Crate generativity
source · [−]Expand description
Create a trusted carrier with a new lifetime that is guaranteed to be
unique among other trusted carriers. When you call make_guard! to make a
unique lifetime, the macro creates a Guard to hold it. This guard can be
converted into an Id, which can be stored in structures to uniquely
“brand” them. A different invocation of the macro will produce a new
lifetime that cannot be unified. The only way to construct these types is
with make_guard! or unsafe code.
use generativity::{Id, make_guard};
struct Struct<'id>(Id<'id>);
make_guard!(a);
Struct(a.into());This is the concept of “generative” lifetime brands. Guard and Id are
invariant over
their lifetime parameter, meaning that it is never valid to substitute or
otherwise coerce Id<'a> into Id<'b>, for any concrete 'a or 'b,
including the 'static lifetime.
Any invariant lifetime can be “trusted” to carry a brand, so long as they
are known to be restricted to carrying a brand, and haven’t been derived
from some untrusted lifetime (or are completely unbound). When using this
library, it is recommended to always use Id<'id> to carry the brand, as
this reduces the risk of accidentally trusting an untrusted lifetime.
Importantly, non-invariant lifetimes cannot be trusted, as the variance
allows lifetimes to be contracted to match and copy the brand lifetime.
Macros
Create a Guard with a unique invariant lifetime (with respect to other
trusted/invariant lifetime brands).