Add initial support for defaults on generic parameters

In the case that the GenericArgs to a type/fn are less than the expected
number of required substitutions we need to look for what is the min
number of required substitutions which might be zero if each TypeParam
has a default.

In the event we have less than expected arguments we can substitute the
defaults if available as the GenericArgumentMappings.

Addresses: #307

3 files changed, 43 insertions, 1 deletions

diff --git a/gcc/rust/hir/tree/rust-hir-item.h b/gcc/rust/hir/tree/rust-hir-item.h
index dbbc743..648999b 100644
--- a/gcc/rust/hir/tree/rust-hir-item.h
+++ b/gcc/rust/hir/tree/rust-hir-item.h
@@ -117,6 +117,12 @@ public:
     return type;
   }
 
+  Analysis::NodeMapping get_type_mappings () const
+  {
+    rust_assert (type != nullptr);
+    return type->get_mappings ();
+  }
+
 protected:
   // Clone function implementation as (not pure) virtual method
   TypeParam *clone_generic_param_impl () const override
diff --git a/gcc/rust/typecheck/rust-tyty.cc b/gcc/rust/typecheck/rust-tyty.cc
index be56dc7..3e9098d 100644
--- a/gcc/rust/typecheck/rust-tyty.cc
+++ b/gcc/rust/typecheck/rust-tyty.cc
@@ -233,7 +233,7 @@ SubstitutionRef::get_mappings_from_generic_args (HIR::GenericArgs &args)
       return SubstitutionArgumentMappings::error ();
     }
 
-  if (args.get_type_args ().size () < substitutions.size ())
+  if (args.get_type_args ().size () < min_required_substitutions ())
     {
       RichLocation r (args.get_locus ());
       r.add_range (substitutions.front ().get_param_locus ());
@@ -260,6 +260,22 @@ SubstitutionRef::get_mappings_from_generic_args (HIR::GenericArgs &args)
       mappings.push_back (std::move (subst_arg));
     }
 
+  // we must need to fill out defaults
+  size_t left_over
+    = num_required_substitutions () - min_required_substitutions ();
+  if (left_over > 0)
+    {
+      for (size_t offs = mappings.size (); offs < substitutions.size (); offs++)
+	{
+	  SubstitutionParamMapping &param = substitutions.at (offs);
+	  rust_assert (param.param_has_default_ty ());
+
+	  BaseType *resolved = param.get_default_ty ();
+	  SubstitutionArg subst_arg (&param, resolved);
+	  mappings.push_back (std::move (subst_arg));
+	}
+    }
+
   return SubstitutionArgumentMappings (mappings, args.get_locus ());
 }
 
diff --git a/gcc/rust/typecheck/rust-tyty.h b/gcc/rust/typecheck/rust-tyty.h
index 5beedda..d6a3aef 100644
--- a/gcc/rust/typecheck/rust-tyty.h
+++ b/gcc/rust/typecheck/rust-tyty.h
@@ -449,6 +449,12 @@ public:
 
   bool param_has_default_ty () const { return generic.has_type (); }
 
+  BaseType *get_default_ty () const
+  {
+    TyVar var (generic.get_type_mappings ().get_hirid ());
+    return var.get_tyty ();
+  }
+
 private:
   const HIR::TypeParam &generic;
   ParamType *param;
@@ -643,6 +649,7 @@ public:
     return used_arguments;
   }
 
+  // this is the count of type params that are not substituted fuly
   size_t num_required_substitutions () const
   {
     size_t n = 0;
@@ -654,6 +661,19 @@ public:
     return n;
   }
 
+  // this is the count of type params that need substituted taking into account
+  // possible defaults
+  size_t min_required_substitutions () const
+  {
+    size_t n = 0;
+    for (auto &p : substitutions)
+      {
+	if (p.needs_substitution () && !p.param_has_default_ty ())
+	  n++;
+      }
+    return n;
+  }
+
   // We are trying to subst <i32, f32> into Struct Foo<X,Y> {}
   // in the case of Foo<i32,f32>{...}
   //