Handle borrows of unions in NLL

text/2094-nll.md

@@ -1694,6 +1694,16 @@ if they meet one of the following criteria:
   - so: writing a path like `a` is illegal if `a.b` is borrowed
   - but: writing `a` is legal if `*a` is borrowed, whether or not `a`
     is a shared or mutable reference
+- the loan path has a **shallow prefix** `<base>.<field1>` that accesses a
+  field of a union, and `lvalue` has a prefix of the form `<base>.<field2>`
+  for a _different_ field of the same base union.
+  - so: if `s.u` is a union with distinct fields `a` and `b`, shallowly
+    accessing a path `s.u.a.x` is illegal if `s.u.b` or `s.u.b.y` is borrowed.
+    In here, `<base>` is `s.u`, `<field1>` is `b` and `<field2>` is `a`.
+  - but: unless `s.u.a` is _also_ a union, the access `s.u.a.x` is _legal_
+    if `s.u.a.z` is borrowed, because the same union field is used in both borrows.
+  - the prefix of `lvalue` can be an arbitrary prefix - if `s.u.b.w` is borrowed,
+    then it is illegal to shallowly access `*(*s.u.a.z).t`
 
 For **deep** accesses to the path `lvalue`, we consider borrows relevant
 if they meet one of the following criteria:
@@ -1707,6 +1717,14 @@ if they meet one of the following criteria:
   - so: reading a path like `a` is illegal if `a.b` is mutably
     borrowed, but -- in contrast with shallow accesses -- reading `a` is also
     illegal if `*a` is mutably borrowed
+- the loan path has a **supporting prefix** `<base>.<field1>` that accesses a
+  field of a union, and `lvalue` has a prefix of the form `<base>.<field2>` for
+  a _different_ field with the same base union.
+  - so: if `s.u` is a union with distinct fields `a` and `b`, deeply accessing
+    a path `s.u.a.x` is illegal if `s.u.b`, `s.u.b.w`, or (in contrast with
+    shallow accesses, and as long as both dereferences are of the form `&mut T`)
+    `*(*s.u.b.w).t` is borrowed. In here, `<base>` is `s.u`, `<field1>` is `b`,
+    and `<field2>` is `a`.
 
 **Dropping an lvalue LV.** Dropping an lvalue can be treated as a DEEP
 WRITE, like a move, but this is overly conservative. The rules here