Generate appropriate constraints for calls to functions with
preconditions, and reject calls where those constraints don't
hold true in the prestate.
...by which I mean that it works for one test case :-)
Before, all aliases were implicitly mutable, and writing
&mutable was the same as writing &. Now, the two are
distinguished, and assignments to regular aliases are
no longer allowed.
Changed function types to include a list of constraints. Added
code for parsing and pretty-printing constraints. This necessitated
splitting pprust into two files (pprust and ppaux) to break a
circulate dependency, as ty_to_str now needs to print out constraints,
which may include literals, but pprust depended on ty.
The old system tried to ensure that the location an alias pointed at
would retain its type. That turned out to not be strong enough in the
face of aliases to the inside of tags.
The new system instead proves that values pointed to by aliases are
not replaced (or invalidated in some other way) at all. It knows of
two sufficient conditions for this, and tries to prove at least of
them:
A) The alias is 'immutably rooted' in a local, and this local is not
reassigned for the lifetime of the alias. Immutably rooted means
the alias refers to the local itself, or to something reachable
from the local through immutable dereferencing.
B) No value whose type might include the type of the 'inner mutable
element' of the thing the alias refers to (for example, the box in
rec(mutable x = @mutable int)) is from the outer scope is accessed
for the lifetime of the alias. This means for functions, no other
argument types may include the alias's inner mutable type. For alt,
for each, and for, it means the body does not refer to any locals
originating from outside their scope that include this type.
The lifetime of an alias in an alt, for each, or for body is defined
as the range from its definition to its last use, not to the point
where it goes out of scope. This makes working around these
restrictions somewhat less annoying. For example, you can assign to
your alt-ed value you don't refer to any bindings afterwards.
