Auto merge of #126130 - compiler-errors:goal-relations, r=lcnr

Make `ObligationEmittingRelation`s emit `Goal` rather than `Obligation`

Helps avoid needing to uplift `Obligation` into the solver. We still can't get rid of `ObligationCause`, but we can keep it as an associated type for `InferCtxtLike` and just give it a `dummy` function.

There's some shuttling between `Goal` and `Obligation` that may be perf-sensitive... Let's see what rust-timer says.

r? lcnr

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -12,9 +12,7 @@ use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::NormalizeExt;
 use crate::traits::SkipLeakCheck;
 use crate::traits::{util, FulfillmentErrorCode};
-use crate::traits::{
-    Obligation, ObligationCause, PredicateObligation, PredicateObligations, SelectionContext,
-};
+use crate::traits::{Obligation, ObligationCause, PredicateObligation, SelectionContext};
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir::def::DefKind;
@@ -305,7 +303,7 @@ fn equate_impl_headers<'tcx>(
     param_env: ty::ParamEnv<'tcx>,
     impl1: &ty::ImplHeader<'tcx>,
     impl2: &ty::ImplHeader<'tcx>,
-) -> Option<PredicateObligations<'tcx>> {
+) -> Option<Vec<PredicateObligation<'tcx>>> {
     let result =
         match (impl1.trait_ref, impl2.trait_ref) {
             (Some(impl1_ref), Some(impl2_ref)) => infcx

compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs

@@ -5,7 +5,7 @@ use crate::infer::{InferCtxt, InferOk};
 use crate::traits::{ObligationCause, ObligationCtxt};
 use rustc_errors::ErrorGuaranteed;
 use rustc_infer::infer::canonical::Certainty;
-use rustc_infer::traits::PredicateObligations;
+use rustc_infer::traits::PredicateObligation;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::{ParamEnvAnd, TyCtxt};
@@ -103,7 +103,7 @@ pub trait QueryTypeOp<'tcx>: fmt::Debug + Copy + TypeFoldable<TyCtxt<'tcx>> + 't
         (
             Self::QueryResponse,
             Option<Canonical<'tcx, ParamEnvAnd<'tcx, Self>>>,
-            PredicateObligations<'tcx>,
+            Vec<PredicateObligation<'tcx>>,
             Certainty,
         ),
         NoSolution,