Safe Transmute: Add alignment tests

tests/ui/transmutability/alignment/align-fail.rs

@@ -0,0 +1,23 @@
+// check-fail
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: false,
+                lifetimes: true,
+                safety: true,
+                validity: true,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    assert::is_maybe_transmutable::<&'static [u8; 0], &'static [u16; 0]>(); //~ ERROR `&[u8; 0]` cannot be safely transmuted into `&[u16; 0]`
+}

tests/ui/transmutability/alignment/align-fail.stderr

@@ -0,0 +1,30 @@
+error[E0277]: `&[u8; 0]` cannot be safely transmuted into `&[u16; 0]` in the defining scope of `assert::Context`
+  --> $DIR/align-fail.rs:22:55
+   |
+LL | ...tatic [u8; 0], &'static [u16; 0]>();
+   |                   ^^^^^^^^^^^^^^^^^ The alignment of `&[u8; 0]` should be stricter than that of `&[u16; 0]`, but it is not
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/align-fail.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: BikeshedIntrinsicFrom<Src, Context, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: false,
+LL | |                 lifetimes: true,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+help: consider removing the leading `&`-reference
+   |
+LL -     assert::is_maybe_transmutable::<&'static [u8; 0], &'static [u16; 0]>();
+LL +     assert::is_maybe_transmutable::<&'static [u8; 0], [u16; 0]>();
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/transmutability/alignment/align-pass.rs

@@ -0,0 +1,23 @@
+// check-pass
+#![feature(transmutability)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+    pub struct Context;
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, {
+            Assume {
+                alignment: false,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+}
+
+fn main() {
+    assert::is_maybe_transmutable::<&'static [u16; 0], &'static [u8; 0]>();
+}