ask people to reach out if we declare too much UB

src/borrow_tracker/stacked_borrows/diagnostics.rs

@@ -6,11 +6,19 @@ use rustc_span::{Span, SpanData};
 use rustc_target::abi::Size;
 
 use crate::borrow_tracker::{
-    stacked_borrows::{err_sb_ub, Permission},
-    AccessKind, GlobalStateInner, ProtectorKind,
+    stacked_borrows::Permission, AccessKind, GlobalStateInner, ProtectorKind,
 };
 use crate::*;
 
+/// Error reporting
+fn err_sb_ub<'tcx>(
+    msg: String,
+    help: Vec<String>,
+    history: Option<TagHistory>,
+) -> InterpError<'tcx> {
+    err_machine_stop!(TerminationInfo::StackedBorrowsUb { msg, help, history })
+}
+
 #[derive(Clone, Debug)]
 pub struct AllocHistory {
     id: AllocId,
@@ -381,9 +389,13 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
             self.history.id,
             self.offset.bytes(),
         );
+        let mut helps = vec![operation_summary(&op.info.summary(), self.history.id, op.range)];
+        if op.info.in_field {
+            helps.push(format!("errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling"));
+        }
         err_sb_ub(
             format!("{action}{}", error_cause(stack, op.orig_tag)),
-            Some(operation_summary(&op.info.summary(), self.history.id, op.range)),
+            helps,
             op.orig_tag.and_then(|orig_tag| self.get_logs_relevant_to(orig_tag, None)),
         )
     }
@@ -406,7 +418,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
         );
         err_sb_ub(
             format!("{action}{}", error_cause(stack, op.tag)),
-            Some(operation_summary("an access", self.history.id, op.range)),
+            vec![operation_summary("an access", self.history.id, op.range)],
             op.tag.and_then(|tag| self.get_logs_relevant_to(tag, None)),
         )
     }
@@ -432,7 +444,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
             Operation::Dealloc(_) =>
                 err_sb_ub(
                     format!("deallocating while item {item:?} is {protected} by call {call_id:?}",),
-                    None,
+                    vec![],
                     None,
                 ),
             Operation::Retag(RetagOp { orig_tag: tag, .. })
@@ -441,7 +453,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
                     format!(
                         "not granting access to tag {tag:?} because that would remove {item:?} which is {protected} because it is an argument of call {call_id:?}",
                     ),
-                    None,
+                    vec![],
                     tag.and_then(|tag| self.get_logs_relevant_to(tag, Some(item.tag()))),
                 ),
         }
@@ -459,7 +471,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
                 alloc_id = self.history.id,
                 cause = error_cause(stack, op.tag),
             ),
-            None,
+            vec![],
             op.tag.and_then(|tag| self.get_logs_relevant_to(tag, None)),
         )
     }

src/borrow_tracker/stacked_borrows/mod.rs

@@ -21,7 +21,7 @@ use rustc_middle::ty::{
 use rustc_target::abi::{Abi, Size};
 
 use crate::borrow_tracker::{
-    stacked_borrows::diagnostics::{AllocHistory, DiagnosticCx, DiagnosticCxBuilder, TagHistory},
+    stacked_borrows::diagnostics::{AllocHistory, DiagnosticCx, DiagnosticCxBuilder},
     AccessKind, GlobalStateInner, ProtectorKind, RetagFields,
 };
 use crate::*;
@@ -170,15 +170,6 @@ impl NewPermission {
     }
 }
 
-/// Error reporting
-pub fn err_sb_ub<'tcx>(
-    msg: String,
-    help: Option<String>,
-    history: Option<TagHistory>,
-) -> InterpError<'tcx> {
-    err_machine_stop!(TerminationInfo::StackedBorrowsUb { msg, help, history })
-}
-
 // # Stacked Borrows Core Begin
 
 /// We need to make at least the following things true:

src/diagnostics.rs

@@ -22,7 +22,7 @@ pub enum TerminationInfo {
     UnsupportedInIsolation(String),
     StackedBorrowsUb {
         msg: String,
-        help: Option<String>,
+        help: Vec<String>,
         history: Option<TagHistory>,
     },
     TreeBorrowsUb {
@@ -224,11 +224,10 @@ pub fn report_error<'tcx, 'mir>(
                     (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")),
                 ],
             StackedBorrowsUb { help, history, .. } => {
-                let url = "https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md";
                 msg.extend(help.clone());
                 let mut helps = vec![
                     (None, format!("this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental")),
-                    (None, format!("see {url} for further information")),
+                    (None, format!("see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information")),
                 ];
                 if let Some(TagHistory {created, invalidated, protected}) = history.clone() {
                     helps.push((Some(created.1), created.0));

tests/fail/both_borrows/buggy_split_at_mut.stack.stderr

@@ -7,8 +7,9 @@ LL | |                 from_raw_parts_mut(ptr.offset(mid as isize), len - mid),
 LL | |             )
    | |             ^
    | |             |
-   | |_____________trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-   |               this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x10]
+   | |             trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   | |_____________this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x10]
+   |               errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/both_borrows/pass_invalid_shr_option.stack.stderr

@@ -6,6 +6,7 @@ LL |     foo(some_xref);
    |         |
    |         trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x4]
+   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/both_borrows/pass_invalid_shr_tuple.stack.stderr

@@ -6,6 +6,7 @@ LL |     foo(pair_xref);
    |         |
    |         trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x4]
+   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/both_borrows/return_invalid_shr_option.stack.stderr

@@ -6,6 +6,7 @@ LL |     ret
    |     |
    |     trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/both_borrows/return_invalid_shr_tuple.stack.stderr

@@ -6,6 +6,7 @@ LL |     ret
    |     |
    |     trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/return_invalid_mut_option.stderr

@@ -6,6 +6,7 @@ LL |     ret
    |     |
    |     trying to retag from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/return_invalid_mut_tuple.stderr

@@ -6,6 +6,7 @@ LL |     ret
    |     |
    |     trying to retag from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information