Rollup merge of #130885 - RalfJung:interp-error-discard, r=oli-obk

panic when an interpreter error gets unintentionally discarded

One important invariant of Miri is that when an interpreter error is raised (*in particular* a UB error), those must not be discarded: it's not okay to just check `foo().is_err()` and then continue executing.

This seems to catch new contributors by surprise fairly regularly, so this PR tries to make it so that *if* this ever happens, we get a panic rather than a silent missed UB bug. The interpreter error type now contains a "guard" that panics on drop, and that is explicitly passed to `mem::forget` when an error is deliberately discarded.

Fixes rust-lang/miri#3855

compiler/rustc_const_eval/src/const_eval/dummy_machine.rs

@@ -6,7 +6,8 @@ use rustc_middle::{bug, span_bug, ty};
 use rustc_span::def_id::DefId;
 
 use crate::interpret::{
-    self, HasStaticRootDefId, ImmTy, Immediate, InterpCx, PointerArithmetic, throw_machine_stop,
+    self, HasStaticRootDefId, ImmTy, Immediate, InterpCx, PointerArithmetic, interp_ok,
+    throw_machine_stop,
 };
 
 /// Macro for machine-specific `InterpError` without allocation.
@@ -79,7 +80,7 @@ impl<'tcx> interpret::Machine<'tcx> for DummyMachine {
             throw_machine_stop_str!("can't access mutable globals in ConstProp");
         }
 
-        Ok(())
+        interp_ok(())
     }
 
     fn find_mir_or_eval_fn(
@@ -127,7 +128,7 @@ impl<'tcx> interpret::Machine<'tcx> for DummyMachine {
         right: &interpret::ImmTy<'tcx, Self::Provenance>,
     ) -> interpret::InterpResult<'tcx, ImmTy<'tcx, Self::Provenance>> {
         use rustc_middle::mir::BinOp::*;
-        Ok(match bin_op {
+        interp_ok(match bin_op {
             Eq | Ne | Lt | Le | Gt | Ge => {
                 // Types can differ, e.g. fn ptrs with different `for`.
                 assert_eq!(left.layout.abi, right.layout.abi);

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -20,7 +20,7 @@ use crate::const_eval::CheckAlignment;
 use crate::interpret::{
     CtfeValidationMode, GlobalId, Immediate, InternKind, InternResult, InterpCx, InterpError,
     InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking, StackPopCleanup, create_static_alloc,
-    eval_nullary_intrinsic, intern_const_alloc_recursive, throw_exhaust,
+    eval_nullary_intrinsic, intern_const_alloc_recursive, interp_ok, throw_exhaust,
 };
 use crate::{CTRL_C_RECEIVED, errors};
 
@@ -98,19 +98,19 @@ fn eval_body_using_ecx<'tcx, R: InterpretationResult<'tcx>>(
             return Err(ecx
                 .tcx
                 .dcx()
-                .emit_err(errors::DanglingPtrInFinal { span: ecx.tcx.span, kind: intern_kind })
-                .into());
+                .emit_err(errors::DanglingPtrInFinal { span: ecx.tcx.span, kind: intern_kind }))
+            .into();
         }
         Err(InternResult::FoundBadMutablePointer) => {
             return Err(ecx
                 .tcx
                 .dcx()
-                .emit_err(errors::MutablePtrInFinal { span: ecx.tcx.span, kind: intern_kind })
-                .into());
+                .emit_err(errors::MutablePtrInFinal { span: ecx.tcx.span, kind: intern_kind }))
+            .into();
         }
     }
 
-    Ok(R::make_result(ret, ecx))
+    interp_ok(R::make_result(ret, ecx))
 }
 
 /// The `InterpCx` is only meant to be used to do field and index projections into constants for
@@ -147,7 +147,8 @@ pub fn mk_eval_cx_for_const_val<'tcx>(
     ty: Ty<'tcx>,
 ) -> Option<(CompileTimeInterpCx<'tcx>, OpTy<'tcx>)> {
     let ecx = mk_eval_cx_to_read_const_val(tcx.tcx, tcx.span, param_env, CanAccessMutGlobal::No);
-    let op = ecx.const_val_to_op(val, ty, None).ok()?;
+    // FIXME: is it a problem to discard the error here?
+    let op = ecx.const_val_to_op(val, ty, None).discard_err()?;
     Some((ecx, op))
 }
 
@@ -185,12 +186,16 @@ pub(super) fn op_to_const<'tcx>(
         _ => false,
     };
     let immediate = if force_as_immediate {
-        match ecx.read_immediate(op) {
+        match ecx.read_immediate(op).report_err() {
             Ok(imm) => Right(imm),
-            Err(err) if !for_diagnostics => {
-                panic!("normalization works on validated constants: {err:?}")
+            Err(err) => {
+                if for_diagnostics {
+                    // This discard the error, but for diagnostics that's okay.
+                    op.as_mplace_or_imm()
+                } else {
+                    panic!("normalization works on validated constants: {err:?}")
+                }
             }
-            _ => op.as_mplace_or_imm(),
         }
     } else {
         op.as_mplace_or_imm()
@@ -283,17 +288,19 @@ pub fn eval_to_const_value_raw_provider<'tcx>(
         let ty::FnDef(_, args) = ty.kind() else {
             bug!("intrinsic with type {:?}", ty);
         };
-        return eval_nullary_intrinsic(tcx, key.param_env, def_id, args).map_err(|error| {
-            let span = tcx.def_span(def_id);
-
-            super::report(
-                tcx,
-                error.into_kind(),
-                span,
-                || (span, vec![]),
-                |span, _| errors::NullaryIntrinsicError { span },
-            )
-        });
+        return eval_nullary_intrinsic(tcx, key.param_env, def_id, args).report_err().map_err(
+            |error| {
+                let span = tcx.def_span(def_id);
+
+                super::report(
+                    tcx,
+                    error.into_kind(),
+                    span,
+                    || (span, vec![]),
+                    |span, _| errors::NullaryIntrinsicError { span },
+                )
+            },
+        );
     }
 
     tcx.eval_to_allocation_raw(key).map(|val| turn_into_const_value(tcx, val, key))
@@ -376,6 +383,7 @@ fn eval_in_interpreter<'tcx, R: InterpretationResult<'tcx>>(
     );
     let res = ecx.load_mir(cid.instance.def, cid.promoted);
     res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, body))
+        .report_err()
         .map_err(|error| report_eval_error(&ecx, cid, error))
 }
 
@@ -400,6 +408,7 @@ fn const_validate_mplace<'tcx>(
             }
         };
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)
+            .report_err()
             // Instead of just reporting the `InterpError` via the usual machinery, we give a more targeted
             // error about the validation failure.
             .map_err(|error| report_validation_error(&ecx, cid, error, alloc_id))?;

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -24,8 +24,8 @@ use crate::fluent_generated as fluent;
 use crate::interpret::{
     self, AllocId, AllocRange, ConstAllocation, CtfeProvenance, FnArg, Frame, GlobalAlloc, ImmTy,
     InterpCx, InterpResult, MPlaceTy, OpTy, Pointer, PointerArithmetic, RangeSet, Scalar,
-    StackPopCleanup, compile_time_machine, err_ub, throw_exhaust, throw_inval, throw_ub_custom,
-    throw_unsup, throw_unsup_format,
+    StackPopCleanup, compile_time_machine, interp_ok, throw_exhaust, throw_inval, throw_ub,
+    throw_ub_custom, throw_unsup, throw_unsup_format,
 };
 
 /// When hitting this many interpreted terminators we emit a deny by default lint
@@ -247,7 +247,7 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
             let msg = Symbol::intern(self.read_str(&msg_place)?);
             let span = self.find_closest_untracked_caller_location();
             let (file, line, col) = self.location_triple_for_span(span);
-            return Err(ConstEvalErrKind::Panic { msg, file, line, col }.into());
+            return Err(ConstEvalErrKind::Panic { msg, file, line, col }).into();
         } else if self.tcx.is_lang_item(def_id, LangItem::PanicFmt) {
             // For panic_fmt, call const_panic_fmt instead.
             let const_def_id = self.tcx.require_lang_item(LangItem::ConstPanicFmt, None);
@@ -259,16 +259,16 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
                 self.cur_span(),
             );
 
-            return Ok(Some(new_instance));
+            return interp_ok(Some(new_instance));
         } else if self.tcx.is_lang_item(def_id, LangItem::AlignOffset) {
             let args = self.copy_fn_args(args);
             // For align_offset, we replace the function call if the pointer has no address.
             match self.align_offset(instance, &args, dest, ret)? {
-                ControlFlow::Continue(()) => return Ok(Some(instance)),
-                ControlFlow::Break(()) => return Ok(None),
+                ControlFlow::Continue(()) => return interp_ok(Some(instance)),
+                ControlFlow::Break(()) => return interp_ok(None),
             }
         }
-        Ok(Some(instance))
+        interp_ok(Some(instance))
     }
 
     /// `align_offset(ptr, target_align)` needs special handling in const eval, because the pointer
@@ -323,25 +323,25 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
                         dest,
                         StackPopCleanup::Goto { ret, unwind: mir::UnwindAction::Unreachable },
                     )?;
-                    Ok(ControlFlow::Break(()))
+                    interp_ok(ControlFlow::Break(()))
                 } else {
                     // Not alignable in const, return `usize::MAX`.
                     let usize_max = Scalar::from_target_usize(self.target_usize_max(), self);
                     self.write_scalar(usize_max, dest)?;
                     self.return_to_block(ret)?;
-                    Ok(ControlFlow::Break(()))
+                    interp_ok(ControlFlow::Break(()))
                 }
             }
             Err(_addr) => {
                 // The pointer has an address, continue with function call.
-                Ok(ControlFlow::Continue(()))
+                interp_ok(ControlFlow::Continue(()))
             }
         }
     }
 
     /// See documentation on the `ptr_guaranteed_cmp` intrinsic.
     fn guaranteed_cmp(&mut self, a: Scalar, b: Scalar) -> InterpResult<'tcx, u8> {
-        Ok(match (a, b) {
+        interp_ok(match (a, b) {
             // Comparisons between integers are always known.
             (Scalar::Int { .. }, Scalar::Int { .. }) => {
                 if a == b {
@@ -403,8 +403,8 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
         instance: ty::InstanceKind<'tcx>,
     ) -> InterpResult<'tcx, &'tcx mir::Body<'tcx>> {
         match instance {
-            ty::InstanceKind::Item(def) => Ok(ecx.tcx.mir_for_ctfe(def)),
-            _ => Ok(ecx.tcx.instance_mir(instance)),
+            ty::InstanceKind::Item(def) => interp_ok(ecx.tcx.mir_for_ctfe(def)),
+            _ => interp_ok(ecx.tcx.instance_mir(instance)),
         }
     }
 
@@ -422,7 +422,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
         // Replace some functions.
         let Some(instance) = ecx.hook_special_const_fn(orig_instance, args, dest, ret)? else {
             // Call has already been handled.
-            return Ok(None);
+            return interp_ok(None);
         };
 
         // Only check non-glue functions
@@ -444,14 +444,14 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
         // This is a const fn. Call it.
         // In case of replacement, we return the *original* instance to make backtraces work out
         // (and we hope this does not confuse the FnAbi checks too much).
-        Ok(Some((ecx.load_mir(instance.def, None)?, orig_instance)))
+        interp_ok(Some((ecx.load_mir(instance.def, None)?, orig_instance)))
     }
 
     fn panic_nounwind(ecx: &mut InterpCx<'tcx, Self>, msg: &str) -> InterpResult<'tcx> {
         let msg = Symbol::intern(msg);
         let span = ecx.find_closest_untracked_caller_location();
         let (file, line, col) = ecx.location_triple_for_span(span);
-        Err(ConstEvalErrKind::Panic { msg, file, line, col }.into())
+        Err(ConstEvalErrKind::Panic { msg, file, line, col }).into()
     }
 
     fn call_intrinsic(
@@ -464,7 +464,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
     ) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
         // Shared intrinsics.
         if ecx.eval_intrinsic(instance, args, dest, target)? {
-            return Ok(None);
+            return interp_ok(None);
         }
         let intrinsic_name = ecx.tcx.item_name(instance.def_id());
 
@@ -541,7 +541,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                         "intrinsic `{intrinsic_name}` is not supported at compile-time"
                     );
                 }
-                return Ok(Some(ty::Instance {
+                return interp_ok(Some(ty::Instance {
                     def: ty::InstanceKind::Item(instance.def_id()),
                     args: instance.args,
                 }));
@@ -550,7 +550,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
 
         // Intrinsic is done, jump to next block.
         ecx.return_to_block(target)?;
-        Ok(None)
+        interp_ok(None)
     }
 
     fn assert_panic(
@@ -581,7 +581,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                 }
             }
         };
-        Err(ConstEvalErrKind::AssertFailure(err).into())
+        Err(ConstEvalErrKind::AssertFailure(err)).into()
     }
 
     fn binary_ptr_op(
@@ -652,7 +652,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
             }
         }
 
-        Ok(())
+        interp_ok(())
     }
 
     #[inline(always)]
@@ -670,7 +670,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
         if !ecx.recursion_limit.value_within_limit(ecx.stack().len() + 1) {
             throw_exhaust!(StackFrameLimitReached)
         } else {
-            Ok(frame)
+            interp_ok(frame)
         }
     }
 
@@ -700,22 +700,22 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
         if is_write {
             // Write access. These are never allowed, but we give a targeted error message.
             match alloc.mutability {
-                Mutability::Not => Err(err_ub!(WriteToReadOnly(alloc_id)).into()),
-                Mutability::Mut => Err(ConstEvalErrKind::ModifiedGlobal.into()),
+                Mutability::Not => throw_ub!(WriteToReadOnly(alloc_id)),
+                Mutability::Mut => Err(ConstEvalErrKind::ModifiedGlobal).into(),
             }
         } else {
             // Read access. These are usually allowed, with some exceptions.
             if machine.can_access_mut_global == CanAccessMutGlobal::Yes {
                 // Machine configuration allows us read from anything (e.g., `static` initializer).
-                Ok(())
+                interp_ok(())
             } else if alloc.mutability == Mutability::Mut {
                 // Machine configuration does not allow us to read statics (e.g., `const`
                 // initializer).
-                Err(ConstEvalErrKind::ConstAccessesMutGlobal.into())
+                Err(ConstEvalErrKind::ConstAccessesMutGlobal).into()
             } else {
                 // Immutable global, this read is fine.
                 assert_eq!(alloc.mutability, Mutability::Not);
-                Ok(())
+                interp_ok(())
             }
         }
     }
@@ -748,9 +748,9 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                 // even when there is interior mutability.)
                 place.map_provenance(CtfeProvenance::as_shared_ref)
             };
-            Ok(ImmTy::from_immediate(new_place.to_ref(ecx), val.layout))
+            interp_ok(ImmTy::from_immediate(new_place.to_ref(ecx), val.layout))
         } else {
-            Ok(val.clone())
+            interp_ok(val.clone())
         }
     }
 
@@ -763,20 +763,20 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
     ) -> InterpResult<'tcx> {
         if range.size == Size::ZERO {
             // Nothing to check.
-            return Ok(());
+            return interp_ok(());
         }
         // Reject writes through immutable pointers.
         if immutable {
-            return Err(ConstEvalErrKind::WriteThroughImmutablePointer.into());
+            return Err(ConstEvalErrKind::WriteThroughImmutablePointer).into();
         }
         // Everything else is fine.
-        Ok(())
+        interp_ok(())
     }
 
     fn before_alloc_read(ecx: &InterpCx<'tcx, Self>, alloc_id: AllocId) -> InterpResult<'tcx> {
         // Check if this is the currently evaluated static.
         if Some(alloc_id) == ecx.machine.static_root_ids.map(|(id, _)| id) {
-            return Err(ConstEvalErrKind::RecursiveStatic.into());
+            return Err(ConstEvalErrKind::RecursiveStatic).into();
         }
         // If this is another static, make sure we fire off the query to detect cycles.
         // But only do that when checks for static recursion are enabled.
@@ -788,7 +788,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                 ecx.ctfe_query(|tcx| tcx.eval_static_initializer(def_id))?;
             }
         }
-        Ok(())
+        interp_ok(())
     }
 
     fn cached_union_data_range<'e>(