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?

[CohenArthur]

so I meant that the const block nodes should always just contain an AnonConst, which is why we do the assertion that it is actually one and then static cast and store an AnonConst instead of a unique_ptr<Expr> in the HIR::ConstBlock

AnonConst should be used within const blocks but also within things like array expressions (e.g. in let x = [15; size] size should technically be represented as an AnonConst) so the visitor needs to be a little generic

[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

[P-E-P]

What does the 414 means here ? Is it supposed to be

rust_error_at (expr.get_locus (), ErrorCode::E0414,
		     "failed to lookup type of BlockExpr");

instead ?

[P-E-P]

Could be replaced with a string_stream

[P-E-P]

We should probably return an unexpected error and let the caller chose instead.