Add initial support for unsized method resolution

In order to support slices, we end up with an operator overload call of:

```
impl<T, I> Index<I> for [T]
where
    I: SliceIndex<[T]>,
{
    type Output = I::Output;

    fn index(&self, index: I) -> &I::Output {
        index.index(self)
    }
}
```

So this means the self in this case is an array[T,capacity] and the index parameter is of type Range<usize>. In order to actually call this method
which has a self parameter of [T] we need to be able to 'unsize' the array
into a slice.

Addresses #849

7 files changed, 82 insertions, 0 deletions

diff --git a/gcc/rust/backend/rust-compile-base.h b/gcc/rust/backend/rust-compile-base.h
index c30aa4d..c7f7f40 100644
--- a/gcc/rust/backend/rust-compile-base.h
+++ b/gcc/rust/backend/rust-compile-base.h
@@ -72,6 +72,9 @@ protected:
   tree resolve_indirection_adjustment (Resolver::Adjustment &adjustment,
 				       tree expression, Location locus);
 
+  tree resolve_unsized_adjustment (Resolver::Adjustment &adjustment,
+				   tree expression, Location locus);
+
   static void setup_attributes_on_fndecl (
     tree fndecl, bool is_main_entry_point, HIR::Visibility &visibility,
     const HIR::FunctionQualifiers &qualifiers, const AST::AttrVec &attrs);
diff --git a/gcc/rust/backend/rust-compile-expr.cc b/gcc/rust/backend/rust-compile-expr.cc
index 03e3c2e..74ab6a4 100644
--- a/gcc/rust/backend/rust-compile-expr.cc
+++ b/gcc/rust/backend/rust-compile-expr.cc
@@ -1204,6 +1204,10 @@ HIRCompileBase::resolve_adjustements (
 	case Resolver::Adjustment::AdjustmentType::INDIRECTION:
 	  e = resolve_indirection_adjustment (adjustment, e, locus);
 	  break;
+
+	case Resolver::Adjustment::AdjustmentType::UNSIZE:
+	  e = resolve_unsized_adjustment (adjustment, e, locus);
+	  break;
 	}
     }
 
@@ -1257,6 +1261,32 @@ HIRCompileBase::resolve_indirection_adjustment (
 						   locus);
 }
 
+tree
+HIRCompileBase::resolve_unsized_adjustment (Resolver::Adjustment &adjustment,
+					    tree expression, Location locus)
+{
+  // assumes this is an array
+  tree expr_type = TREE_TYPE (expression);
+  rust_assert (TREE_CODE (expr_type) == ARRAY_TYPE);
+
+  // takes an array and returns a fat-pointer so this becomes a constructor
+  // expression
+  rust_assert (adjustment.get_expected ()->get_kind ()
+	       == TyTy::TypeKind::SLICE);
+  tree fat_pointer
+    = TyTyResolveCompile::compile (ctx, adjustment.get_expected ());
+
+  // make a constructor for this
+  tree data = address_expression (expression, locus);
+
+  // fetch the size from the domain
+  tree domain = TYPE_DOMAIN (expr_type);
+  tree size = TYPE_MAX_VALUE (domain);
+
+  return ctx->get_backend ()->constructor_expression (fat_pointer, false,
+						      {data, size}, -1, locus);
+}
+
 void
 CompileExpr::visit (HIR::IdentifierExpr &expr)
 {
diff --git a/gcc/rust/typecheck/rust-autoderef.cc b/gcc/rust/typecheck/rust-autoderef.cc
index 2862252..67c1908 100644
--- a/gcc/rust/typecheck/rust-autoderef.cc
+++ b/gcc/rust/typecheck/rust-autoderef.cc
@@ -98,6 +98,27 @@ Adjuster::try_raw_deref_type (const TyTy::BaseType *ty)
   return Adjustment (Adjustment::AdjustmentType::INDIRECTION, infered);
 }
 
+Adjustment
+Adjuster::try_unsize_type (const TyTy::BaseType *ty)
+{
+  bool is_valid_type = ty->get_kind () == TyTy::TypeKind::ARRAY;
+  if (!is_valid_type)
+    return Adjustment::get_error ();
+
+  auto mappings = Analysis::Mappings::get ();
+  auto context = TypeCheckContext::get ();
+
+  const auto ref_base = static_cast<const TyTy::ArrayType *> (ty);
+  auto slice_elem = ref_base->get_element_type ();
+
+  auto slice
+    = new TyTy::SliceType (mappings->get_next_hir_id (), ty->get_ident ().locus,
+			   TyTy::TyVar (slice_elem->get_ref ()));
+  context->insert_implicit_type (slice);
+
+  return Adjustment (Adjustment::AdjustmentType::INDIRECTION, slice);
+}
+
 static bool
 resolve_operator_overload_fn (
   Analysis::RustLangItem::ItemType lang_item_type, const TyTy::BaseType *ty,
diff --git a/gcc/rust/typecheck/rust-autoderef.h b/gcc/rust/typecheck/rust-autoderef.h
index f389aff..369744e 100644
--- a/gcc/rust/typecheck/rust-autoderef.h
+++ b/gcc/rust/typecheck/rust-autoderef.h
@@ -36,6 +36,7 @@ public:
     DEREF,
     DEREF_MUT,
     INDIRECTION,
+    UNSIZE,
   };
 
   // ctor for all adjustments except derefs
@@ -78,6 +79,8 @@ public:
 	return "DEREF_MUT";
       case AdjustmentType::INDIRECTION:
 	return "INDIRECTION";
+      case AdjustmentType::UNSIZE:
+	return "UNSIZE";
       }
     gcc_unreachable ();
     return "";
@@ -135,6 +138,8 @@ public:
 
   static Adjustment try_raw_deref_type (const TyTy::BaseType *ty);
 
+  static Adjustment try_unsize_type (const TyTy::BaseType *ty);
+
 private:
   const TyTy::BaseType *base;
 };
diff --git a/gcc/rust/typecheck/rust-hir-dot-operator.cc b/gcc/rust/typecheck/rust-hir-dot-operator.cc
index 84bbc0c..ce7c4e1 100644
--- a/gcc/rust/typecheck/rust-hir-dot-operator.cc
+++ b/gcc/rust/typecheck/rust-hir-dot-operator.cc
@@ -40,6 +40,21 @@ MethodResolver::Probe (const TyTy::BaseType *receiver,
       if (autoderef_flag)
 	return MethodCandidate::get_error ();
 
+      // try unsize
+      Adjustment unsize = Adjuster::try_unsize_type (r);
+      if (!unsize.is_error ())
+	{
+	  adjustments.push_back (unsize);
+	  auto unsize_r = unsize.get_expected ();
+	  auto res = Try (unsize_r, segment_name, adjustments);
+	  if (!res.is_error ())
+	    {
+	      return res;
+	    }
+
+	  adjustments.pop_back ();
+	}
+
       Adjustment deref
 	= Adjuster::try_deref_type (r, Analysis::RustLangItem::ItemType::DEREF);
       if (!deref.is_error ())
diff --git a/gcc/rust/typecheck/rust-hir-type-check.h b/gcc/rust/typecheck/rust-hir-type-check.h
index 49141b3..e63ef9f 100644
--- a/gcc/rust/typecheck/rust-hir-type-check.h
+++ b/gcc/rust/typecheck/rust-hir-type-check.h
@@ -103,6 +103,7 @@ public:
 
   void insert_type (const Analysis::NodeMapping &mappings,
 		    TyTy::BaseType *type);
+  void insert_implicit_type (TyTy::BaseType *type);
   bool lookup_type (HirId id, TyTy::BaseType **type);
 
   void insert_implicit_type (HirId id, TyTy::BaseType *type);
diff --git a/gcc/rust/typecheck/rust-tyctx.cc b/gcc/rust/typecheck/rust-tyctx.cc
index bd8e14f..434809c 100644
--- a/gcc/rust/typecheck/rust-tyctx.cc
+++ b/gcc/rust/typecheck/rust-tyctx.cc
@@ -84,6 +84,13 @@ TypeCheckContext::insert_type (const Analysis::NodeMapping &mappings,
 }
 
 void
+TypeCheckContext::insert_implicit_type (TyTy::BaseType *type)
+{
+  rust_assert (type != nullptr);
+  resolved[type->get_ref ()] = type;
+}
+
+void
 TypeCheckContext::insert_implicit_type (HirId id, TyTy::BaseType *type)
 {
   rust_assert (type != nullptr);