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create Coercion
obligations given 2 unbound type variables
Motivation: in upcoming commits, we are going to create a graph of the coercion relationships between variables. We want to distinguish *coercion* specifically from other sorts of subtyping, as it indicates values flowing from one place to another via assignment.
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@ -42,6 +42,7 @@ use rustc_hir as hir;
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use rustc_hir::def_id::DefId;
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use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
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use rustc_infer::infer::{Coercion, InferOk, InferResult};
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use rustc_infer::traits::Obligation;
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use rustc_middle::lint::in_external_macro;
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use rustc_middle::ty::adjustment::{
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Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
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@ -50,7 +51,7 @@ use rustc_middle::ty::error::TypeError;
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use rustc_middle::ty::fold::TypeFoldable;
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use rustc_middle::ty::relate::RelateResult;
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use rustc_middle::ty::subst::SubstsRef;
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use rustc_middle::ty::{self, Ty, TypeAndMut};
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use rustc_middle::ty::{self, ToPredicate, Ty, TypeAndMut};
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use rustc_session::parse::feature_err;
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use rustc_span::symbol::sym;
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use rustc_span::{self, BytePos, Span};
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@ -172,7 +173,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
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kind: TypeVariableOriginKind::AdjustmentType,
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span: self.cause.span,
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});
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self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
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self.coerce_from_inference_variable(diverging_ty, b, simple(Adjust::NeverToAny))
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} else {
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success(simple(Adjust::NeverToAny)(b), b, vec![])
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};
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@ -182,7 +183,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
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// we have no information about the source type. This will always
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// ultimately fall back to some form of subtyping.
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if a.is_ty_var() {
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return self.coerce_from_inference_variable(a, b);
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return self.coerce_from_inference_variable(a, b, identity);
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}
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// Consider coercing the subtype to a DST
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@ -245,11 +246,53 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
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/// Coercing *from* an inference variable. In this case, we have no information
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/// about the source type, so we can't really do a true coercion and we always
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/// fall back to subtyping (`unify_and`).
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fn coerce_from_inference_variable(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
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fn coerce_from_inference_variable(
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&self,
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a: Ty<'tcx>,
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b: Ty<'tcx>,
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make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
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) -> CoerceResult<'tcx> {
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debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
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assert!(a.is_ty_var() && self.infcx.shallow_resolve(a) == a);
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assert!(self.infcx.shallow_resolve(b) == b);
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self.unify_and(a, b, identity)
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if b.is_ty_var() {
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// Two unresolved type variables: create a `Coerce` predicate.
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let target_ty = if self.use_lub {
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self.infcx.next_ty_var(TypeVariableOrigin {
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kind: TypeVariableOriginKind::LatticeVariable,
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span: self.cause.span,
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})
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} else {
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b
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};
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let mut obligations = Vec::with_capacity(2);
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for &source_ty in &[a, b] {
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if source_ty != target_ty {
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obligations.push(Obligation::new(
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self.cause.clone(),
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self.param_env,
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ty::PredicateKind::Coerce(ty::CoercePredicate {
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a: source_ty,
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b: target_ty,
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})
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.to_predicate(self.tcx()),
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));
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}
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}
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debug!(
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"coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
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target_ty, obligations
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);
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let adjustments = make_adjustments(target_ty);
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InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
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} else {
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// One unresolved type variable: just apply subtyping, we may be able
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// to do something useful.
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self.unify_and(a, b, make_adjustments)
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}
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}
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/// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
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