Safe Transmute: Enable handling references, including recursive types

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>

compiler/rustc_transmute/src/lib.rs

@@ -8,7 +8,7 @@ extern crate tracing;
 
 pub(crate) use rustc_data_structures::fx::{FxIndexMap as Map, FxIndexSet as Set};
 
-pub(crate) mod layout;
+pub mod layout;
 pub(crate) mod maybe_transmutable;
 
 #[derive(Default)]
@@ -21,10 +21,7 @@ pub struct Assume {
 
 /// 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,
-{
+pub enum Answer<R> {
     /// `Src` is transmutable into `Dst`.
     Yes,
 
@@ -54,6 +51,10 @@ pub enum Reason {
     DstIsPrivate,
     /// `Dst` is larger than `Src`, and the excess bytes were not exclusively uninitialized.
     DstIsTooBig,
+    /// Src should have a stricter alignment than Dst, but it does not.
+    DstHasStricterAlignment,
+    /// Can't go from shared pointer to unique pointer
+    DstIsMoreUnique,
 }
 
 #[cfg(feature = "rustc")]