Auto merge of rust-lang#118118 - spastorino:do-not-erase-late-bound-regions-on-iat, r=<try>

Do not erase late bound regions when selecting inherent associated types

In the fix for rust-lang#97156 we would want the following code:

```rust
#![feature(inherent_associated_types)]
#![allow(incomplete_features)]

struct Foo<T>(T);

impl Foo<fn(&'static ())> {
    type Assoc = u32;
}

trait Other {}
impl Other for u32 {}

// FIXME(inherent_associated_types): Avoid emitting two diagnostics (they only differ in span).
// FIXME(inherent_associated_types): Enhancement: Spruce up the diagnostic by saying something like
// "implementation is not general enough" as is done for traits via
// `try_report_trait_placeholder_mismatch`.

fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}
//~^ ERROR mismatched types
//~| ERROR mismatched types

fn main() {}
```

to fail with ...

```
error[E0220]: associated type `Assoc` not found for `Foo<for<'a> fn(&'a ())>` in the current scope
  --> tests/ui/associated-inherent-types/issue-109789.rs:18:36
   |
4  | struct Foo<T>(T);
   | ------------- associated item `Assoc` not found for this struct
...
18 | fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}
   |                                    ^^^^^ associated item not found in `Foo<for<'a> fn(&'a ())>`
   |
   = note: the associated type was found for
           - `Foo<fn(&'static ())>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0220`.
```

This PR fixes the ICE we are currently getting "was a subtype of Foo<Binder(fn(&ReStatic ()), [])> during selection but now it is not"

Also fixes rust-lang#112631

r? `@lcnr`

Author: bors

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -27,20 +27,21 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::{walk_generics, Visitor as _};
 use rustc_hir::{GenericArg, GenericArgs, OpaqueTyOrigin};
 use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
-use rustc_infer::traits::ObligationCause;
+use rustc_infer::traits::{Obligation, ObligationCause};
 use rustc_middle::middle::stability::AllowUnstable;
 use rustc_middle::ty::GenericParamDefKind;
 use rustc_middle::ty::{
-    self, Const, GenericArgKind, GenericArgsRef, IsSuggestable, Ty, TyCtxt, TypeVisitableExt,
+    self, Const, GenericArgKind, GenericArgsRef, IsSuggestable, ParamEnv, Predicate, Ty, TyCtxt,
+    TypeFoldable, TypeVisitableExt,
 };
 use rustc_session::lint::builtin::AMBIGUOUS_ASSOCIATED_ITEMS;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{kw, Ident, Symbol};
 use rustc_span::{sym, BytePos, Span, DUMMY_SP};
 use rustc_target::spec::abi;
+use rustc_trait_selection::solve::eval_ctxt::canonical::EagerResolver;
 use rustc_trait_selection::traits::wf::object_region_bounds;
 use rustc_trait_selection::traits::{self, NormalizeExt, ObligationCtxt};
-use rustc_type_ir::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 
 use std::fmt::Display;
 use std::slice;
@@ -1608,134 +1609,122 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let param_env = tcx.param_env(block.owner);
         let cause = ObligationCause::misc(span, block.owner.def_id);
 
-        let mut fulfillment_errors = Vec::new();
-        let mut applicable_candidates: Vec<_> = infcx.probe(|_| {
-            // Regions are not considered during selection.
-            let self_ty = self_ty
-                .fold_with(&mut BoundVarEraser { tcx, universe: infcx.create_next_universe() });
-
-            struct BoundVarEraser<'tcx> {
-                tcx: TyCtxt<'tcx>,
-                universe: ty::UniverseIndex,
-            }
-
-            // FIXME(non_lifetime_binders): Don't assign the same universe to each placeholder.
-            impl<'tcx> TypeFolder<TyCtxt<'tcx>> for BoundVarEraser<'tcx> {
-                fn interner(&self) -> TyCtxt<'tcx> {
-                    self.tcx
-                }
+        let mut universes = if self_ty.has_escaping_bound_vars() {
+            vec![None; self_ty.outer_exclusive_binder().as_usize()]
+        } else {
+            vec![]
+        };
 
-                fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-                    // FIXME(@lcnr): This is broken, erasing bound regions
-                    // impacts selection as it results in different types.
-                    if r.is_bound() { self.tcx.lifetimes.re_erased } else { r }
-                }
+        crate::traits::project::with_replaced_escaping_bound_vars(
+            infcx,
+            &mut universes,
+            self_ty,
+            |self_ty| {
+                let tcx = self.tcx();
+                let InferOk { value: self_ty, obligations } =
+                    infcx.at(&cause, param_env).normalize(self_ty);
+
+                let (impl_, (assoc_item, def_scope)) = self.select_inherent_assoc_type_candidates(
+                    infcx,
+                    name,
+                    span,
+                    self_ty,
+                    cause,
+                    param_env,
+                    obligations,
+                    candidates,
+                )?;
+
+                self.check_assoc_ty(assoc_item, name, def_scope, block, span);
+
+                // FIXME(fmease): Currently creating throwaway `parent_args` to please
+                // `create_args_for_associated_item`. Modify the latter instead (or sth. similar) to
+                // not require the parent args logic.
+                let parent_args = ty::GenericArgs::identity_for_item(tcx, impl_);
+                let args =
+                    self.create_args_for_associated_item(span, assoc_item, segment, parent_args);
+                let args = tcx.mk_args_from_iter(
+                    std::iter::once(ty::GenericArg::from(self_ty))
+                        .chain(args.into_iter().skip(parent_args.len())),
+                );
 
-                fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-                    match *ty.kind() {
-                        ty::Bound(_, bv) => Ty::new_placeholder(
-                            self.tcx,
-                            ty::PlaceholderType { universe: self.universe, bound: bv },
-                        ),
-                        _ => ty.super_fold_with(self),
-                    }
-                }
+                let ty = Ty::new_alias(tcx, ty::Inherent, ty::AliasTy::new(tcx, assoc_item, args));
+                let ty = ty.fold_with(&mut EagerResolver { infcx });
 
-                fn fold_const(
-                    &mut self,
-                    ct: ty::Const<'tcx>,
-                ) -> <TyCtxt<'tcx> as rustc_type_ir::Interner>::Const {
-                    assert!(!ct.ty().has_escaping_bound_vars());
-
-                    match ct.kind() {
-                        ty::ConstKind::Bound(_, bv) => ty::Const::new_placeholder(
-                            self.tcx,
-                            ty::PlaceholderConst { universe: self.universe, bound: bv },
-                            ct.ty(),
-                        ),
-                        _ => ct.super_fold_with(self),
-                    }
-                }
-            }
+                Ok(Some((ty, assoc_item)))
+            },
+        )
+    }
 
-            let InferOk { value: self_ty, obligations } =
-                infcx.at(&cause, param_env).normalize(self_ty);
+    fn select_inherent_assoc_type_candidates(
+        &self,
+        infcx: &InferCtxt<'tcx>,
+        name: Ident,
+        span: Span,
+        self_ty: Ty<'tcx>,
+        cause: ObligationCause<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        obligations: Vec<Obligation<'tcx, Predicate<'tcx>>>,
+        candidates: Vec<(DefId, (DefId, DefId))>,
+    ) -> Result<(DefId, (DefId, DefId)), ErrorGuaranteed> {
+        let tcx = self.tcx();
+        let mut fulfillment_errors = Vec::new();
 
-            candidates
-                .iter()
-                .copied()
-                .filter(|&(impl_, _)| {
-                    infcx.probe(|_| {
-                        let ocx = ObligationCtxt::new(infcx);
-                        ocx.register_obligations(obligations.clone());
-
-                        let impl_args = infcx.fresh_args_for_item(span, impl_);
-                        let impl_ty = tcx.type_of(impl_).instantiate(tcx, impl_args);
-                        let impl_ty = ocx.normalize(&cause, param_env, impl_ty);
-
-                        // Check that the self types can be related.
-                        // FIXME(inherent_associated_types): Should we use `eq` here? Method probing uses
-                        // `sup` for this situtation, too. What for? To constrain inference variables?
-                        if ocx.sup(&ObligationCause::dummy(), param_env, impl_ty, self_ty).is_err()
-                        {
-                            return false;
-                        }
+        let applicable_candidates: Vec<_> = candidates
+            .iter()
+            .copied()
+            .filter(|&(impl_, _)| {
+                infcx.probe(|_| {
+                    let ocx = ObligationCtxt::new(infcx);
+                    ocx.register_obligations(obligations.clone());
+
+                    let impl_args = infcx.fresh_args_for_item(span, impl_);
+                    let impl_ty = tcx.type_of(impl_).instantiate(tcx, impl_args);
+                    let impl_ty = ocx.normalize(&cause, param_env, impl_ty);
+
+                    // Check that the self types can be related.
+                    if ocx.eq(&ObligationCause::dummy(), param_env, impl_ty, self_ty).is_err() {
+                        return false;
+                    }
 
-                        // Check whether the impl imposes obligations we have to worry about.
-                        let impl_bounds = tcx.predicates_of(impl_).instantiate(tcx, impl_args);
-                        let impl_bounds = ocx.normalize(&cause, param_env, impl_bounds);
-                        let impl_obligations = traits::predicates_for_generics(
-                            |_, _| cause.clone(),
-                            param_env,
-                            impl_bounds,
-                        );
-                        ocx.register_obligations(impl_obligations);
+                    // Check whether the impl imposes obligations we have to worry about.
+                    let impl_bounds = tcx.predicates_of(impl_).instantiate(tcx, impl_args);
+                    let impl_bounds = ocx.normalize(&cause, param_env, impl_bounds);
+                    let impl_obligations = traits::predicates_for_generics(
+                        |_, _| cause.clone(),
+                        param_env,
+                        impl_bounds,
+                    );
+                    ocx.register_obligations(impl_obligations);
 
-                        let mut errors = ocx.select_where_possible();
-                        if !errors.is_empty() {
-                            fulfillment_errors.append(&mut errors);
-                            return false;
-                        }
+                    let mut errors = ocx.select_where_possible();
+                    if !errors.is_empty() {
+                        fulfillment_errors.append(&mut errors);
+                        return false;
+                    }
 
-                        true
-                    })
+                    true
                 })
-                .collect()
-        });
+            })
+            .collect();
 
-        if applicable_candidates.len() > 1 {
-            return Err(self.complain_about_ambiguous_inherent_assoc_type(
+        match &applicable_candidates[..] {
+            &[] => Err(self.complain_about_inherent_assoc_type_not_found(
                 name,
-                applicable_candidates.into_iter().map(|(_, (candidate, _))| candidate).collect(),
+                self_ty,
+                candidates,
+                fulfillment_errors,
                 span,
-            ));
-        }
-
-        if let Some((impl_, (assoc_item, def_scope))) = applicable_candidates.pop() {
-            self.check_assoc_ty(assoc_item, name, def_scope, block, span);
-
-            // FIXME(fmease): Currently creating throwaway `parent_args` to please
-            // `create_args_for_associated_item`. Modify the latter instead (or sth. similar) to
-            // not require the parent args logic.
-            let parent_args = ty::GenericArgs::identity_for_item(tcx, impl_);
-            let args = self.create_args_for_associated_item(span, assoc_item, segment, parent_args);
-            let args = tcx.mk_args_from_iter(
-                std::iter::once(ty::GenericArg::from(self_ty))
-                    .chain(args.into_iter().skip(parent_args.len())),
-            );
+            )),
 
-            let ty = Ty::new_alias(tcx, ty::Inherent, ty::AliasTy::new(tcx, assoc_item, args));
+            &[applicable_candidate] => Ok(applicable_candidate),
 
-            return Ok(Some((ty, assoc_item)));
+            &[_, ..] => Err(self.complain_about_ambiguous_inherent_assoc_type(
+                name,
+                applicable_candidates.into_iter().map(|(_, (candidate, _))| candidate).collect(),
+                span,
+            )),
         }
-
-        Err(self.complain_about_inherent_assoc_type_not_found(
-            name,
-            self_ty,
-            candidates,
-            fulfillment_errors,
-            span,
-        ))
     }
 
     fn lookup_assoc_ty(

compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs

@@ -365,8 +365,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 }
 
 /// Resolves ty, region, and const vars to their inferred values or their root vars.
-struct EagerResolver<'a, 'tcx> {
-    infcx: &'a InferCtxt<'tcx>,
+pub struct EagerResolver<'a, 'tcx> {
+    pub infcx: &'a InferCtxt<'tcx>,
 }
 
 impl<'tcx> TypeFolder<TyCtxt<'tcx>> for EagerResolver<'_, 'tcx> {

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -33,7 +33,7 @@ use super::{search_graph, GoalEvaluationKind};
 use super::{search_graph::SearchGraph, Goal};
 pub use select::InferCtxtSelectExt;
 
-mod canonical;
+pub mod canonical;
 mod commit_if_ok;
 mod probe;
 mod select;

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -31,7 +31,7 @@ use rustc_middle::ty::{
 mod alias_relate;
 mod assembly;
 mod canonicalize;
-mod eval_ctxt;
+pub mod eval_ctxt;
 mod fulfill;
 pub mod inspect;
 mod normalize;

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1001,7 +1001,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
         &mut self,
         t: ty::Binder<'tcx, T>,
     ) -> ty::Binder<'tcx, T> {
-        if !t.has_placeholders() && !t.has_infer_regions() {
+        if !t.has_placeholders() && !t.has_infer() {
             return t;
         }
         self.current_index.shift_in(1);
@@ -1048,6 +1048,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
     }
 
     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let ty = self.infcx.shallow_resolve(ty);
         match *ty.kind() {
             ty::Placeholder(p) => {
                 let replace_var = self.mapped_types.get(&p);
@@ -1063,16 +1064,23 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
                         );
                         Ty::new_bound(self.infcx.tcx, db, *replace_var)
                     }
-                    None => ty,
+                    None => {
+                        if ty.has_infer() {
+                            ty.super_fold_with(self)
+                        } else {
+                            ty
+                        }
+                    }
                 }
             }
 
-            _ if ty.has_placeholders() || ty.has_infer_regions() => ty.super_fold_with(self),
+            _ if ty.has_placeholders() || ty.has_infer() => ty.super_fold_with(self),
             _ => ty,
         }
     }
 
     fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        let ct = self.infcx.shallow_resolve(ct);
         if let ty::ConstKind::Placeholder(p) = ct.kind() {
             let replace_var = self.mapped_consts.get(&p);
             match replace_var {
@@ -1087,7 +1095,13 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
                     );
                     ty::Const::new_bound(self.infcx.tcx, db, *replace_var, ct.ty())
                 }
-                None => ct,
+                None => {
+                    if ct.has_infer() {
+                        ct.super_fold_with(self)
+                    } else {
+                        ct
+                    }
+                }
             }
         } else {
             ct.super_fold_with(self)

tests/ui/associated-inherent-types/issue-111404-1.rs

@@ -9,5 +9,6 @@ impl<'a> Foo<fn(&'a ())> {
 
 fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
 //~^ ERROR higher-ranked subtype error
+//~| ERROR higher-ranked subtype error
 
 fn main() {}

tests/ui/associated-inherent-types/issue-111404-1.stderr

@@ -4,5 +4,13 @@ error: higher-ranked subtype error
 LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: higher-ranked subtype error
+  --> $DIR/issue-111404-1.rs:10:1
+   |
+LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 2 previous errors