Make alignment checks a future incompat lint

compiler/rustc_lint_defs/src/builtin.rs

@@ -1019,6 +1019,42 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `invalid_alignment` lint detects dereferences of misaligned pointers during
+    /// constant evluation.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// const FOO: () = unsafe {
+    ///     let x = [0_u8; 10];
+    ///     let y = x.as_ptr() as *const u32;
+    ///     *y; // the address of a `u8` array is unknown and thus we don't know if
+    ///     // it is aligned enough for reading a `u32`.
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The compiler allowed dereferencing raw pointers irrespective of alignment
+    /// during const eval due to the const evaluator at the time not making it easy
+    /// or cheap to check. Now that it is both, this is not accepted anymore.
+    ///
+    /// Since it was undefined behaviour to begin with, this breakage does not violate
+    /// Rust's stability guarantees. Using undefined behaviour can cause arbitrary
+    /// behaviour, including failure to build.
+    ///
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub INVALID_ALIGNMENT,
+    Deny,
+    "raw pointers must be aligned before dereferencing",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #68585 <https://github.com/rust-lang/rust/issues/104616>",
+    };
+}
+
 declare_lint! {
     /// The `exported_private_dependencies` lint detects private dependencies
     /// that are exposed in a public interface.