In the `Tree` and `Dfa` representations of a type's layout, store byte
ranges rather than needing to separately store each byte value. This
permits us to, for example, represent a `u8` using a single 0..=255 edge
in the DFA rather than using 256 separate edges.
This leads to drastic performance improvements. For example, on the
author's 2024 MacBook Pro, the time to convert the `Tree` representation
of a `u64` to its equivalent DFA representation drops from ~8.5ms to
~1us, a reduction of ~8,500x. See `bench_dfa_from_tree`.
Similarly, the time to execute a transmutability query from `u64` to
`u64` drops from ~35us to ~1.7us, a reduction of ~20x. See
`bench_transmute`.
Prior to this commit, the transmutability analysis used an intermediate
NFA representation of type layout. We then determinized this
representation into a DFA, upon which we ran the core transmutability
analysis. Unfortunately, determinizing NFAs is expensive. In this
commit, we avoid NFAs entirely by observing that Rust `union`s are the
only source of nondeterminism and that it is comparatively cheap to
compute the DFA union of DFAs.
We also implement Graphviz DOT debug formatting of DFAs.
Fixesrust-lang/project-safe-transmute#23Fixesrust-lang/project-safe-transmute#24
Modifies `BikeshedIntrinsicFrom` to forbid lifetime extensions on
references. This static check can be opted out of with the
`Assume::lifetimes` flag.
Fixes#129097
The source referent absolutely must be smaller than the destination
referent of a ref-to-ref transmute; the excess bytes referenced
cannot arise from thin air, even if those bytes are uninitialized.
- Only create dst -> src obligation if Dst is mutable
- Add some long comments to explain parts of the transmutability code that were
unclear to me when reading
- Update/add tests
This patch enables support for references in Safe Transmute, by generating
nested obligations during trait selection. Specifically, when we call
`confirm_transmutability_candidate(...)`, we now recursively traverse the
`rustc_transmute::Answer` tree and create obligations for all the `Answer`
variants, some of which include multiple nested `Answer`s.
Also, to handle recursive types, enable support for coinduction for the Safe
Transmute trait (`BikeshedIntrinsicFrom`) by adding the `#[rustc_coinduction]`
annotation.
Also fix some small logic issues when reducing the `or` and `and` combinations
in `rustc_transmute`, so that we don't end up with additional redundant
`Answer`s in the tree.
Co-authored-by: Jack Wrenn <jack@wrenn.fyi>
Convert all the crates that have had their diagnostic migration
completed (except save_analysis because that will be deleted soon and
apfloat because of the licensing problem).
This initial implementation handles transmutations between types with specified layouts, except when references are involved.
Co-authored-by: Igor null <m1el.2027@gmail.com>
