Allow mutable borrows to coerce into immutable

reid/src/mir/typecheck/mod.rs

@@ -152,7 +152,7 @@ impl TypeKind {
             (TypeKind::Borrow(val1, mut1), TypeKind::Borrow(val2, mut2)) => {
                 // Extracted to give priority for other collapse-error
                 let collapsed = val1.narrow_into(val2)?;
-                if mut1 == mut2 {
+                if mut1 == mut2 || (*mut1 && !mut2) {
                     Ok(TypeKind::Borrow(Box::new(collapsed), *mut1 && *mut2))
                 } else {
                     Err(ErrorKind::TypesDifferMutability(self.clone(), other.clone()))