Handle const blocks

[CohenArthur]

This adds some handling towards const { ... } expressions. I haven't added specific handling for const folding them, but they are properly handled by the const checker

[philberty]

should this not be in the visitor above?

[philberty]

LGTM implementing this should be straight forward in the backend if you look at rust-compile-item.cc ConstantDecl we have wrapper for supporting constant blocks.

  ctx->push_const_context ();
  tree value
    = compile_constant_item (var.get_mappings ().get_hirid (), expr_type,
			     resolved_type, *canonical_path, const_value_expr,
			     var.get_locus (), const_value_expr.get_locus ());
  ctx->pop_const_context ();

Then you can do whatever you need with the value for your case you will be implementing this in rust-compile-expr.cc so it will just be:

  TyTy::BaseType *expr_type = nullptr;
  bool ok = ctx->get_tyctx ()->lookup_type (const.get_mappings ().get_hirid (),
					    &expr_type);
  rust_assert (ok);
  
    Resolver::CanonicalPath canonical_path
    = Resolver::CanonicalPath::create_empty ();
    
    HIR::Expr* const_value_expr = const.get_const_block ();

  ctx->push_const_context ();
  translated
    = compile_constant_item (const.get_mappings ().get_hirid (), expr_type,
			     expr_type, *canonical_path, const_value_expr,
			     const.get_locus (), const_value_expr.get_locus ());
  ctx->pop_const_context ();

something like that should just do it