don't ICE when encountering an extern type field during validation

compiler/rustc_const_eval/messages.ftl

@@ -89,6 +89,8 @@ const_eval_exact_div_has_remainder =
 
 const_eval_extern_static =
     cannot access extern static ({$did})
+const_eval_extern_type_field = `extern type` field does not have a known offset
+
 const_eval_fn_ptr_call =
     function pointers need an RFC before allowed to be called in {const_eval_const_context}s
 const_eval_for_loop_into_iter_non_const =

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -386,33 +386,8 @@ fn eval_in_interpreter<'tcx, R: InterpretationResult<'tcx>>(
         CompileTimeMachine::new(CanAccessMutGlobal::from(is_static), CheckAlignment::Error),
     );
     let res = ecx.load_mir(cid.instance.def, cid.promoted);
-    res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, body)).map_err(|error| {
-        let (error, backtrace) = error.into_parts();
-        backtrace.print_backtrace();
-
-        let (kind, instance) = if ecx.tcx.is_static(cid.instance.def_id()) {
-            ("static", String::new())
-        } else {
-            // If the current item has generics, we'd like to enrich the message with the
-            // instance and its args: to show the actual compile-time values, in addition to
-            // the expression, leading to the const eval error.
-            let instance = &cid.instance;
-            if !instance.args.is_empty() {
-                let instance = with_no_trimmed_paths!(instance.to_string());
-                ("const_with_path", instance)
-            } else {
-                ("const", String::new())
-            }
-        };
-
-        super::report(
-            *ecx.tcx,
-            error,
-            DUMMY_SP,
-            || super::get_span_and_frames(ecx.tcx, ecx.stack()),
-            |span, frames| ConstEvalError { span, error_kind: kind, instance, frame_notes: frames },
-        )
-    })
+    res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, body))
+        .map_err(|error| report_eval_error(&ecx, cid, error))
 }
 
 #[inline(always)]
@@ -438,24 +413,61 @@ fn const_validate_mplace<'tcx>(
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)
             // 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, error, alloc_id))?;
+            .map_err(|error| report_validation_error(&ecx, cid, error, alloc_id))?;
         inner = true;
     }
 
     Ok(())
 }
 
-#[inline(always)]
+#[inline(never)]
+fn report_eval_error<'tcx>(
+    ecx: &InterpCx<'tcx, CompileTimeMachine<'tcx>>,
+    cid: GlobalId<'tcx>,
+    error: InterpErrorInfo<'tcx>,
+) -> ErrorHandled {
+    let (error, backtrace) = error.into_parts();
+    backtrace.print_backtrace();
+
+    let (kind, instance) = if ecx.tcx.is_static(cid.instance.def_id()) {
+        ("static", String::new())
+    } else {
+        // If the current item has generics, we'd like to enrich the message with the
+        // instance and its args: to show the actual compile-time values, in addition to
+        // the expression, leading to the const eval error.
+        let instance = &cid.instance;
+        if !instance.args.is_empty() {
+            let instance = with_no_trimmed_paths!(instance.to_string());
+            ("const_with_path", instance)
+        } else {
+            ("const", String::new())
+        }
+    };
+
+    super::report(
+        *ecx.tcx,
+        error,
+        DUMMY_SP,
+        || super::get_span_and_frames(ecx.tcx, ecx.stack()),
+        |span, frames| ConstEvalError { span, error_kind: kind, instance, frame_notes: frames },
+    )
+}
+
+#[inline(never)]
 fn report_validation_error<'tcx>(
     ecx: &InterpCx<'tcx, CompileTimeMachine<'tcx>>,
+    cid: GlobalId<'tcx>,
     error: InterpErrorInfo<'tcx>,
     alloc_id: AllocId,
 ) -> ErrorHandled {
+    if !matches!(error.kind(), InterpError::UndefinedBehavior(_)) {
+        // Some other error happened during validation, e.g. an unsupported operation.
+        return report_eval_error(ecx, cid, error);
+    }
+
     let (error, backtrace) = error.into_parts();
     backtrace.print_backtrace();
 
-    let ub_note = matches!(error, InterpError::UndefinedBehavior(_)).then(|| {});
-
     let bytes = ecx.print_alloc_bytes_for_diagnostics(alloc_id);
     let (size, align, _) = ecx.get_alloc_info(alloc_id);
     let raw_bytes = errors::RawBytesNote { size: size.bytes(), align: align.bytes(), bytes };
@@ -465,6 +477,11 @@ fn report_validation_error<'tcx>(
         error,
         DUMMY_SP,
         || crate::const_eval::get_span_and_frames(ecx.tcx, ecx.stack()),
-        move |span, frames| errors::ValidationFailure { span, ub_note, frames, raw_bytes },
+        move |span, frames| errors::ValidationFailure {
+            span,
+            ub_note: Some(()),
+            frames,
+            raw_bytes,
+        },
     )
 }

compiler/rustc_const_eval/src/errors.rs

@@ -825,6 +825,7 @@ impl ReportErrorExt for UnsupportedOpInfo {
         use crate::fluent_generated::*;
         match self {
             UnsupportedOpInfo::Unsupported(s) => s.clone().into(),
+            UnsupportedOpInfo::ExternTypeField => const_eval_extern_type_field,
             UnsupportedOpInfo::UnsizedLocal => const_eval_unsized_local,
             UnsupportedOpInfo::OverwritePartialPointer(_) => const_eval_partial_pointer_overwrite,
             UnsupportedOpInfo::ReadPartialPointer(_) => const_eval_partial_pointer_copy,
@@ -845,7 +846,10 @@ impl ReportErrorExt for UnsupportedOpInfo {
             // `ReadPointerAsInt(Some(info))` is never printed anyway, it only serves as an error to
             // be further processed by validity checking which then turns it into something nice to
             // print. So it's not worth the effort of having diagnostics that can print the `info`.
-            UnsizedLocal | Unsupported(_) | ReadPointerAsInt(_) => {}
+            UnsizedLocal
+            | UnsupportedOpInfo::ExternTypeField
+            | Unsupported(_)
+            | ReadPointerAsInt(_) => {}
             OverwritePartialPointer(ptr) | ReadPartialPointer(ptr) => {
                 diag.arg("ptr", ptr);
             }

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -21,7 +21,7 @@ use rustc_target::abi::{self, VariantIdx};
 use tracing::{debug, instrument};
 
 use super::{
-    throw_ub, throw_unsup_format, InterpCx, InterpResult, MPlaceTy, Machine, MemPlaceMeta, OpTy,
+    throw_ub, throw_unsup, InterpCx, InterpResult, MPlaceTy, Machine, MemPlaceMeta, OpTy,
     Provenance, Scalar,
 };
 
@@ -186,8 +186,8 @@ where
                     (base_meta, offset)
                 }
                 None => {
-                    // We don't know the alignment of this field, so we cannot adjust.
-                    throw_unsup_format!("`extern type` does not have a known offset")
+                    // We cannot know the alignment of this field, so we cannot adjust.
+                    throw_unsup!(ExternTypeField)
                 }
             }
         } else {

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -5,6 +5,7 @@
 //! to be const-safe.
 
 use std::fmt::Write;
+use std::hash::Hash;
 use std::num::NonZero;
 
 use either::{Left, Right};
@@ -17,7 +18,8 @@ use rustc_hir as hir;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::{
     ExpectedKind, InterpError, InvalidMetaKind, Misalignment, PointerKind, Provenance,
-    ValidationErrorInfo, ValidationErrorKind, ValidationErrorKind::*,
+    UnsupportedOpInfo, ValidationErrorInfo,
+    ValidationErrorKind::{self, *},
 };
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::ty::{self, Ty};
@@ -26,8 +28,6 @@ use rustc_target::abi::{
     Abi, FieldIdx, Scalar as ScalarAbi, Size, VariantIdx, Variants, WrappingRange,
 };
 
-use std::hash::Hash;
-
 use super::{
     err_ub, format_interp_error, machine::AllocMap, throw_ub, AllocId, AllocKind, CheckInAllocMsg,
     GlobalAlloc, ImmTy, Immediate, InterpCx, InterpResult, MPlaceTy, Machine, MemPlaceMeta, OpTy,
@@ -1028,7 +1028,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             Err(err)
                 if matches!(
                     err.kind(),
-                    err_ub!(ValidationError { .. }) | InterpError::InvalidProgram(_)
+                    err_ub!(ValidationError { .. })
+                        | InterpError::InvalidProgram(_)
+                        | InterpError::Unsupported(UnsupportedOpInfo::ExternTypeField)
                 ) =>
             {
                 Err(err)

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -520,6 +520,8 @@ pub enum UnsupportedOpInfo {
     Unsupported(String),
     /// Unsized local variables.
     UnsizedLocal,
+    /// Extern type field with an indeterminate offset.
+    ExternTypeField,
     //
     // The variants below are only reachable from CTFE/const prop, miri will never emit them.
     //

tests/ui/consts/const-eval/validation-ice-extern-type-field.rs

@@ -0,0 +1,15 @@
+#![feature(extern_types)]
+
+extern {
+    type Opaque;
+}
+
+struct ThinDst {
+    x: u8,
+    tail: Opaque,
+}
+
+const C1: &ThinDst = unsafe { std::mem::transmute(b"d".as_ptr()) };
+//~ERROR: evaluation of constant value failed
+
+fn main() {}

tests/ui/consts/const-eval/validation-ice-extern-type-field.stderr

@@ -0,0 +1,9 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/validation-ice-extern-type-field.rs:12:1
+   |
+LL | const C1: &ThinDst = unsafe { std::mem::transmute(b"d".as_ptr()) };
+   | ^^^^^^^^^^^^^^^^^^ `extern type` field does not have a known offset
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.