Move ReverseMapper logic onto OpaqueHiddenType

Author: oli-obk

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -5,8 +5,7 @@ use rustc_hir::OpaqueTyOrigin;
 use rustc_infer::infer::TyCtxtInferExt as _;
 use rustc_infer::infer::{DefiningAnchor, InferCtxt};
 use rustc_infer::traits::{Obligation, ObligationCause, TraitEngine};
-use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
-use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts};
+use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts};
 use rustc_middle::ty::visit::TypeVisitable;
 use rustc_middle::ty::{
     self, OpaqueHiddenType, OpaqueTypeKey, ToPredicate, Ty, TyCtxt, TypeFoldable,
@@ -15,8 +14,6 @@ use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::TraitEngineExt as _;
 
-use crate::session_diagnostics::ConstNotUsedTraitAlias;
-
 use super::RegionInferenceContext;
 
 impl<'tcx> RegionInferenceContext<'tcx> {
@@ -228,29 +225,9 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
             return self.tcx.ty_error();
         }
 
-        let OpaqueTypeKey { def_id, substs } = opaque_type_key;
-
-        // Use substs to build up a reverse map from regions to their
-        // identity mappings. This is necessary because of `impl
-        // Trait` lifetimes are computed by replacing existing
-        // lifetimes with 'static and remapping only those used in the
-        // `impl Trait` return type, resulting in the parameters
-        // shifting.
-        let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id.to_def_id());
-        debug!(?id_substs);
-        let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> =
-            substs.iter().enumerate().map(|(index, subst)| (subst, id_substs[index])).collect();
-        debug!("map = {:#?}", map);
-
-        // Convert the type from the function into a type valid outside
-        // the function, by replacing invalid regions with 'static,
-        // after producing an error for each of them.
-        let definition_ty = instantiated_ty.ty.fold_with(&mut ReverseMapper::new(
-            self.tcx,
-            map,
-            instantiated_ty.span,
-        ));
-        debug!(?definition_ty);
+        let definition_ty = instantiated_ty
+            .remap_generic_params_to_declaration_params(opaque_type_key, self.tcx)
+            .ty;
 
         if !check_opaque_type_parameter_valid(
             self.tcx,
@@ -266,6 +243,7 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
         let OpaqueTyOrigin::TyAlias = origin else {
             return definition_ty;
         };
+        let def_id = opaque_type_key.def_id;
         // This logic duplicates most of `check_opaque_meets_bounds`.
         // FIXME(oli-obk): Also do region checks here and then consider removing `check_opaque_meets_bounds` entirely.
         let param_env = self.tcx.param_env(def_id);
@@ -281,6 +259,8 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
             .to_predicate(infcx.tcx);
         let mut fulfillment_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
 
+        let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id.to_def_id());
+
         // Require that the hidden type actually fulfills all the bounds of the opaque type, even without
         // the bounds that the function supplies.
         match infcx.register_hidden_type(
@@ -421,200 +401,3 @@ fn check_opaque_type_parameter_valid(
     }
     true
 }
-
-struct ReverseMapper<'tcx> {
-    tcx: TyCtxt<'tcx>,
-    map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
-    do_not_error: bool,
-
-    /// Span of function being checked.
-    span: Span,
-}
-
-impl<'tcx> ReverseMapper<'tcx> {
-    fn new(
-        tcx: TyCtxt<'tcx>,
-        map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
-        span: Span,
-    ) -> Self {
-        Self { tcx, map, do_not_error: false, span }
-    }
-
-    fn fold_kind_no_missing_regions_error(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
-        assert!(!self.do_not_error);
-        self.do_not_error = true;
-        let kind = kind.fold_with(self);
-        self.do_not_error = false;
-        kind
-    }
-
-    fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
-        assert!(!self.do_not_error);
-        kind.fold_with(self)
-    }
-}
-
-impl<'tcx> TypeFolder<'tcx> for ReverseMapper<'tcx> {
-    fn tcx(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    #[instrument(skip(self), level = "debug")]
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        match *r {
-            // Ignore bound regions and `'static` regions that appear in the
-            // type, we only need to remap regions that reference lifetimes
-            // from the function declaration.
-            // This would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
-            ty::ReLateBound(..) | ty::ReStatic => return r,
-
-            // If regions have been erased (by writeback), don't try to unerase
-            // them.
-            ty::ReErased => return r,
-
-            // The regions that we expect from borrow checking.
-            ty::ReEarlyBound(_) | ty::ReFree(_) => {}
-
-            ty::RePlaceholder(_) | ty::ReVar(_) => {
-                // All of the regions in the type should either have been
-                // erased by writeback, or mapped back to named regions by
-                // borrow checking.
-                bug!("unexpected region kind in opaque type: {:?}", r);
-            }
-        }
-
-        match self.map.get(&r.into()).map(|k| k.unpack()) {
-            Some(GenericArgKind::Lifetime(r1)) => r1,
-            Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
-            None if self.do_not_error => self.tcx.lifetimes.re_static,
-            None => {
-                self.tcx
-                    .sess
-                    .struct_span_err(self.span, "non-defining opaque type use in defining scope")
-                    .span_label(
-                        self.span,
-                        format!(
-                            "lifetime `{}` is part of concrete type but not used in \
-                                 parameter list of the `impl Trait` type alias",
-                            r
-                        ),
-                    )
-                    .emit();
-
-                self.tcx().lifetimes.re_static
-            }
-        }
-    }
-
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        match *ty.kind() {
-            ty::Closure(def_id, substs) => {
-                // I am a horrible monster and I pray for death. When
-                // we encounter a closure here, it is always a closure
-                // from within the function that we are currently
-                // type-checking -- one that is now being encapsulated
-                // in an opaque type. Ideally, we would
-                // go through the types/lifetimes that it references
-                // and treat them just like we would any other type,
-                // which means we would error out if we find any
-                // reference to a type/region that is not in the
-                // "reverse map".
-                //
-                // **However,** in the case of closures, there is a
-                // somewhat subtle (read: hacky) consideration. The
-                // problem is that our closure types currently include
-                // all the lifetime parameters declared on the
-                // enclosing function, even if they are unused by the
-                // closure itself. We can't readily filter them out,
-                // so here we replace those values with `'empty`. This
-                // can't really make a difference to the rest of the
-                // compiler; those regions are ignored for the
-                // outlives relation, and hence don't affect trait
-                // selection or auto traits, and they are erased
-                // during codegen.
-
-                let generics = self.tcx.generics_of(def_id);
-                let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
-                    if index < generics.parent_count {
-                        // Accommodate missing regions in the parent kinds...
-                        self.fold_kind_no_missing_regions_error(kind)
-                    } else {
-                        // ...but not elsewhere.
-                        self.fold_kind_normally(kind)
-                    }
-                }));
-
-                self.tcx.mk_closure(def_id, substs)
-            }
-
-            ty::Generator(def_id, substs, movability) => {
-                let generics = self.tcx.generics_of(def_id);
-                let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
-                    if index < generics.parent_count {
-                        // Accommodate missing regions in the parent kinds...
-                        self.fold_kind_no_missing_regions_error(kind)
-                    } else {
-                        // ...but not elsewhere.
-                        self.fold_kind_normally(kind)
-                    }
-                }));
-
-                self.tcx.mk_generator(def_id, substs, movability)
-            }
-
-            ty::Param(param) => {
-                // Look it up in the substitution list.
-                match self.map.get(&ty.into()).map(|k| k.unpack()) {
-                    // Found it in the substitution list; replace with the parameter from the
-                    // opaque type.
-                    Some(GenericArgKind::Type(t1)) => t1,
-                    Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
-                    None => {
-                        debug!(?param, ?self.map);
-                        self.tcx
-                            .sess
-                            .struct_span_err(
-                                self.span,
-                                &format!(
-                                    "type parameter `{}` is part of concrete type but not \
-                                          used in parameter list for the `impl Trait` type alias",
-                                    ty
-                                ),
-                            )
-                            .emit();
-
-                        self.tcx().ty_error()
-                    }
-                }
-            }
-
-            _ => ty.super_fold_with(self),
-        }
-    }
-
-    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        trace!("checking const {:?}", ct);
-        // Find a const parameter
-        match ct.kind() {
-            ty::ConstKind::Param(..) => {
-                // Look it up in the substitution list.
-                match self.map.get(&ct.into()).map(|k| k.unpack()) {
-                    // Found it in the substitution list, replace with the parameter from the
-                    // opaque type.
-                    Some(GenericArgKind::Const(c1)) => c1,
-                    Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
-                    None => {
-                        self.tcx.sess.emit_err(ConstNotUsedTraitAlias {
-                            ct: ct.to_string(),
-                            span: self.span,
-                        });
-
-                        self.tcx().const_error(ct.ty())
-                    }
-                }
-            }
-
-            _ => ct,
-        }
-    }
-}

compiler/rustc_borrowck/src/session_diagnostics.rs

@@ -52,15 +52,6 @@ pub(crate) struct VarNeedNotMut {
     #[suggestion_short(applicability = "machine-applicable", code = "")]
     pub span: Span,
 }
-
-#[derive(Diagnostic)]
-#[diag(borrowck::const_not_used_in_type_alias)]
-pub(crate) struct ConstNotUsedTraitAlias {
-    pub ct: String,
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(borrowck::var_cannot_escape_closure)]
 #[note]

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -511,3 +511,11 @@ impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx> {
         c.super_visit_with(self)
     }
 }
+
+#[derive(Diagnostic)]
+#[diag(borrowck::const_not_used_in_type_alias)]
+pub(super) struct ConstNotUsedTraitAlias {
+    pub ct: String,
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_middle/src/ty/mod.rs

@@ -131,6 +131,7 @@ mod generics;
 mod impls_ty;
 mod instance;
 mod list;
+mod opaque_types;
 mod parameterized;
 mod rvalue_scopes;
 mod structural_impls;
@@ -1300,6 +1301,32 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
             sub: sub_diag,
         });
     }
+
+    #[instrument(level = "debug", skip(tcx), ret)]
+    pub fn remap_generic_params_to_declaration_params(
+        self,
+        opaque_type_key: OpaqueTypeKey<'tcx>,
+        tcx: TyCtxt<'tcx>,
+    ) -> Self {
+        let OpaqueTypeKey { def_id, substs } = opaque_type_key;
+
+        // Use substs to build up a reverse map from regions to their
+        // identity mappings. This is necessary because of `impl
+        // Trait` lifetimes are computed by replacing existing
+        // lifetimes with 'static and remapping only those used in the
+        // `impl Trait` return type, resulting in the parameters
+        // shifting.
+        let id_substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
+        debug!(?id_substs);
+        let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> =
+            substs.iter().enumerate().map(|(index, subst)| (subst, id_substs[index])).collect();
+        debug!("map = {:#?}", map);
+
+        // Convert the type from the function into a type valid outside
+        // the function, by replacing invalid regions with 'static,
+        // after producing an error for each of them.
+        self.fold_with(&mut opaque_types::ReverseMapper::new(tcx, map, self.span))
+    }
 }
 
 /// The "placeholder index" fully defines a placeholder region, type, or const. Placeholders are