Bind the structure of the value a mutable borrow prophesies

[coord-e]

Fixes #176.

Cause

Env::borrow_var rebinds the borrowed variable to the borrow's prophecy variable, which stands for the value the place takes once the borrow ends. That variable comes from push_temp_var, so it is bound as a value (TempVarBinding::Type), not with the FlowBinding that locate_place and borrow_var walk to reach through a reference, a box, a tuple, or an enum. A place whose value has structure therefore lost that structure as soon as it was borrowed.

Writing to a mut local is elaborated into a borrow of the local's slot (ReborrowVisitor::visit_assign), which is why the issue's straight-line reassignment hit exactly that: r = &mut b left r's content standing for the prophecy, and elaborating *r = 20 into a reborrow of *r reached borrow_var with no binding — borrowing unbound var. The same reassignment of a tuple local panics with deref unbound var when a field is projected afterwards. Reading through the reassigned local was fine, since reads go through place_type, which needs no flow binding; and the cross-basic-block form was fine too, since a block entry re-binds its locals from the block type.

Fix

bind_prophesied_value binds a fresh variable refined to equal the prophecy through bind_impl, so the prophesied value carries the structure of the value it replaces. A value that had no flow binding to begin with keeps standing for the prophecy variable directly, so the constraints generated for a borrow of a scalar are unchanged.

Tests

tests/ui/{pass,fail}/reassign_mut_ref.rs write through a &mut local both before and after reassigning it, and check both referents afterwards; the failing side breaks the assertion on the reassigned referent.

Also checked by hand, all verifying as expected after the fix and all but the last panicking before it:

Program	Before	After
r = &mut b; *r = 20; assert!(b == 20)	borrowing unbound var	passes
r = &mut b; *r = 20; assert!(b == 21)	borrowing unbound var	Unsat
r = &mut b; *r = 20; assert!(a == 20)	borrowing unbound var	Unsat
r = &mut a; *r = 5; assert!(a == 5) (same referent)	borrowing unbound var	passes
t = (3, 4); t.0 = 5; assert!(t.0 == 5 && t.1 == 4)	deref unbound var	passes
x = Box::new(2); *x = 3; assert!(*x == 3)	passes	passes

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