Merge #1268

1268: Fix Slice Type Layout r=philberty a=philberty

Slices in Rust are represented by TypePaths such as '[i32]'. Though if you
actually try to use this explicit type-path you will hit errors such as
this type has an unknown size at compile time. This is because this is
actually what Rust calls a dynamically sized type. This means when you use
types such as: '&[i32]' it is not actually a reference type to a slice. Its
a slice in its entirety this means for lack of a better word when you use
'*const [i32]' or '&mut [i32]' we end up actually passing around a struct
by value _not_ at pointer/reference to it.

This patch changes the type-layout so that we handle this layout change
properly. This patch will also need to be applied to str types which I
believe have a similar layout for safety.

The patch also sets up TYPE_MAIN_VARIANT so that we can avoid unnessecary
view_convert_expressions between *const [i32] and &mut [i32] which will
have the same layout.

Reference:

https://doc.rust-lang.org/reference/dynamically-sized-types.html

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=672adac002939a2dab43b8d231adc1dc

Fixes #1232


Co-authored-by: Philip Herron <philip.herron@embecosm.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/gcc/rust/backend/rust-compile-context.h b/gcc/rust/backend/rust-compile-context.h
index 8378554..096b65f 100644
--- a/gcc/rust/backend/rust-compile-context.h
+++ b/gcc/rust/backend/rust-compile-context.h
@@ -67,6 +67,17 @@ public:
     return type;
   }
 
+  tree insert_main_variant (tree type)
+  {
+    hashval_t h = type_hasher (type);
+    auto it = main_variants.find (h);
+    if (it != main_variants.end ())
+      return it->second;
+
+    main_variants.insert ({h, type});
+    return type;
+  }
+
   ::Backend *get_backend () { return backend; }
   Resolver::Resolver *get_resolver () { return resolver; }
   Resolver::TypeCheckContext *get_tyctx () { return tyctx; }
@@ -314,6 +325,7 @@ private:
   std::map<DefId, std::vector<std::pair<const TyTy::BaseType *, tree>>>
     mono_fns;
   std::map<HirId, tree> implicit_pattern_bindings;
+  std::map<hashval_t, tree> main_variants;
 
   // To GCC middle-end
   std::vector<tree> type_decls;