Function rustc_trait_selection::traits::project::poly_project_and_unify_type

pub(in traits) fn poly_project_and_unify_type<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>, 
    obligation: &Obligation<'tcx, PolyProjectionPredicate<'tcx>>
) -> Result<Result<Option<Vec<PredicateObligation<'tcx>>>, InProgress>, MismatchedProjectionTypes<'tcx>>

Evaluates constraints of the form:

for<...> <T as Trait>::U == V

If successful, this may result in additional obligations. Also returns the projection cache key used to track these additional obligations.

Returns

• Err(_): the projection can be normalized, but is not equal to the expected type.
• Ok(Err(InProgress)): this is called recursively while normalizing the same projection.
• Ok(Ok(None)): The projection cannot be normalized due to ambiguity (resolving some inference variables in the projection may fix this).
• Ok(Ok(Some(obligations))): The projection bound holds subject to the given obligations. If the projection cannot be normalized because the required trait bound doesn't hold this returned with obligations being a predicate that cannot be proven.