method.def_id
|
|
);
|
|
|
|
// create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
|
|
// `U: ?Sized` is already implied here
|
|
let param_env = {
|
|
let mut param_env = self.param_env(method.def_id);
|
|
|
|
// Self: Unsize<U>
|
|
let unsize_predicate = ty::TraitRef {
|
|
def_id: unsize_did,
|
|
substs: self.mk_substs_trait(self.types.self_param, &[unsized_self_ty.into()]),
|
|
}.to_predicate();
|
|
|
|
// U: Trait<Arg1, ..., ArgN>
|
|
let trait_predicate = {
|
|
let substs = InternalSubsts::for_item(
|
|
self,
|
|
method.container.assert_trait(),
|
|
|param, _| {
|
|
if param.index == 0 {
|
|
unsized_self_ty.into()
|
|
} else {
|
|
self.mk_param_from_def(param)
|
|
}
|
|
},
|
|
);
|
|
|
|
ty::TraitRef {
|
|
def_id: unsize_did,
|
|
substs,
|
|
}.to_predicate()
|
|
};
|
|
|
|
let caller_bounds: Vec<Predicate<'tcx>> = param_env.caller_bounds.iter().cloned()
|
|
.chain(iter::once(unsize_predicate))
|
|
.chain(iter::once(trait_predicate))
|
|
.collect();
|
|
|
|
param_env.caller_bounds = self.intern_predicates(&caller_bounds);
|
|
|
|
param_env
|
|
};
|
|
|
|
// Receiver: DispatchFromDyn<Receiver[Self => U]>
|
|
let obligation = {
|
|
let predicate = ty::TraitRef {
|
|
def_id: dispatch_from_dyn_did,
|
|
substs: self.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
|
|
}.to_predicate();
|
|
|
|
Obligation::new(
|
|
ObligationCause::dummy(),
|
|
param_env,
|
|
predicate,
|
|
)
|
|
};
|
|
|
|
self.infer_ctxt().enter(|ref infcx| {
|
|
// the receiver is dispatchable iff the obligation holds
|
|
infcx.predicate_must_hold_modulo_regions(&obligation)
|
|
})
|
|
}
|
|
|
|
fn contains_illegal_self_type_reference(
|
|
self,
|
|
trait_def_id: DefId,
|
|
ty: Ty<'tcx>,
|
|
) -> bool {
|
|
// This is somewhat subtle. In general, we want to forbid
|
|
// references to `Self` in the argument and return types,
|
|
// since the value of `Self` is erased. However, there is one
|
|
// exception: it is ok to reference `Self` in order to access
|
|
// an associated type of the current trait, since we retain
|
|
// the value of those associated types in the object type
|
|
// itself.
|
|
//
|
|
// ```rust
|
|
// trait SuperTrait {
|
|
// type X;
|
|
// }
|
|
//
|
|
// trait Trait : SuperTrait {
|
|
// type Y;
|
|
// fn foo(&self, x: Self) // bad
|
|
// fn foo(&self) -> Self // bad
|
|
// fn foo(&self) -> Option<Self> // bad
|
|
// fn foo(&self) -> Self::Y // OK, desugars to next example
|
|
// fn foo(&self) -> <Self as Trait>::Y // OK
|
|
// fn foo(&self) -> Self::X // OK, desugars to next example
|
|
// fn foo(&self) -> <Self as SuperTrait>::X // OK
|
|
// }
|
|
// ```
|
|
//
|
|
// However, it is not as simple as allowing `Self` in a projected
|
|
// type, because there are illegal ways to use `Self` as well:
|
|
//
|
|
// ```rust
|
|
// trait Trait : SuperTrait {
|
|
// ...
|
|
// fn foo(&self) -> <Self as SomeOtherTrait>::X;
|
|
// }
|
|
// ```
|
|
//
|
|
// Here we will not have the type of `X` recorded in the
|
|
// object type, and we cannot resolve `Self as SomeOtherTrait`
|
|
// without knowing what `Self` is.
|
|
|
|
let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
|
|
let mut error = false;
|
|
let self_ty = self.types.self_param;
|
|
ty.maybe_walk(|ty| {
|
|
match ty.kind {
|
|
ty::Param(_) => {
|
|
if ty == self_ty {
|
|
error = true;
|
|
}
|
|
|
|
false // no contained types to walk
|
|
}
|
|
|
|
ty::Projection(ref data) => {
|
|
// This is a projected type `<Foo as SomeTrait>::X`.
|
|
|
|
// Compute supertraits of current trait lazily.
|
|
if supertraits.is_none() {
|
|
let trait_ref = ty::Binder::bind(
|
|
ty::TraitRef::identity(self, trait_def_id),
|
|
);
|
|
supertraits = Some(traits::supertraits(self, trait_ref).collect());
|
|
}
|
|
|
|
// Determine whether the trait reference `Foo as
|
|
// SomeTrait` is in fact a supertrait of the
|
|
// current trait. In that case, this type is
|
|
// legal, because the type `X` will be specified
|
|
// in the object type. Note that we can just use
|
|
// direct equality here because all of these types
|
|
// are part of the formal parameter listing, and
|
|
// hence there should be no inference variables.
|
|
let projection_trait_ref = ty::Binder::bind(data.trait_ref(self));
|
|
let is_supertrait_of_current_trait =
|
|
supertraits.as_ref().unwrap().contains(&projection_trait_ref);
|
|
|
|
if is_supertrait_of_current_trait {
|
|
false // do not walk contained types, do not report error, do collect $200
|
|
} else {
|
|
true // DO walk contained types, POSSIBLY reporting an error
|
|
}
|
|
}
|
|
|
|
_ => true, // walk contained types, if any
|
|
}
|
|
});
|
|
|
|
error
|
|
}
|
|
}
|
|
|
|
pub(super) fn is_object_safe_provider(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
|
|
tcx.object_safety_violations(trait_def_id).is_empty()
|
|
}
|