Print spans where tags are created and invalidated

src/diagnostics.rs

@@ -7,7 +7,8 @@ use log::trace;
 use rustc_middle::ty;
 use rustc_span::{source_map::DUMMY_SP, Span, SpanData, Symbol};
 
-use crate::stacked_borrows::{AccessKind, SbTag};
+use crate::helpers::HexRange;
+use crate::stacked_borrows::{diagnostics::TagHistory, AccessKind, SbTag};
 use crate::*;
 
 /// Details of premature program termination.
@@ -19,6 +20,7 @@ pub enum TerminationInfo {
         msg: String,
         help: Option<String>,
         url: String,
+        history: Option<TagHistory>,
     },
     Deadlock,
     MultipleSymbolDefinitions {
@@ -155,12 +157,46 @@ pub fn report_error<'tcx, 'mir>(
                         (None, format!("pass the flag `-Zmiri-disable-isolation` to disable isolation;")),
                         (None, format!("or pass `-Zmiri-isolation-error=warn` to configure Miri to return an error code from isolated operations (if supported for that operation) and continue with a warning")),
                     ],
-                ExperimentalUb { url, help, .. } => {
+                ExperimentalUb { url, help, history, .. } => {
                     msg.extend(help.clone());
-                    vec![
+                    let mut helps = vec![
                         (None, format!("this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental")),
-                        (None, format!("see {} for further information", url))
-                    ]
+                        (None, format!("see {} for further information", url)),
+                    ];
+                    match history {
+                        Some(TagHistory::Tagged {tag, created: (created_range, created_span), invalidated, protected }) => {
+                            let msg = format!("{:?} was created by a retag at offsets {}", tag, HexRange(*created_range));
+                            helps.push((Some(created_span.clone()), msg));
+                            if let Some((invalidated_range, invalidated_span)) = invalidated {
+                                let msg = format!("{:?} was later invalidated at offsets {}", tag, HexRange(*invalidated_range));
+                                helps.push((Some(invalidated_span.clone()), msg));
+                            }
+                            if let Some((protecting_tag, protecting_tag_span, protection_span)) = protected {
+                                helps.push((Some(protecting_tag_span.clone()), format!("{:?} was protected due to {:?} which was created here", tag, protecting_tag)));
+                                helps.push((Some(protection_span.clone()), "this protector is live for this call".to_string()));
+                            }
+                        }
+                        Some(TagHistory::Untagged{ recently_created, recently_invalidated, matching_created, protected }) => {
+                            if let Some((range, span)) = recently_created {
+                                let msg = format!("tag was most recently created at offsets {}", HexRange(*range));
+                                helps.push((Some(span.clone()), msg));
+                            }
+                            if let Some((range, span)) = recently_invalidated {
+                                let msg = format!("tag was later invalidated at offsets {}", HexRange(*range));
+                                helps.push((Some(span.clone()), msg));
+                            }
+                            if let Some((range, span)) = matching_created {
+                                let msg = format!("this tag was also created here at offsets {}", HexRange(*range));
+                                helps.push((Some(span.clone()), msg));
+                            }
+                            if let Some((protecting_tag, protecting_tag_span, protection_span)) = protected {
+                                helps.push((Some(protecting_tag_span.clone()), format!("{:?} was protected due to a tag which was created here", protecting_tag)));
+                                helps.push((Some(protection_span.clone()), "this protector is live for this call".to_string()));
+                            }
+                        }
+                        None => {}
+                    }
+                    helps
                 }
                 MultipleSymbolDefinitions { first, first_crate, second, second_crate, .. } =>
                     vec![

src/eval.rs

@@ -15,6 +15,7 @@ use rustc_target::spec::abi::Abi;
 
 use rustc_session::config::EntryFnType;
 
+use rustc_span::DUMMY_SP;
 use std::collections::HashSet;
 
 use crate::*;
@@ -310,6 +311,9 @@ pub fn eval_entry<'tcx>(
             let info = ecx.preprocess_diagnostics();
             match ecx.schedule()? {
                 SchedulingAction::ExecuteStep => {
+                    if let Some(sb) = ecx.machine.stacked_borrows.as_mut() {
+                        sb.get_mut().current_span = DUMMY_SP;
+                    }
                     assert!(ecx.step()?, "a terminated thread was scheduled for execution");
                 }
                 SchedulingAction::ExecuteTimeoutCallback => {

src/helpers.rs

@@ -813,3 +813,12 @@ pub fn get_local_crates(tcx: &TyCtxt<'_>) -> Vec<CrateNum> {
     }
     local_crates
 }
+
+/// Formats an AllocRange like [0x1..0x3], for use in diagnostics.
+pub struct HexRange(pub AllocRange);
+
+impl std::fmt::Display for HexRange {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "[{:#x}..{:#x}]", self.0.start.bytes(), self.0.end().bytes())
+    }
+}

src/machine.rs

@@ -25,6 +25,7 @@ use rustc_middle::{
 };
 use rustc_span::def_id::{CrateNum, DefId};
 use rustc_span::symbol::{sym, Symbol};
+use rustc_span::DUMMY_SP;
 use rustc_target::abi::Size;
 use rustc_target::spec::abi::Abi;
 
@@ -561,6 +562,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         alloc: Cow<'b, Allocation>,
         kind: Option<MemoryKind<Self::MemoryKind>>,
     ) -> Cow<'b, Allocation<Self::PointerTag, Self::AllocExtra>> {
+        set_current_span(&ecx.machine);
         if ecx.machine.tracked_alloc_ids.contains(&id) {
             register_diagnostic(NonHaltingDiagnostic::CreatedAlloc(id));
         }
@@ -589,6 +591,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         ecx: &MiriEvalContext<'mir, 'tcx>,
         ptr: Pointer<AllocId>,
     ) -> Pointer<Tag> {
+        set_current_span(&ecx.machine);
         let absolute_addr = intptrcast::GlobalStateInner::rel_ptr_to_addr(ecx, ptr);
         let sb_tag = if let Some(stacked_borrows) = &ecx.machine.stacked_borrows {
             stacked_borrows.borrow_mut().base_tag(ptr.provenance)
@@ -624,6 +627,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         (alloc_id, tag): (AllocId, Self::TagExtra),
         range: AllocRange,
     ) -> InterpResult<'tcx> {
+        set_current_span(&machine);
         if let Some(data_race) = &alloc_extra.data_race {
             data_race.read(alloc_id, range, machine.data_race.as_ref().unwrap())?;
         }
@@ -647,6 +651,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         (alloc_id, tag): (AllocId, Self::TagExtra),
         range: AllocRange,
     ) -> InterpResult<'tcx> {
+        set_current_span(&machine);
         if let Some(data_race) = &mut alloc_extra.data_race {
             data_race.write(alloc_id, range, machine.data_race.as_mut().unwrap())?;
         }
@@ -655,7 +660,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
                 alloc_id,
                 tag,
                 range,
-                machine.stacked_borrows.as_mut().unwrap(),
+                machine.stacked_borrows.as_ref().unwrap(),
             )
         } else {
             Ok(())
@@ -670,6 +675,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         (alloc_id, tag): (AllocId, Self::TagExtra),
         range: AllocRange,
     ) -> InterpResult<'tcx> {
+        set_current_span(&machine);
         if machine.tracked_alloc_ids.contains(&alloc_id) {
             register_diagnostic(NonHaltingDiagnostic::FreedAlloc(alloc_id));
         }
@@ -681,7 +687,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
                 alloc_id,
                 tag,
                 range,
-                machine.stacked_borrows.as_mut().unwrap(),
+                machine.stacked_borrows.as_ref().unwrap(),
             )
         } else {
             Ok(())
@@ -694,7 +700,12 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         kind: mir::RetagKind,
         place: &PlaceTy<'tcx, Tag>,
     ) -> InterpResult<'tcx> {
-        if ecx.machine.stacked_borrows.is_some() { ecx.retag(kind, place) } else { Ok(()) }
+        if ecx.machine.stacked_borrows.is_some() {
+            set_current_span(&ecx.machine);
+            ecx.retag(kind, place)
+        } else {
+            Ok(())
+        }
     }
 
     #[inline(always)]
@@ -740,7 +751,12 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
 
     #[inline(always)]
     fn after_stack_push(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
-        if ecx.machine.stacked_borrows.is_some() { ecx.retag_return_place() } else { Ok(()) }
+        if ecx.machine.stacked_borrows.is_some() {
+            set_current_span(&ecx.machine);
+            ecx.retag_return_place()
+        } else {
+            Ok(())
+        }
     }
 
     #[inline(always)]
@@ -757,3 +773,30 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         res
     }
 }
+
+// This is potentially a performance hazard.
+// Factoring it into its own function lets us keep an eye on how much it shows up in a profile.
+///
+fn set_current_span<'mir, 'tcx: 'mir>(machine: &Evaluator<'mir, 'tcx>) {
+    if let Some(sb) = machine.stacked_borrows.as_ref() {
+        if sb.borrow().current_span != DUMMY_SP {
+            return;
+        }
+        let current_span = machine
+            .threads
+            .active_thread_stack()
+            .into_iter()
+            .rev()
+            .find(|frame| {
+                let info = FrameInfo {
+                    instance: frame.instance,
+                    span: frame.current_span(),
+                    lint_root: None,
+                };
+                machine.is_local(&info)
+            })
+            .map(|frame| frame.current_span())
+            .unwrap_or(rustc_span::DUMMY_SP);
+        sb.borrow_mut().current_span = current_span;
+    }
+}