Add code generation for the slice type

This type must respect the layout of the FatPtr type in libcore. Rust
implements slices using Rustc types in libcore and uses a neat trick.

The slice is generated into the FatPtr which contains the pointer and
length of the slice. This is then placed into a union called Repr which
has 3 variants a mutable and immutable pointer to the FatPtr and a final
variant which is the raw FatPtr. This means we can use unsafe access to
the union to gain a pointer to the FatPtr.

Addresses #849

gcc/rust/backend/rust-compile-type.cc

@@ -349,8 +349,38 @@ TyTyResolveCompile::visit (const TyTy::ArrayType &type)
 void
 TyTyResolveCompile::visit (const TyTy::SliceType &type)
 {
-  // TODO
-  gcc_unreachable ();
+  if (ctx->lookup_compiled_types (type.get_ty_ref (), &translated, &type))
+    return;
+
+  std::vector<Backend::typed_identifier> fields;
+
+  tree element_type
+    = TyTyResolveCompile::compile (ctx, type.get_element_type ());
+  tree data_field_ty = build_pointer_type (element_type);
+  Backend::typed_identifier data_field ("data", data_field_ty, Location ());
+  fields.push_back (std::move (data_field));
+
+  // lookup usize
+  TyTy::BaseType *usize = nullptr;
+  bool ok = ctx->get_tyctx ()->lookup_builtin ("usize", &usize);
+  rust_assert (ok);
+
+  tree len_field_ty = TyTyResolveCompile::compile (ctx, usize);
+  Backend::typed_identifier len_field ("len", len_field_ty, Location ());
+  fields.push_back (std::move (len_field));
+
+  tree type_record = ctx->get_backend ()->struct_type (fields);
+
+  std::string named_struct_str
+    = std::string ("[") + type.get_element_type ()->get_name () + "]";
+  tree named_struct
+    = ctx->get_backend ()->named_type (named_struct_str, type_record,
+				       type.get_ident ().locus);
+
+  ctx->push_type (named_struct);
+  translated = named_struct;
+
+  ctx->insert_compiled_type (type.get_ty_ref (), named_struct, &type);
 }
 
 void

gcc/testsuite/rust/execute/torture/slice1.rs

@@ -0,0 +1,27 @@
+// { dg-additional-options "-w" }
+struct FatPtr<T> {
+    data: *const T,
+    len: usize,
+}
+
+union Repr<T> {
+    rust: *const [T],
+    rust_mut: *mut [T],
+    raw: FatPtr<T>,
+}
+
+const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
+    unsafe {
+        let a = FatPtr { data, len };
+        let b = Repr { raw: a };
+        b.rust
+    }
+}
+
+fn main() -> i32 {
+    let a = 123;
+    let b: *const i32 = &a;
+    let c = slice_from_raw_parts(b, 1);
+
+    0
+}