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/maybe_transmutable/tests.rs

@@ -0,0 +1,115 @@
+use super::query_context::test::{Def, UltraMinimal};
+use crate::maybe_transmutable::MaybeTransmutableQuery;
+use crate::{layout, Answer, Reason, Set};
+use itertools::Itertools;
+
+mod bool {
+    use super::*;
+
+    #[test]
+    fn should_permit_identity_transmutation_tree() {
+        println!("{:?}", layout::Tree::<!, !>::bool());
+        let answer = crate::maybe_transmutable::MaybeTransmutableQuery::new(
+            layout::Tree::<Def, !>::bool(),
+            layout::Tree::<Def, !>::bool(),
+            (),
+            crate::Assume { alignment: false, lifetimes: false, validity: true, visibility: false },
+            UltraMinimal,
+        )
+        .answer();
+        assert_eq!(answer, Answer::Yes);
+    }
+
+    #[test]
+    fn should_permit_identity_transmutation_dfa() {
+        let answer = crate::maybe_transmutable::MaybeTransmutableQuery::new(
+            layout::Dfa::<!>::bool(),
+            layout::Dfa::<!>::bool(),
+            (),
+            crate::Assume { alignment: false, lifetimes: false, validity: true, visibility: false },
+            UltraMinimal,
+        )
+        .answer();
+        assert_eq!(answer, Answer::Yes);
+    }
+
+    #[test]
+    fn should_permit_validity_expansion_and_reject_contraction() {
+        let un = layout::Tree::<Def, !>::uninhabited();
+        let b0 = layout::Tree::<Def, !>::from_bits(0);
+        let b1 = layout::Tree::<Def, !>::from_bits(1);
+        let b2 = layout::Tree::<Def, !>::from_bits(2);
+
+        let alts = [b0, b1, b2];
+
+        let into_layout = |alts: Vec<_>| {
+            alts.into_iter().fold(layout::Tree::<Def, !>::uninhabited(), layout::Tree::<Def, !>::or)
+        };
+
+        let into_set = |alts: Vec<_>| {
+            #[cfg(feature = "rustc")]
+            let mut set = Set::default();
+            #[cfg(not(feature = "rustc"))]
+            let mut set = Set::new();
+            set.extend(alts);
+            set
+        };
+
+        for src_alts in alts.clone().into_iter().powerset() {
+            let src_layout = into_layout(src_alts.clone());
+            let src_set = into_set(src_alts.clone());
+
+            for dst_alts in alts.clone().into_iter().powerset().filter(|alts| !alts.is_empty()) {
+                let dst_layout = into_layout(dst_alts.clone());
+                let dst_set = into_set(dst_alts.clone());
+
+                if src_set.is_subset(&dst_set) {
+                    assert_eq!(
+                        Answer::Yes,
+                        MaybeTransmutableQuery::new(
+                            src_layout.clone(),
+                            dst_layout.clone(),
+                            (),
+                            crate::Assume { validity: false, ..crate::Assume::default() },
+                            UltraMinimal,
+                        )
+                        .answer(),
+                        "{:?} SHOULD be transmutable into {:?}",
+                        src_layout,
+                        dst_layout
+                    );
+                } else if !src_set.is_disjoint(&dst_set) {
+                    assert_eq!(
+                        Answer::Yes,
+                        MaybeTransmutableQuery::new(
+                            src_layout.clone(),
+                            dst_layout.clone(),
+                            (),
+                            crate::Assume { validity: true, ..crate::Assume::default() },
+                            UltraMinimal,
+                        )
+                        .answer(),
+                        "{:?} SHOULD be transmutable (assuming validity) into {:?}",
+                        src_layout,
+                        dst_layout
+                    );
+                } else {
+                    assert_eq!(
+                        Answer::No(Reason::DstIsBitIncompatible),
+                        MaybeTransmutableQuery::new(
+                            src_layout.clone(),
+                            dst_layout.clone(),
+                            (),
+                            crate::Assume { validity: false, ..crate::Assume::default() },
+                            UltraMinimal,
+                        )
+                        .answer(),
+                        "{:?} should NOT be transmutable into {:?}",
+                        src_layout,
+                        dst_layout
+                    );
+                }
+            }
+        }
+    }
+}