improve cast handling - this fixes test failures
improve cast handling - this fixes test failures

the problem is that now "type_is_known_to_be_sized" now returns
false when called on a type with ty_err inside - this prevents
spurious errors (we may want to move the check to check::cast
anyway - see #12894).

graydon/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust