[WIP] transmute_from!(@allow_shrink)

gherrit-pr-id: I10874e2bc703fb6b7fcdea050b8971de869a850a

Author: joshlf

src/layout.rs

@@ -694,8 +694,8 @@ mod cast_from_raw {
         /// `Src`'s alignment must not be smaller than `Dst`'s alignment.
         #[derive(Copy, Clone)]
         struct CastParams {
-            offset_delta_elems: usize,
-            elem_multiple: usize,
+            offset_delta_elems: Fraction,
+            elem_multiple: Fraction,
         }
 
         impl CastParams {
@@ -718,32 +718,41 @@ mod cast_from_raw {
                     return None;
                 };
 
+                // TODO: When doing size-lossy casts, we can permit ZST trailing
+                // slice elements in the destination type, although we'd need to
+                // think carefully about what we promise about the metadata.
                 let dst_elem_size = if let Some(e) = NonZeroUsize::new(dst.elem_size) {
                     e
                 } else {
                     return None;
                 };
 
-                // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
-                #[allow(clippy::arithmetic_side_effects)]
-                let delta_mod_other_elem = offset_delta % dst_elem_size.get();
+                let offset_delta_elems =
+                    Fraction { num: offset_delta, denom: dst_elem_size }.simplify();
 
-                // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
-                #[allow(clippy::arithmetic_side_effects)]
-                let elem_remainder = src.elem_size % dst_elem_size.get();
+                let elem_multiple =
+                    Fraction { num: src.elem_size, denom: dst_elem_size }.simplify();
 
-                if delta_mod_other_elem != 0 || src.elem_size < dst.elem_size || elem_remainder != 0
-                {
-                    return None;
-                }
+                // // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
+                // #[allow(clippy::arithmetic_side_effects)]
+                // let delta_mod_other_elem = offset_delta % dst_elem_size.get();
 
-                // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
-                #[allow(clippy::arithmetic_side_effects)]
-                let offset_delta_elems = offset_delta / dst_elem_size.get();
+                // // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
+                // #[allow(clippy::arithmetic_side_effects)]
+                // let elem_remainder = src.elem_size % dst_elem_size.get();
 
-                // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
-                #[allow(clippy::arithmetic_side_effects)]
-                let elem_multiple = src.elem_size / dst_elem_size.get();
+                // if delta_mod_other_elem != 0 || src.elem_size < dst.elem_size || elem_remainder != 0
+                // {
+                //     return None;
+                // }
+
+                // // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
+                // #[allow(clippy::arithmetic_side_effects)]
+                // let offset_delta_elems = offset_delta / dst_elem_size.get();
+
+                // // PANICS: `dst_elem_size: NonZeroUsize`, so this won't div by zero.
+                // #[allow(clippy::arithmetic_side_effects)]
+                // let elem_multiple = src.elem_size / dst_elem_size.get();
 
                 // SAFETY: We checked above that `src.align >= dst.align`.
                 Some(CastParams {
@@ -816,6 +825,24 @@ mod cast_from_raw {
         // subset of those bytes, and has provenance for those bytes.
         unsafe { PtrInner::new(dst) }
     }
+
+    #[derive(Copy, Clone)]
+    struct Fraction {
+        num: usize,
+        denom: NonZeroUsize,
+    }
+
+    impl Fraction {
+        const fn simplify(self) -> Fraction {
+            todo!() // https://stackoverflow.com/a/7777146/836390
+        }
+
+        const fn mul(self, other: usize) -> Fraction {
+            // TODO: How can we prove that it's sound to use `unchecked_mul`
+            // when the denom is 1?
+            todo!()
+        }
+    }
 }
 
 // TODO(#67): For some reason, on our MSRV toolchain, this `allow` isn't