gccrs: Remove solve_missing_mappings_from_this to handle covariants

change how we monomorphize dyn-items when we need to compute the generics

We might have a trait item such as:

  impl<'a, T> FnLike<&'a T, &'a T> for Identity {
      fn call(&self, arg: &'a T) -> &'a T { ... }
  }

Which ended up monomorphized badly to:

  const isize & const &
    <example::Identity as example::FnLike::<& T, & T>>::call<& isize>
    (const struct example::Identity & const self,
     const isize & const & const arg)

This is wrong because it turned into a double reference type becasuse this
bug was consistent bugs were not picked up but this is not correct. We now
reuse our type inference infrastructure to solve the parameters instead.

Fixes #1984

gcc/rust/ChangeLog:

	* backend/rust-compile.cc: use unify_and instead
	* typecheck/rust-tyty-subst.cc (SubstitutionRef::solve_missing_mappings_from_this): remove
	* typecheck/rust-tyty-subst.h: update header

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

3 files changed, 16 insertions, 65 deletions

diff --git a/gcc/rust/backend/rust-compile.cc b/gcc/rust/backend/rust-compile.cc
index a01662c..2c46992 100644
--- a/gcc/rust/backend/rust-compile.cc
+++ b/gcc/rust/backend/rust-compile.cc
@@ -21,6 +21,8 @@
 #include "rust-compile-implitem.h"
 #include "rust-hir-type-bounds.h"
 #include "rust-compile-type.h"
+#include "rust-substitution-mapper.h"
+#include "rust-type-util.h"
 
 namespace Rust {
 namespace Compile {
@@ -299,11 +301,6 @@ HIRCompileBase::compute_address_for_trait_item (
     = self_bound->lookup_associated_item (ref->get_identifier ());
   rust_assert (!associated_self_item.is_error ());
 
-  TyTy::BaseType *mono1 = associated_self_item.get_tyty_for_receiver (self);
-  rust_assert (mono1 != nullptr);
-  rust_assert (mono1->get_kind () == TyTy::TypeKind::FNDEF);
-  TyTy::FnType *assocated_item_ty1 = static_cast<TyTy::FnType *> (mono1);
-
   // Lookup the impl-block for the associated impl_item if it exists
   HIR::Function *associated_function = nullptr;
   for (auto &impl_item : associated_impl_block->get_impl_items ())
@@ -333,10 +330,14 @@ HIRCompileBase::compute_address_for_trait_item (
 
       if (lookup_fntype->needs_substitution ())
 	{
-	  TyTy::SubstitutionArgumentMappings mappings
-	    = assocated_item_ty1->solve_missing_mappings_from_this (
-	      *trait_item_fntype, *lookup_fntype);
-	  lookup_fntype = lookup_fntype->handle_substitions (mappings);
+	  TyTy::BaseType *infer
+	    = Resolver::SubstMapper::InferSubst (lookup_fntype, Location ());
+	  infer
+	    = Resolver::unify_site (infer->get_ref (),
+				    TyTy::TyWithLocation (trait_item_fntype),
+				    TyTy::TyWithLocation (infer), Location ());
+	  rust_assert (infer->get_kind () == TyTy::TypeKind::FNDEF);
+	  lookup_fntype = static_cast<TyTy::FnType *> (infer);
 	}
 
       return CompileInherentImplItem::Compile (associated_function, ctx,
diff --git a/gcc/rust/typecheck/rust-tyty-subst.cc b/gcc/rust/typecheck/rust-tyty-subst.cc
index 6b77a9e..db31ff9 100644
--- a/gcc/rust/typecheck/rust-tyty-subst.cc
+++ b/gcc/rust/typecheck/rust-tyty-subst.cc
@@ -821,57 +821,6 @@ SubstitutionRef::solve_mappings_from_receiver_for_self (
 				       mappings.get_locus ());
 }
 
-SubstitutionArgumentMappings
-SubstitutionRef::solve_missing_mappings_from_this (SubstitutionRef &ref,
-						   SubstitutionRef &to)
-{
-  rust_assert (!ref.needs_substitution ());
-  rust_assert (needs_substitution ());
-  rust_assert (get_num_substitutions () == ref.get_num_substitutions ());
-
-  Location locus = used_arguments.get_locus ();
-  std::vector<SubstitutionArg> resolved_mappings;
-
-  std::map<HirId, std::pair<ParamType *, BaseType *>> substs;
-  for (size_t i = 0; i < get_num_substitutions (); i++)
-    {
-      SubstitutionParamMapping &a = substitutions.at (i);
-      SubstitutionParamMapping &b = ref.substitutions.at (i);
-
-      if (a.need_substitution ())
-	{
-	  const BaseType *root = a.get_param_ty ()->resolve ()->get_root ();
-	  rust_assert (root->get_kind () == TyTy::TypeKind::PARAM);
-	  const ParamType *p = static_cast<const TyTy::ParamType *> (root);
-
-	  substs[p->get_ty_ref ()] = {static_cast<ParamType *> (p->clone ()),
-				      b.get_param_ty ()->resolve ()};
-	}
-    }
-
-  for (auto it = substs.begin (); it != substs.end (); it++)
-    {
-      HirId param_id = it->first;
-      BaseType *arg = it->second.second;
-
-      const SubstitutionParamMapping *associate_param = nullptr;
-      for (SubstitutionParamMapping &p : to.substitutions)
-	{
-	  if (p.get_param_ty ()->get_ty_ref () == param_id)
-	    {
-	      associate_param = &p;
-	      break;
-	    }
-	}
-
-      rust_assert (associate_param != nullptr);
-      SubstitutionArg argument (associate_param, arg);
-      resolved_mappings.push_back (std::move (argument));
-    }
-
-  return SubstitutionArgumentMappings (resolved_mappings, {}, locus);
-}
-
 Resolver::AssociatedImplTrait *
 SubstitutionRef::lookup_associated_impl (const SubstitutionParamMapping &subst,
 					 const TypeBoundPredicate &bound,
diff --git a/gcc/rust/typecheck/rust-tyty-subst.h b/gcc/rust/typecheck/rust-tyty-subst.h
index 365fdb6..b5a4181 100644
--- a/gcc/rust/typecheck/rust-tyty-subst.h
+++ b/gcc/rust/typecheck/rust-tyty-subst.h
@@ -291,11 +291,12 @@ public:
   SubstitutionArgumentMappings solve_mappings_from_receiver_for_self (
     SubstitutionArgumentMappings &mappings) const;
 
-  // TODO comment
-  SubstitutionArgumentMappings
-  solve_missing_mappings_from_this (SubstitutionRef &ref, SubstitutionRef &to);
-
-  // TODO comment
+  // Given a type such as:
+  //
+  //   fn<X,Y>(a:&X, b:Y) -> (...)
+  //
+  // This function will inject implicit inference variables for the type
+  // parameters X and Y
   BaseType *infer_substitions (Location locus);
 
   // this clears any possible projections from higher ranked trait bounds which