Rollup merge of #94057 - lcnr:simplify_type-uwu, r=nikomatsakis

improve comments for `simplify_type`

Should now correctly describe what's going on. Experimented with checking the invariant for projections
but that ended up requiring fairly involved changes. I assume that it is not possible to get unsoundness here,
at least for now and I can pretty much guarantee that it's impossible to trigger it by accident.

r? `````@nikomatsakis````` cc #92721

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -20,7 +20,7 @@ use rustc_hir::CRATE_HIR_ID;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::TraitEngine;
 use rustc_middle::traits::specialization_graph::OverlapMode;
-use rustc_middle::ty::fast_reject::{self, SimplifyParams};
+use rustc_middle::ty::fast_reject::{self, TreatParams};
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -87,8 +87,8 @@ where
         impl2_ref.iter().flat_map(|tref| tref.substs.types()),
     )
     .any(|(ty1, ty2)| {
-        let t1 = fast_reject::simplify_type(tcx, ty1, SimplifyParams::No);
-        let t2 = fast_reject::simplify_type(tcx, ty2, SimplifyParams::No);
+        let t1 = fast_reject::simplify_type(tcx, ty1, TreatParams::AsPlaceholders);
+        let t2 = fast_reject::simplify_type(tcx, ty2, TreatParams::AsPlaceholders);
 
         if let (Some(t1), Some(t2)) = (t1, t2) {
             // Simplified successfully

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -36,7 +36,7 @@ use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
-use rustc_middle::ty::fast_reject::{self, SimplifyParams};
+use rustc_middle::ty::fast_reject::{self, TreatParams};
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
@@ -2176,8 +2176,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn fast_reject_trait_refs(
         &mut self,
-        obligation: &TraitObligation<'_>,
-        impl_trait_ref: &ty::TraitRef<'_>,
+        obligation: &TraitObligation<'tcx>,
+        impl_trait_ref: &ty::TraitRef<'tcx>,
     ) -> bool {
         // We can avoid creating type variables and doing the full
         // substitution if we find that any of the input types, when
@@ -2193,10 +2193,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         let simplified_obligation_ty = fast_reject::simplify_type(
                             self.tcx(),
                             obligation_ty,
-                            SimplifyParams::Yes,
+                            TreatParams::AsBoundTypes,
+                        );
+                        let simplified_impl_ty = fast_reject::simplify_type(
+                            self.tcx(),
+                            impl_ty,
+                            TreatParams::AsPlaceholders,
                         );
-                        let simplified_impl_ty =
-                            fast_reject::simplify_type(self.tcx(), impl_ty, SimplifyParams::No);
 
                         simplified_obligation_ty.is_some()
                             && simplified_impl_ty.is_some()

compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs

@@ -2,7 +2,7 @@ use super::OverlapError;
 
 use crate::traits;
 use rustc_hir::def_id::DefId;
-use rustc_middle::ty::fast_reject::{self, SimplifiedType, SimplifyParams};
+use rustc_middle::ty::fast_reject::{self, SimplifiedType, TreatParams};
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
 
@@ -49,7 +49,9 @@ impl ChildrenExt<'_> for Children {
     /// Insert an impl into this set of children without comparing to any existing impls.
     fn insert_blindly(&mut self, tcx: TyCtxt<'_>, impl_def_id: DefId) {
         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
-        if let Some(st) = fast_reject::simplify_type(tcx, trait_ref.self_ty(), SimplifyParams::No) {
+        if let Some(st) =
+            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsPlaceholders)
+        {
             debug!("insert_blindly: impl_def_id={:?} st={:?}", impl_def_id, st);
             self.non_blanket_impls.entry(st).or_default().push(impl_def_id)
         } else {
@@ -64,7 +66,9 @@ impl ChildrenExt<'_> for Children {
     fn remove_existing(&mut self, tcx: TyCtxt<'_>, impl_def_id: DefId) {
         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
         let vec: &mut Vec<DefId>;
-        if let Some(st) = fast_reject::simplify_type(tcx, trait_ref.self_ty(), SimplifyParams::No) {
+        if let Some(st) =
+            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsPlaceholders)
+        {
             debug!("remove_existing: impl_def_id={:?} st={:?}", impl_def_id, st);
             vec = self.non_blanket_impls.get_mut(&st).unwrap();
         } else {
@@ -312,7 +316,8 @@ impl GraphExt for Graph {
 
         let mut parent = trait_def_id;
         let mut last_lint = None;
-        let simplified = fast_reject::simplify_type(tcx, trait_ref.self_ty(), SimplifyParams::No);
+        let simplified =
+            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsPlaceholders);
 
         // Descend the specialization tree, where `parent` is the current parent node.
         loop {