Initial (incomplete) implementation of transmutability trait.

This initial implementation handles transmutations between types with specified layouts, except when references are involved.

Co-authored-by: Igor null <m1el.2027@gmail.com>

compiler/rustc_transmute/src/lib.rs

@@ -0,0 +1,114 @@
+#![feature(
+    alloc_layout_extra,
+    control_flow_enum,
+    decl_macro,
+    iterator_try_reduce,
+    never_type,
+    result_into_ok_or_err
+)]
+#![allow(dead_code, unused_variables)]
+
+#[cfg(feature = "rustc")]
+pub(crate) use rustc_data_structures::fx::{FxHashMap as Map, FxHashSet as Set};
+
+#[cfg(not(feature = "rustc"))]
+pub(crate) use std::collections::{HashMap as Map, HashSet as Set};
+
+pub(crate) mod layout;
+pub(crate) mod maybe_transmutable;
+
+#[derive(Default)]
+pub struct Assume {
+    pub alignment: bool,
+    pub lifetimes: bool,
+    pub validity: bool,
+    pub visibility: bool,
+}
+
+/// The type encodes answers to the question: "Are these types transmutable?"
+#[derive(Debug, Hash, Eq, PartialEq, PartialOrd, Ord, Clone)]
+pub enum Answer<R>
+where
+    R: layout::Ref,
+{
+    /// `Src` is transmutable into `Dst`.
+    Yes,
+
+    /// `Src` is NOT transmutable into `Dst`.
+    No(Reason),
+
+    /// `Src` is transmutable into `Dst`, if `src` is transmutable into `dst`.
+    IfTransmutable { src: R, dst: R },
+
+    /// `Src` is transmutable into `Dst`, if all of the enclosed requirements are met.
+    IfAll(Vec<Answer<R>>),
+
+    /// `Src` is transmutable into `Dst` if any of the enclosed requirements are met.
+    IfAny(Vec<Answer<R>>),
+}
+
+/// Answers: Why wasn't the source type transmutable into the destination type?
+#[derive(Debug, Hash, Eq, PartialEq, PartialOrd, Ord, Clone)]
+pub enum Reason {
+    /// The layout of the source type is unspecified.
+    SrcIsUnspecified,
+    /// The layout of the destination type is unspecified.
+    DstIsUnspecified,
+    /// The layout of the destination type is bit-incompatible with the source type.
+    DstIsBitIncompatible,
+    /// There aren't any public constructors for `Dst`.
+    DstIsPrivate,
+    /// `Dst` is larger than `Src`, and the excess bytes were not exclusively uninitialized.
+    DstIsTooBig,
+}
+
+#[cfg(feature = "rustc")]
+mod rustc {
+    use rustc_infer::infer::InferCtxt;
+    use rustc_macros::{TypeFoldable, TypeVisitable};
+    use rustc_middle::traits::ObligationCause;
+    use rustc_middle::ty::Binder;
+    use rustc_middle::ty::Ty;
+
+    /// The source and destination types of a transmutation.
+    #[derive(TypeFoldable, TypeVisitable, Debug, Clone, Copy)]
+    pub struct Types<'tcx> {
+        /// The source type.
+        pub src: Ty<'tcx>,
+        /// The destination type.
+        pub dst: Ty<'tcx>,
+    }
+
+    pub struct TransmuteTypeEnv<'cx, 'tcx> {
+        infcx: &'cx InferCtxt<'cx, 'tcx>,
+    }
+
+    impl<'cx, 'tcx> TransmuteTypeEnv<'cx, 'tcx> {
+        pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> Self {
+            Self { infcx }
+        }
+
+        #[allow(unused)]
+        pub fn is_transmutable(
+            &mut self,
+            cause: ObligationCause<'tcx>,
+            src_and_dst: Binder<'tcx, Types<'tcx>>,
+            scope: Ty<'tcx>,
+            assume: crate::Assume,
+        ) -> crate::Answer<crate::layout::rustc::Ref<'tcx>> {
+            let src = src_and_dst.map_bound(|types| types.src).skip_binder();
+            let dst = src_and_dst.map_bound(|types| types.dst).skip_binder();
+            crate::maybe_transmutable::MaybeTransmutableQuery::new(
+                src,
+                dst,
+                scope,
+                assume,
+                self.infcx.tcx,
+            )
+            .answer()
+        }
+    }
+}
+
+#[cfg(feature = "rustc")]
+pub use rustc::*;