There was a bug that would cause the alias analyser to allow you to
invalidate an alias that was no longer directly referred to, even if
another alias was rooted in it. It now properly tracks dependencies
between live aliases.
Required another case of copying values in map.rs.
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.
Some of the vec utilities now only work on immutable vecs, since they
would have to be rewritten to do a lot more copying to be alias-safe.
Some forced copying was added to map.rs, showing a weakness in the
alias checker (or maybe the alias system): when fn args are passed
into a function, calling them must assume all aliases that are not
immutably rooted (directly connected to a local or temporary without
any mutable edges) become invalid. This will be a drag on functional
programming in Rust.
Work around alias issues in the stdlib
The alias checker works by ensuring that any value to which an alias
is created is rooted in some way that ensures it outlives the alias.
It is now disallowed to create an alias to the content of a mutable
box, or to a box hanging off a mutable field. There is also machinery
in place to prevent assignment to local variables whenever they are
the root of a live alias.
This litters aberrations like 'alt({foo.bar}) { ... }' and f({*baz})
though the code (mostly in trans.rs). These are a way to explicitly
copy the given value so that it can be safely aliased. At some point
we'll probably want a more explicit copy operator.
