get back the more precise error message

Author: RalfJung

src/librustc_mir/const_eval/machine.rs

@@ -8,6 +8,7 @@ use std::hash::Hash;
 use rustc_data_structures::fx::FxHashMap;
 
 use rustc::mir::AssertMessage;
+use rustc_ast::ast::Mutability;
 use rustc_span::symbol::Symbol;
 use rustc_span::{def_id::DefId, Span};
 
@@ -347,11 +348,14 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter {
 
     fn before_access_global(
         memory_extra: &MemoryExtra,
-        _allocation: &Allocation,
+        alloc_id: AllocId,
+        allocation: &Allocation,
         def_id: Option<DefId>,
         is_write: bool,
     ) -> InterpResult<'tcx> {
-        if is_write {
+        if is_write && allocation.mutability == Mutability::Not {
+            Err(err_ub!(WriteToReadOnly(alloc_id)).into())
+        } else if is_write {
             Err(ConstEvalErrKind::ModifiedGlobal.into())
         } else if memory_extra.can_access_statics || def_id.is_none() {
             Ok(())

src/librustc_mir/interpret/machine.rs

@@ -212,6 +212,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
     #[inline]
     fn before_access_global(
         _memory_extra: &Self::MemoryExtra,
+        _alloc_id: AllocId,
         _allocation: &Allocation,
         _def_id: Option<DefId>,
         _is_write: bool,

src/librustc_mir/interpret/memory.rs

@@ -456,7 +456,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                 (allocation, Some(def_id))
             }
         };
-        M::before_access_global(memory_extra, alloc, def_id, is_write)?;
+        M::before_access_global(memory_extra, id, alloc, def_id, is_write)?;
         let alloc = Cow::Borrowed(alloc);
         // We got tcx memory. Let the machine initialize its "extra" stuff.
         let (alloc, tag) = M::init_allocation_extra(

src/librustc_mir/transform/const_prop.rs

@@ -272,6 +272,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 
     fn before_access_global(
         _memory_extra: &(),
+        _alloc_id: AllocId,
         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
         _def_id: Option<DefId>,
         is_write: bool,

src/test/ui/consts/miri_unleashed/mutable_const.stderr

@@ -11,7 +11,7 @@ LL | / const MUTATING_BEHIND_RAW: () = {
 LL | |     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
 LL | |     unsafe {
 LL | |         *MUTABLE_BEHIND_RAW = 99
-   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ modifying a static's initial value from another static's initializer
+   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ writing to alloc1 which is read-only
 LL | |     }
 LL | | };
    | |__-