Miri: let machine hook dynamically decide about alignment checks

Author: RalfJung

src/librustc_mir/const_eval/machine.rs

@@ -179,9 +179,12 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter {
 
     const GLOBAL_KIND: Option<!> = None; // no copying of globals from `tcx` to machine memory
 
-    // We do not check for alignment to avoid having to carry an `Align`
-    // in `ConstValue::ByRef`.
-    const CHECK_ALIGN: bool = false;
+    #[inline(always)]
+    fn enforce_alignment(_memory_extra: &Self::MemoryExtra) -> bool {
+        // We do not check for alignment to avoid having to carry an `Align`
+        // in `ConstValue::ByRef`.
+        false
+    }
 
     #[inline(always)]
     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {

src/librustc_mir/interpret/machine.rs

@@ -118,7 +118,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
     const GLOBAL_KIND: Option<Self::MemoryKind>;
 
     /// Whether memory accesses should be alignment-checked.
-    const CHECK_ALIGN: bool;
+    fn enforce_alignment(memory_extra: &Self::MemoryExtra) -> bool;
 
     /// Whether to enforce the validity invariant
     fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

src/librustc_mir/interpret/memory.rs

@@ -323,12 +323,12 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         size: Size,
         align: Align,
     ) -> InterpResult<'tcx, Option<Pointer<M::PointerTag>>> {
-        let align = M::CHECK_ALIGN.then_some(align);
+        let align = M::enforce_alignment(&self.extra).then_some(align);
         self.check_ptr_access_align(sptr, size, align, CheckInAllocMsg::MemoryAccessTest)
     }
 
     /// Like `check_ptr_access`, but *definitely* checks alignment when `align`
-    /// is `Some` (overriding `M::CHECK_ALIGN`). Also lets the caller control
+    /// is `Some` (overriding `M::enforce_alignment`). Also lets the caller control
     /// the error message for the out-of-bounds case.
     pub fn check_ptr_access_align(
         &self,

src/librustc_mir/transform/const_prop.rs

@@ -173,7 +173,10 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 
     const GLOBAL_KIND: Option<!> = None; // no copying of globals from `tcx` to machine memory
 
-    const CHECK_ALIGN: bool = false;
+    #[inline(always)]
+    fn enforce_alignment(_memory_extra: &Self::MemoryExtra) -> bool {
+        false
+    }
 
     #[inline(always)]
     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {