Function translate_user_annotation 

pub fn translate_user_annotation<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    owner: RDefId,
    annotation: &CanonicalUserTypeAnnotation<'tcx>,
    projections: &[UserAnnotationProj],
) -> Result<UserAnnotation<'tcx>, String>

Convert a type annotation into data we can use, by fetching the types that rustc inferred and gathering the predicates that still need to be checked at borrowck time.