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/layout/mod.rs

@@ -0,0 +1,71 @@
+use std::fmt::{self, Debug};
+use std::hash::Hash;
+
+pub(crate) mod tree;
+pub(crate) use tree::Tree;
+
+pub(crate) mod nfa;
+pub(crate) use nfa::Nfa;
+
+pub(crate) mod dfa;
+pub(crate) use dfa::Dfa;
+
+#[derive(Debug)]
+pub(crate) struct Uninhabited;
+
+/// An instance of a byte is either initialized to a particular value, or uninitialized.
+#[derive(Hash, Eq, PartialEq, Clone, Copy)]
+pub(crate) enum Byte {
+    Uninit,
+    Init(u8),
+}
+
+impl fmt::Debug for Byte {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match &self {
+            Self::Uninit => f.write_str("??u8"),
+            Self::Init(b) => write!(f, "{:#04x}u8", b),
+        }
+    }
+}
+
+pub(crate) trait Def: Debug + Hash + Eq + PartialEq + Copy + Clone {}
+pub trait Ref: Debug + Hash + Eq + PartialEq + Copy + Clone {}
+
+impl Def for ! {}
+impl Ref for ! {}
+
+#[cfg(feature = "rustc")]
+pub(crate) mod rustc {
+    use rustc_middle::mir::Mutability;
+    use rustc_middle::ty;
+    use rustc_middle::ty::Region;
+    use rustc_middle::ty::Ty;
+
+    /// A reference in the layout [`Nfa`].
+    #[derive(Debug, Hash, Eq, PartialEq, PartialOrd, Ord, Clone, Copy)]
+    pub struct Ref<'tcx> {
+        lifetime: Region<'tcx>,
+        ty: Ty<'tcx>,
+        mutability: Mutability,
+    }
+
+    impl<'tcx> super::Ref for Ref<'tcx> {}
+
+    impl<'tcx> Ref<'tcx> {
+        pub fn min_align(&self) -> usize {
+            todo!()
+        }
+    }
+
+    /// A visibility node in the layout [`Nfa`].
+    #[derive(Debug, Hash, Eq, PartialEq, Clone, Copy)]
+    pub enum Def<'tcx> {
+        Adt(ty::AdtDef<'tcx>),
+        Variant(&'tcx ty::VariantDef),
+        Field(&'tcx ty::FieldDef),
+        Primitive,
+    }
+
+    impl<'tcx> super::Def for Def<'tcx> {}
+}