Auto merge of #104616 - RalfJung:ctfe-alignment, r=oli-obk,RalfJung

always check alignment during CTFE

We originally disabled alignment checks because they got in the way -- there are some things we do with the interpreter during CTFE which does not correspond to actually running user-written code, but is purely administrative, and we didn't want alignment checks there, so we just disabled them entirely. But with `-Zextra-const-ub-checks` we anyway had to figure out how to disable those alignment checks while doing checks in regular code. So now it is easy to enable CTFE alignment checking by default. Let's see what the perf consequences of that are.

r? `@oli-obk`

compiler/rustc_mir_transform/src/const_prop.rs

@@ -6,6 +6,7 @@ use std::cell::Cell;
 use either::Right;
 
 use rustc_ast::Mutability;
+use rustc_const_eval::const_eval::CheckAlignment;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def::DefKind;
 use rustc_index::bit_set::BitSet;
@@ -22,7 +23,7 @@ use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayo
 use rustc_middle::ty::InternalSubsts;
 use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisitable};
 use rustc_span::{def_id::DefId, Span};
-use rustc_target::abi::{self, HasDataLayout, Size, TargetDataLayout};
+use rustc_target::abi::{self, Align, HasDataLayout, Size, TargetDataLayout};
 use rustc_target::spec::abi::Abi as CallAbi;
 use rustc_trait_selection::traits;
 
@@ -186,16 +187,27 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
     type MemoryKind = !;
 
     #[inline(always)]
-    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment {
         // We do not check for alignment to avoid having to carry an `Align`
         // in `ConstValue::ByRef`.
-        false
+        CheckAlignment::No
     }
 
     #[inline(always)]
     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         false // for now, we don't enforce validity
     }
+    fn alignment_check_failed(
+        ecx: &InterpCx<'mir, 'tcx, Self>,
+        _has: Align,
+        _required: Align,
+        _check: CheckAlignment,
+    ) -> InterpResult<'tcx, ()> {
+        span_bug!(
+            ecx.cur_span(),
+            "`alignment_check_failed` called when no alignment check requested"
+        )
+    }
 
     fn load_mir(
         _ecx: &InterpCx<'mir, 'tcx, Self>,

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -2,6 +2,7 @@
 //!
 //! Currently, this pass only propagates scalar values.
 
+use rustc_const_eval::const_eval::CheckAlignment;
 use rustc_const_eval::interpret::{ConstValue, ImmTy, Immediate, InterpCx, Scalar};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::mir::visit::{MutVisitor, Visitor};
@@ -10,6 +11,7 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_mir_dataflow::value_analysis::{Map, State, TrackElem, ValueAnalysis, ValueOrPlace};
 use rustc_mir_dataflow::{lattice::FlatSet, Analysis, ResultsVisitor, SwitchIntEdgeEffects};
 use rustc_span::DUMMY_SP;
+use rustc_target::abi::Align;
 
 use crate::MirPass;
 
@@ -448,13 +450,21 @@ impl<'mir, 'tcx> rustc_const_eval::interpret::Machine<'mir, 'tcx> for DummyMachi
     type MemoryKind = !;
     const PANIC_ON_ALLOC_FAIL: bool = true;
 
-    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment {
         unimplemented!()
     }
 
     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         unimplemented!()
     }
+    fn alignment_check_failed(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        _has: Align,
+        _required: Align,
+        _check: CheckAlignment,
+    ) -> interpret::InterpResult<'tcx, ()> {
+        unimplemented!()
+    }
 
     fn find_mir_or_eval_fn(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,